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GREATEST MYSTERIES OF THE UNIVERSE by David J. Eicher COLLECTOR’S EDITION contents PAGE PAGE A universe of limitless wonder ...... 6 26. What creates gravitational waves?...... 54 1. How old is the universe? ...... 8 27. What happens when black holes collide? . . . 55 2. How big is the universe? ...... 10 28. Why does antimatter exist?...... 57 3. How did the Big Bang happen?...... 12 29. Are there other planets like Earth?...... 58 4. What is dark matter? ...... 14 30. Does every big galaxy have a central black hole?...... 60 5. How did galaxies form? ...... 15 31. Does inflation theory govern the universe? . . 62 6. How common are black holes? ...... 17 32. Should Pluto be considered a planet? ...... 64 7. How many planets are in the solar system? . . 19 33. Why did Venus turn itself inside-out?...... 66 8. Are we alone?...... 21 34. How could we recognize life elsewhere 9. What is the fate of the universe?...... 23 in the cosmos?...... 69 10. What will happen to life on Earth?...... 24 35. What created Saturn’s rings? ...... 70 11. What is dark energy?...... 26 36. Could a distant, dark body end life on Earth? ...... 72 12. What are gamma-ray bursts? ...... 28 37. Do we live in a multiple universe? ...... 74 13. Will asteroids threaten life on Earth?...... 30 38. How did the Milky Way Galaxy form?...... 75 14. Is water necessary for life?...... 32 39. How did the solar system form? ...... 77 15. Is there life on Mars, Titan, or Europa?...... 34 40. What happens when galaxies collide? ...... 78 16. Why did Mars dry out? ...... 36 41. How do massive stars explode? ...... 80 17. How did the Moon form?...... 37 42. What will happen to the Sun? ...... 82 18. Where do meteorites come from? ...... 39 43. Did comets bring life to Earth? ...... 84 19. Can light escape from black holes? ...... 40 44. How did quasars form?...... 85 20. Did stars, galaxies, or black holes come first?...... 42 45. Will the Milky Way merge with another galaxy? ...... 86 21. Where do cosmic rays come from?...... 44 46. How many brown dwarfs exist? ...... 88 22. How are comets and asteroids related?..... 46 47. What happens at the cores of 23. How many planets surround other galaxy clusters? ...... 90 star systems? ...... 48 48. Is Jupiter a failed star?...... 92 24. How many asteroids are locked up in the Kuiper Belt?...... 50 49. How many galaxies are in our Local Group?. . 94 25. Does string theory control the universe?.... 52 50. Do neutrinos hold secrets to the cosmos?. . . 96

BEAUTIFUL UNIVERSE. Towering columns of cosmic gas and dust make up the famous Pillars of Creation, located in the heart of the Eagle Nebula. NASA/ESA/HUBBLE HERITAGE TEAM (STSCI/AURA) GREATEST MYSTERIES OF THE UNIVERSE

COLLECTOR’S EDITION

Editor David J. Eicher Art Director LuAnn Williams Belter EDITORIAL Senior Editors Michael E. Bakich, Richard Talcott Production Editor Elisa R. Neckar A universe of Associate Editors Alison Klesman, Jake Parks Copy Editors Karri Ferron, Dave Lee Editorial Assistant Amber Jorgenson I/AURA; LEFT: WILLIAM ZUBACK WILLIAM LEFT: I/AURA;

ART C limitless wonder Graphic Designer Kelly Katlaps Illustrator Roen Kelly Production Specialist Jodi Jeranek We live in a golden age of astronomy. Consider the history of CONTRIBUTING EDITORS Bob Berman, Adam Block, Glenn F. Chaple, Jr., Martin George, modern humans: We struggled for 1,000 centuries to secure Tony Hallas, Phil Harrington, Korey Haynes, Jeff Hester, Liz Kruesi, Ray Jayawardhana, Alister Ling, Steve Nadis, I: FAR LEFT: NASA/ESA/STS LEFT: FAR I:

Stephen James O’Meara, Tom Polakis, Martin Ratcliffe, Mike D. C food and shelter. We spent another 50 centuries forming Reynolds, Sheldon Reynolds, Erika Rix, Raymond Shubinski SCIENCE GROUP early civilizations and building cities. And in but more often relies on countless small General Manager Tim Paulson just the last century alone, technology has Executive Editor Becky Lang steps. After reading 50 Greatest Mysteries of Design Director Dan Bishop helped us unravel the mysteries of the cosmos the Universe, you’ll better understand why EDITORIAL ADVISORY BOARD in ways our ancestors could not have imag- these burning questions captivate astrono- Buzz Aldrin, Marcia Bartusiak, Timothy Ferris, Alex Filippenko,

Adam Frank, John S. Gallagher lll, Daniel W. E. Green, William K. NASA/ESA/STS IMAGE: COVER ined. That's pretty humbling. Despite the fact mers, as well as grasp how researchers are Hartmann, Paul Hodge, Edward Kolb, Stephen P. Maran, that 5,000 generations of modern humans attempting to answer them. Whether you’re a Brian May, S. Alan Stern, James Trefil have pondered the heavens above, we’re seasoned observer or an armchair enthusiast, Kalmbach Media really just beginning to comprehend the story after reading this special issue, you'll have a Chief Executive Officer Dan Hickey Senior Vice President, Finance Christine Metcalf of the universe — and it’s mind-boggling. much deeper understanding of where Vice President, Content Stephen C. George Drawing on Astronomy magazine's long astronomy currently stands. Vice President, Consumer Marketing Nicole McGuire Vice President, Operations Brian J. Schmidt heritage as the leading publication in its field, But most importantly, you’ll have fun. Vice President, Human Resources Sarah A. Horner 50 Greatest Mysteries of the Universe explores Astronomical news seems to make headlines Senior Director, Advertising Sales and Events David T. Sherman Advertising Sales Director Scott Redmond the cosmos’ biggest questions. We published more often now than ever before, and Circulation Director Liz Runyon the first iteration of this special issue just over equipped with this special issue, you’ll have Art and Production Manager Michael Soliday New Business Manager Cathy Daniels a decade ago, but even in the short time everything you need to insightfully discuss the Retention Manager Kathy Steele since, the field of astronomy has marched cosmos in any situation. Did you stumble upon Single Copy Specialist Kim Redmond ADVERTISING DEPARTMENT forward faster than ever before. Because of a pop-up observing session in a park? Now Phone (888) 558-1544 Advertising Sales Manager Steve Meni this, we’ve updated our answers to include you can chime in about the creation of Saturn’s Advertising Sales Representative the latest theories and cutting-edge research. rings, talk about why Venus turned itself inside Dina Johnston, [email protected] Ad Services Representative The first few questions you’ll encounter are out, or discuss how many asteroids are Christa Burbank, [email protected] the classics: How old is the universe? How big trapped in the Kuiper Belt. Did someone bring RETAIL TRADE ORDERS AND INQUIRIES Selling Astronomy magazine or products in your store: is it? Are we alone? How did galaxies form? up exoplanets at a cocktail party? Now you can Phone (800) 558-1544 What will happen to life on Earth? regale them with your knowledge of hot Outside U.S. and Canada (262) 796-8776, ext. 818 Fax (262) 798-6592 Many of these questions undoubtedly Jupiters, brown dwarfs, and pulsar planets. Email [email protected] Website www.Retailers.Kalmbach.com darted through the minds of our early ascen- After all, that’s often the most satisfying part of CUSTOMER SALES AND SERVICE dants as they gazed upward at the shimmering this hobby: embarking on a quest for knowl- Phone (877) 246-4835 Outside U.S. and Canada (903) 636-1125 night sky. But despite the long history of these edge that allows you to place what we see into Customer Service [email protected] questions, most are answered only in part, and some sort of meaningful context. CONTACT US Ad Sales [email protected] some remain completely shrouded in mystery. By investing in this compelling issue, Ask Astro [email protected] Then there are the finer queries of the you'll gain a valuable perspective on the Books [email protected] Letters [email protected] cosmos, questions unimaginable until recent deep, dark universe that surrounds us all — a Products [email protected] Reader Gallery [email protected] times: Can light escape from black holes? perspective that our ancient ancestors could Editorial Phone (262) 796-8776 What are gamma-ray bursts? Why did Mars only dream of.

dry out? Where do cosmic rays come from? 50 Greatest Mysteries of the Universe 2018 (ISBN 978-0-89024-721-1) is published by Kalmbach Media Co., 21027 Crossroads Circle, P. O. Does string theory control the universe? Yours truly, Box 1612, Waukesha, WI 53187-1612. SINGLE COPY PRICE: U.S. $9.99, What creates gravitational waves? Canada and foreign $10.99. Canadian price includes GST. BN 12271 3209 RT. Canadian and foreign orders payable in U.S. funds. Copyright This special issue does not claim to hold © 2018 Kalmbach Media Co. Title registered as trademark; all rights reserved. Material in this publication may not be reproduced in any all the answers. Astronomy, like all sciences, is form without permission. Printed in the U.S.A. an ever-evolving field — one that occasionally takes giant leaps toward understanding, Jake Parks

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WWW.ASTRONOMY.COM 7 How old is the universe?

Over the past century, astronomers have deduced several per megaparsec and the other up at 80 km/ sec/Mpc. (A megaparsec equals 3.26 million ways to estimate the age of the universe. Yet at the dawn of light-years, or about 20 billion billion miles.) this century, the universe’s age remained far from certain. Therefore, the two groups estimated a range for the age of the universe of about 10 to Fortunately, the launch of NASA’s Wilkinson well as how fast they recede from Earth. 16 billion years. (Higher values of the Hubble Microwave Anisotropy Probe (WMAP) in 2001 They then determine the Hubble constant constant produce younger age values for the and the European Space Agency’s Planck sat- by dividing the galaxy’s speed of recession universe.) Research by various groups, includ- ellite in 2009 changed all that. Still, astrono- by its distance. Once they decide on a value ing Wendy Freedman and her colleagues in mers’ attempts during the previous century to for the Hubble constant, they can estimate the Key Project — astronomers who were narrow the age estimates makes for a fasci- the maximum age of the universe by calcu- using the Hubble Space Telescope to mea- nating detective story. lating the constant’s reciprocal. sure the distances to many galaxies — Before WMAP and Planck, the best But there was a problem. The values narrowed in on a value toward the faster, approach for determining the universe’s age astronomers got for the Hubble constant and thus younger, end of the scale. But relied on the much-debated Hubble con- depended on various assumptions about the uncertainties still remained. stant, a figure that describes the rate at universe’s density and composition and the The other series of approaches for deter- which the universe is expanding. To find the method used to determine distances. So mining the universe’s age attempted to Hubble constant, astronomers observe dis- astronomers of different mindsets got differ- directly measure the ages of the oldest tant galaxies and measure their distances ent values for the constant. objects in the universe. Astronomers can esti- (by using Cepheid variable stars or other They generally divided into two camps, mate the age of the cosmos by measuring objects of known intrinsic brightness) as one in the range of 50 kilometers per second the decay of radioactive elements. This

8 50 GREATEST MYSTERIES STAR BLAST. When the technique yields ages of 4.4 billion years for calculations pointed to a universe between Such studies of universe was less than a the oldest rocks on Earth (zircons found in 12 and 15 billion years old, with a large uncer- numerous globulars, billion years old, it was little more than a sea of hydrogen Jack Hills, Australia) and 4.6 billion years for tainty of plus or minus 3 to 4 billion years. based on distance mea- and helium. A sudden blast the oldest meteorites, effectively dating the Alternatively, astronomers measured the surements provided by of star formation turned on solar system but not the universe. ages of white-dwarf stars, the shrunken rem- the European Space lights within the darkness. Applying this method to gas in the Milky nants of stars that are as heavy as the Sun but Agency’s Hipparcos and NASA/ESA/ADOLF SCHALLER Way or to old stars is less precise, however, only as large as Earth. By finding the faintest, Gaia missions, suggested due to assumptions about the primordial and thus oldest, white dwarfs, astronomers an age for many of the oldest stars of around abundances of various isotopes. These estimated how long they have been cooling. 13 billion years. And astronomers think the Comprehensive attempts at cataloging white age of globulars gives a pretty good indica- dwarfs and measuring their ages yielded tion for the age of the universe. That’s because about 10 billion years for the age of the Milky globulars contain hardly any elements heavier Way’s disk. The galaxy’s disk formed about than hydrogen and helium, and so had to be 2 billion years after the Big Bang, yielding an among the first objects to form. age of the universe of about 12 billion years. Any discrepancies narrowed significantly Measuring the ages of ancient star clus- with the release of WMAP data, before ters offers yet another avenue for exploring essentially disappearing when researchers the age of the universe. By looking at the announced Planck’s latest findings in 2015. most luminous stars in a globular cluster, By carefully examining the microwave back-

NASA/ADOLF SCHALLER NASA/ADOLF astronomers can determine an upper limit for ground radiation, astronomers have pinned the cluster’s age. They look at the brightest down the universe’s age to 13.8 billion years, BUILDING PROJECT. When our galaxy formed, its spiral arms had not coalesced, stars on the so-called main sequence — the accurate to better than 1 percent. The and the sky was a sea of globular clusters. primary track on a plot of stellar brightnesses results pretty much ended the debate, versus temperatures. but what a debate it was.

WWW.ASTRONOMY.COM 9 of t UNIVERSAL SIZE. Since the nt age he un rre ive Cu rse Big Bang, the expansion of the universe has slowed and then sped up. In this illustration, How big is the concentric red Expansion Expansion slows down speeds up circles show that galaxies migrated universe? Big Bang apart slowly during the first half of cosmic history, and then a mysterious Two great debates have taken center stage in the force — dark energy — accelerated the Galaxies search to answer this age-old question. In April expansion. NASA/ANN FEILD (STSCI) 1920, Harlow Shapley and Heber Curtis argued over the scale of the universe in the great audito- which would indicate an older, larger be the case in the highly likely event that rium of the Smithsonian Institution’s Natural universe. the inflation hypothesis, put forth in 1980 History Museum in Washington, D.C. As was the case with Shapley and by MIT’s Alan Guth, proves correct. This In this discussion, which preceded Edwin Curtis, the antagonists van den Bergh and idea suggests that the extremely young Hubble’s discovery of the nature of galaxies Tammann each provided crisp, clear-cut universe experienced a brief period of by just a few years, Curtis argued that the arguments and data supporting his side, hypergrowth so severe that it ballooned cosmos consists of many separate “island and neither succeeded in convincing astron- from the size of a subatomic particle to the universes,” claiming that the so-called spiral omers from the other camp. As yet, astron- size of a softball almost instantly. If inflation nebulae were distant systems of stars out- omers are limited by both assumptions occurred, then the universe is much larger side our Milky Way. Meanwhile, Shapley and a lack of adequate data to agree on the than we might expect based on current argued that spiral nebulae were merely gas cosmic distance scale. observations. clouds in the Milky Way. Shapley further Despite this, astronomers can still set Here’s where it gets weird: If inflation hap- placed the Sun toward the edge of our gal- some limits on what must be true based on pened, then it may have occurred in many axy — which, in his view, was the entire uni- the observations they have collected and places (perhaps an infinite number) beyond verse — whereas Curtis believed the Sun to refined over the past century. Using today’s the visible horizon and the limits of the be near the galaxy’s center. Curtis was right most powerful telescopes, astronomers see space-time continuum we are familiar with. If about the large size of the universe but galaxies located over 13 billion light-years this is so, then other universes might exist wrong about the Sun’s place within it. On from Earth. (A light-year equals about 6 tril- beyond our ability to detect them. Science the other hand, Shapley was wrong about lion miles, or 10 trillion kilometers.) Since begs off this question, as by definition sci- the small size of the universe but right they see these distant galaxies in all direc- ence is about creating and experimenting about the Sun’s location within it. tions, the current “horizon” of visibility is at with testable ideas. For now, it’s wondrous With the advent of many extragalactic least 26 billion light-years in diameter. enough to know we live in a universe that’s at distance measurements and two camps But the universe is probably much least 550 billion trillion miles across, and it arguing for different results on the critical larger than the portion we can see. This will may be much bigger than that. number called the Hubble constant — the ULTRA DEEP FIELD. expansion rate of the universe — astrono- In 2014, when mers staged a second great debate in 1996. astronomers added The age and size of the universe are, of ultraviolet data to the course, interrelated, and both depend Hubble Ultra Deep Field, critically on the Hubble constant. they were able to study In the same auditorium used by Shapley some of the youngest, largest, and hottest and Curtis, galaxy researchers Sidney van stars located between den Bergh and Gustav Tammann argued 5 billion and 10 billion over the question. Van den Bergh offered light-years from Earth, evidence supporting a high value of the ultimately finding that Hubble constant (about 80 kilometers per young galaxies grow by second per megaparsec), suggesting a forming small groups of young age and therefore small size of the very hot stars. In this deep portrait of the universe. Tammann argued for a low value universe, we see of the constant (about 55 km/sec/Mpc), FAINT MYSTERIES. Strange, young star-forming galaxies (circled) in roughly 13 billion years this Hubble Space Telescope image from 2009 are less than a billion of cosmic history. NASA/ESA/G. ILLINGWORTH AND R. BOUWENS (UNIVERSITY OF CALIFORNIA, SANTA CRUZ)/THE years old. NASA/ESA HUDF09 TEAM

10 50 GREATEST MYSTERIES

BIG-TIME THEORY. The discovery of the cosmic microwave background (CMB) confirmed the Big Bang theory. The CMB’s clumpiness gives astronomers evidence for theories ranging from what our universe’s contents are to how modern structures formed. ASTRONOMY: ROEN KELLY

How did the down into a few key eras and events. Standard cosmology, the set of ideas that are most reliable in helping decipher the uni- Big Bang verse’s history, applies from the present time back to about a hundredth of second after the Big Bang. But before then, particle physics happen? and quantum cosmology ruled the universe. When the Big Bang occurred, matter, Virtually all astronomers and cosmologists agree the universe energy, space, and time were all formed, and the universe was infinitely dense and incredi- began with a “big bang” — a tremendously powerful genesis bly hot. The often-asked question “What came before the Big Bang?” is outside the of space-time that sent matter and energy reeling outward. realm of science because it can’t be answered The evidence is clear, ranging from the ancient space-time from the Cosmic by scientific means. In fact, science says little underpinnings of Albert Einstein’s general Background Explorer (COBE) satellite in about the way the universe behaved until theory of relativity, to the detection of the 1992. But the devil is in the details, and some 10–43 second after the Big Bang, when cosmic microwave background by Arno that’s where figuring out how Big Bang cos- the Grand Unification Epoch began (and Penzias and Robert Wilson in the 1960s, to mology really works gets interesting. lasted only until about 10–35 second). Matter the confirmation of ripples in the fabric of The Big Bang model is typically broken and energy were interchangeable and in

12 50 GREATEST MYSTERIES equilibrium during this period, and the weak HIGH-RES ECHO. Evidence for the Big Bang and strong nuclear forces and electromag- comes from detailed data collected by the netism were all equivalent. Cosmic Background Explorer (COBE; top), The universe cooled rapidly as it blew which launched in 1989; the Wilkinson Microwave Anisotropy Probe (WMAP; mid- outward, however, and by 10–35 second after dle), which launched in 2001; and the Planck the Big Bang, the epoch of inflation space observatory (bottom), which launched in occurred, enlarging the universe by a factor 2009. NASA/WMAP SCIENCE TEAM/ESA/PLANCK COLLABORATION of 1050 in only 10–34 second. During this wild period, cosmic strings, monopoles, and other exotic species likely came to be. As transparent. Matter and radiation were sensational as inflation sounds, it explains finally separate. several observations that would otherwise Observational astronomers consider be difficult to reconcile. After inflat- much of the history of the early ing, the universe slowed down universe the province of par- its expansion rate but con- When ticle physicists, describing tinued to grow, as it does what happened up to still. It also cooled sig- the Big Bang the formation of galaxies, nificantly, allowing for stars, and black holes as the formation of matter occurred, matter, “a lot of messy physics.” — first neutrinos, elec- They are more inter- trons, quarks, and pho- energy, space, ested in how the tons, followed by first astronomical protons and neutrons. and time were objects, the large- Likewise, antiparticles were all formed. scale inhabitants of the produced in abundance, car- universe, came to be about 1 rying the opposite charge of their billion years after the Big Bang. But corresponding particles (positrons along before these astronomers can gain a clear with electrons, for example). picture of that process, they need to consider As time went on and particles’ rest-mass the role of the wild card — dark matter. energy was greater than the thermal energy of the universe, many were annihilated with their partners, producing gamma rays in the COSMIC DISCOVERY. process. As more time crept by, these anni- Robert Wilson (left) and hilations left an excess of ordinary matter Arno Penzias unexpectedly over antimatter. discovered the cosmic Chemistry has its roots deep in the his- microwave background radiation with this horn- tory of the universe. At a key moment about shaped antenna. one second after the Big Bang, nucleosynthesis took place and created deuterium along with the light elements helium and lithium. After some 10,000 years, the temperature of the universe cooled to the point where massive particles contributed more to the universe’s overall energy density than light and other radiation, which had dominated until then. This turned on gravity as a key player, and the little irregularities in the density of matter were magnified into structures as the universe expanded. The relic radiation of the Big Bang decou- pled (picture heavy traffic suddenly clear- ing) nearly 400,000 years later, creating the resonant echo of radiation observed by Penzias and Wilson with their radio telescope. This decoupling moment witnessed

the universe changing from opaque to PACIFIC THE SOCIETYASTRONOMICAL OF

WWW.ASTRONOMY.COM 13 makes up this stuff? Over the years, scientists have proposed many different possibilities, and each has its own strengths and weaknesses in terms of explaining astronomers’ observations. They include massive numbers of normal neutrinos; MACHOs (massive compact halo objects) such as brown dwarfs, neutron stars, and black DARK-MATTER MAP. holes; and WIMPs (weakly interacting mas- Gravitational lensing sive particles) such as exotic particles, within galaxy cluster axions, massive neutrinos, and photinos. CL0025+1654, mapped Whatever it consists of, dark matter car- by the Hubble Space Telescope, revealed that ries enormous implications for the structure enormous halos of dark and future of the cosmos, as it accounts matter (in blue) surround for 26 percent of the universe’s total the brightest galaxies. mass-energy. About 85 percent of the matter in the universe consists of both baryonic J.-P. KNEIB/ESA/NASA J.-P. and non-baryonic dark matter. (Baryons are particles consisting of three quarks that interact through the strong nuclear force — including protons and neutrons.) What is Of non-baryonic dark matter, two candidates exist: hot dark matter (HDM) and cold dark matter (CDM). The “temperature” in dark matter? each model refers to the particles’ velocities. Neutrinos represent the likeliest HDM candi- Astronomers might be more confident about their picture date, while WIMPs are the favorite of the CDM possibilities. MACHOs, which are a form of the universe were it not for dark matter. Observations of baryonic dark matter, only constitute a small percentage of the total. show that the universe is populated with some unseen An HDM-dominated universe would sug- form of matter — and plenty of it. galaxy orbit at a steady velocity indepen- gest little matter exists between clusters of Astronomers attempt to “weigh” the uni- dent of how far from the galaxy’s center they galaxies; however, observations over the past verse in a variety of ways. They observe the are. The most logical explanation for this is decade have shown this is not the case, largely effects of dark matter on astronomical that massive spherical halos of dark matter discrediting the HDM model of the universe. objects that vary from small to large. surround the visible matter in galaxies. Had massive numbers of neutrinos been Dark matter was posited by Dutch astron- Other clues for the existence of dark mat- created in the early universe, they likely omer Jan Oort in the 1930 when he studied ter come from studying galaxy clusters. Also would have smoothed out the ripples in the star motions in the Sun’s neighborhood. in the 1930s, American astronomer Fritz cosmic microwave background. Because the galaxy was not flying apart, he Zwicky deduced that much larger clouds of This didn’t happen. The vast majority of reasoned, enough matter must reside in the dark matter exist in the Coma cluster of gal- dark matter, therefore, must exist in some disk to keep the stars from moving away axies, about 300 million light-years from form of CDM. The odds are leaning toward from the galaxy’s center. Oort postulated that Earth. By looking at the Doppler shifts of massive, exotic, relatively slow-moving par- in the Sun’s neighborhood, three times as individual galaxies in the cluster, Zwicky ticles. But astronomers must make great much dark matter existed as bright matter. concluded that 10 times more mass than strides before we’ll know the exact identity Stronger evidence came later as astrono- was detected via visible light must be pres- of dark matter, one of the century’s greatest mers examined the luminous disks and ent to keep the galaxies gravitationally astronomical mysteries. halos of galaxies. By studying galactic rota- bound. COSMIC TRAIN WRECK. tion curves, astronomers can glimpse how One of the The Bullet Cluster, which some dark matter is distributed. great mysteries represents a spectacular The process works like this: Newton’s of recent collision between two law of gravity says stars revolving about the decades centers galaxy clusters, affords center of a galaxy should slow dramatically on a very basic astronomers the chance to the farther away they are from the galactic question about study dark matter in a natural center. But the rotation curves of galaxies dark matter: laboratory. Since dark matter does not interact with are “flat,” meaning the stars in an individual What exactly baryonic matter, when two galaxy clusters collide, their dark matter (blue) passes

through unimpeded. TEAM HERITAGE HUBBLE NASA/THE 14 50 GREATEST MYSTERIES How did galaxies form? While observational tests on the details of cosmology proceed apace, astronomers are focusing on the mechanics of how matter came together in the early universe. The fundamental question is: Did galaxies, galaxies and tried to interpret how the gal- stars, or black holes come first? The infant axies formed. One of the premier research- universe was a relatively uniform sea of ers at California’s Mount Wilson Observatory several-thousand-degree gas and dark in the 1950s, Baade discovered a group of matter — the unseen, mysterious, and pre- stars around the Milky Way with few metals dominant form of matter that is indirectly (elements heavier than hydrogen and

known to exist because of its gravitational helium). These stars are ancient, probably 13 TEAM HERITAGE HUBBLE NASA/THE influence. But how galaxies, stars, and black billion years old. Metals thrown out into WHEEL IN THE SKY. Hoag’s Object is a holes came together is the key to under- interstellar space by supernovae and other ring of hot blue stars wheeling around a standing the puzzle of the early universe. processes were eventually incorporated into cooler yellow nucleus. The whole galaxy measures about 120,000 light-years across, Based on cosmic microwave background a younger generation of stars in our galaxy. slightly larger than the Milky Way. data, astronomers think matter coalesced Baade’s discovery led to a model of gal- when the universe cooled and became axy formation in the 1960s nicknamed ELS, “transparent” 380,000 years after the Big after Olin Eggen, Donald Lynden-Bell, and turn of the century is the merger theory. It Bang. And according to recent studies, Allan Sandage. The ELS model says galaxies could have been hatched on Wall Street structures like stars and galaxies formed as collapsed as single objects out of gas when the merger buzz was about AOL with early as 200 million years after the Big Bang. clouds. As the gas fell in by gravity, it first Time-Warner and Exxon with Mobil. But But exactly how matter clumped is open to formed a spherical halo. As more gas those mergers are minuscule when com- future research. coalesced, it began spinning and was pared to the unions of protogalaxies — Deciphering galaxy formation goes enriched with metals, creating galactic disks. blobs of starless, gravitationally bound gas back to Walter Baade, who studied stars in A different idea proposed around the that formed galaxies in the early universe — and galaxies of various sizes that merged together later on. Indeed, over the years it has become increasingly clear that many galaxies, perhaps the vast majority, formed when small gas clouds came together, merging into larger and larger structures as time went on. This process is called the bottom-up path to galaxy formation. “We don’t really know which is the dominant path yet,” says John S.

A STAR-LADEN SOMBRERO. Beautifully formed spiral galaxies like the Sombrero Galaxy, seen from our line of sight as edge-on, coalesced as clumps of matter aggregated in the

NASA/THE HUBBLE HERITAGE TEAM HERITAGE HUBBLE NASA/THE early universe.

WWW.ASTRONOMY.COM 15 I C P. GOUDFROOIJ/STS P.

LOOPS AND BLOBS. Gallagher III, an emeritus galaxies we see around us. Many galaxy Just four decades ago, astronomers The galaxy NGC 1316 professor at the University experts now believe the Milky Way may have thought black holes — regions of intense shows hints of its chaotic of Wisconsin-Madison. formed from the mergers of 100 or gravity from which no matter or past. Probably the result “There’s a strong theoretical more small galaxies over time. light can escape — were math- of a head-on collision prejudice to make small The question of whether ematical oddities. But now between two galaxies, Black NGC 1316 exhibits great things and have them grow galaxies came together as astronomers armed with turbulence in its core. bigger by having gas fall gas and then com- holes exist in large telescopes infer into them or capturing menced forming stars or their presence in the their neighbors. But astronomers haven’t yet whether stars formed the centers of centers of most large- proven that this is the main way it happens.” from little pockets of and medium-sized However, astronomers have accumulated gas and then aggre- most large- and galaxies. They are the decades’ worth of circumstantial evidence gated into galaxies is driving engines of qua- that points to mergers being the primary unclear. A third possibility medium-sized sars, which are highly path to forming galaxies. is that black holes formed energetic objects seen in Multiple “deep field” images by Hubble initially as dense pockets of galaxies. the most distant galaxies. show distant galaxies and reveal numerous matter. They then swept up mate- Astronomers are now close to a blob-like objects that appear to be proto- rial around themselves, and galaxies consensus that supermassive black holes galaxies. These are likely the fragments that formed from the surrounding gas that inhabit the cores of most galaxies, but per- clung together to form the larger “normal” didn’t get sucked in by the black holes. haps not the small ones.

16 50 GREATEST MYSTERIES you could throw a baseball at a velocity of How common 7 miles per second, you could hurl it into space, overcoming Earth’s gravitational tug. As massive objects are crushed into smaller are black holes? volumes, their gravitational pull increases dramatically. In a black hole, the escape velocity exceeds 186,000 miles per second On the heels of Einstein’s general theory of relativity, German — the speed of light — and everything inside the hole is trapped. theoretical astrophysicist Karl Schwarzschild provided a So, if black holes are black, how do astronomers know they exist? Since they are detailed proposal on the existence of black holes in 1916. not directly visible, black holes must be The concept of black holes goes all the way star systems in the 1970s and early 1980s detected by their effects on nearby stars, back to the 1780s when John Michell and that it became obvious black holes must gas, dust, or even the fabric of space-time Pierre Simon Laplace envisioned “dark stars” exist. In the 1990s, it became clear to itself. In the Milky Way, many stellar-mass whose gravity was so strong that not even astronomers that black holes not only exist, black holes with masses ranging up to light could escape. As with many startling but are plentiful. about 10 times that of the Sun exist in ideas, the acceptance of black holes as real A black hole is a region of space-time binary star systems. objects took a long time. affected by such a dense gravitational field When a massive star dies, it explodes as a It wasn’t until astronomers were able to that nothing, not even light, can escape it. supernova. But the core of the exploded star observe lots of galaxies and massive binary Consider the escape velocity on Earth: If either remains behind as a neutron star or, if

BLAST BORN. A stellar- mass black hole in the binary system Cygnus X-1 hurtles through the plane of the Milky Way after the black hole’s creation in a supernova explosion. This imaginative artwork also reveals the bipolar jet emanating from the black hole’s center. NASA/CXC/M. WEISS gravitational wells. In any event, the “active” phases of some galaxies occur when fresh material falls into the central black holes, feeding them and emitting vast amounts of radiation we see with telescopes. Such an idea is akin to a “healthy” galaxy going through a periodic bout of “the flu,” which upsets and reconfigures its system. The idea that black holes are common got a boost in 2001 when the Chandra X-ray Observatory completed a survey of the X-ray sky and found an abundance of supermassive black holes in two “deep fields.” The Chandra data showed that these giant black holes were more active in the past than they are today, fitting the evolutionary picture nicely. Furthermore, since September 2015, researchers using the Laser Interferometer Gravitational-wave Observatory (LIGO) have detected the echoes of five pairs of merging black holes, proving that stellar-mass black holes are at least somewhat common throughout the universe, if not ubiquitous. In addition to ordinary black holes, theo- reticians have proposed “wormholes,” black holes with different degrees of rotation and electric charge. Science-fiction literature and movies suggest a wormhole could lead to travel through the space-time continuum, but no evidence exists as yet for wormholes. And, besides, the ride would be a little rough. Encountering a black hole of any type would “spaghettify” your body (and spacecraft, etc.) by pulling it into a very long line of protons

ESA/NASA/ADOLF SCHALLER ESA/NASA/ADOLF — before it’s fried by X-rays and gamma rays, which would make getting anywhere you BLACK-HOLE NEIGHBOR. it’s heavy enough (more of material. In fact, the Milky Way holds a rela- went a most unpleasant journey. A disk of young blue stars than 2.16 solar masses), tively modest 4.6-million-solar-mass black encircles a supermassive turns into a black hole. hole. Many hundreds of galaxies are what black hole at the center of Black holes become visi- astronomers call “active,” producing high-en- the Andromeda Galaxy ble when they exist in ergy emission from their cores, and are also (M31) in this artwork. The region of the black hole lies X-ray binaries, twin star suspected of harboring black holes. at the center of the disk, systems in which one of As observations of distant galaxies accu- barely visible. the stars has become a mulate, it has become clear that extragalac- black hole and the other tic black holes are common in the universe. is still there. The black hole shreds or perturbs It may be that black holes formed within all its companion, which ultimately unleashes a medium- and large-sized galaxies early in torrent of X-ray energy. the universe. And although supermassive But star-sized black holes aren’t the only black holes are probably not commonplace type. Research with the Hubble Space in dwarf galaxies, where there’s insufficient Telescope and large ground-based instru- mass, a handful of ultra-compact dwarf gal-

ments has uncovered several dozen clear-cut axies have recently been found to house (SUBMILLIMETER); ET AL. WEISS MPIFR/ESO/APEX/A. (OPTICAL); ESO/WFI (X-RAY) ET AL. KRAFT NASA/CXC/R. cases of supermassive black holes in the cen- monstrous black holes in their cores as well. COSMIC SEARCHLIGHT. A supermassive ters of galaxies. These are monstrously pow- It’s possible that “seed” black holes black hole at the center of galaxy Centaurus erful black holes that contain anywhere from even predated the formation of galaxies A fires lobed jets of material outward. a million to more than a billion solar masses and stars, which formed around these

18 50 GREATEST MYSTERIES How many planets are in the solar system? You might think astronomers know the solar system pretty well. And they do. But they might not know its whole story. In fact, it’s possible another planet lurks beyond Neptune (other than the once-planet Pluto), or even a Does the number 8 really constitute the faint, distant companion star to the Sun. whole inventory of the Sun’s planets? Many Hypothetical planets in the solar system, mathematical and observational exercises along with real ones, have turned up in have led astronomers to suspect other some pretty strange places. major bodies orbit the Sun. As early as Of course, the naked-eye planets — 1841, astronomers commenced a search for Mercury, Venus, Mars, Jupiter, and Saturn — various “Planet Xs.” The first one turned out were all known in antiquity and revered as to be Neptune. The second was Pluto, after

gods because they showed free will to move some seven different trans-Neptunian plan- NASA/JPL-CALTECH among the stars. The first telescopically ets (with different masses and orbits) had PLANET MAKER. In this artwork showing discovered planet was Uranus, found by been proposed by the most active searcher, the early solar system, an enormous dust William Herschel in 1781. After orbital calcu- E.C. Pickering, alone. disk stretches around the Sun and serves lations suggested a massive tug on Uranus But even after Pluto’s discovery, astrono- as a breeding ground for new planets as being applied farther out, Johann Galle, mers predicted planets beyond, mostly on material clumps together. with some help from Heinrich D’Arrest, dis- theoretical grounds. In 1946, Francis Sevin covered Neptune in 1846. predicted the existence of “Transpluto,” a Perturbations in Neptune’s orbit sug- planet 7 billion miles (11 billion kilometers) and 11 spacecraft and concluded a more gested yet another, more distant planet, from the Sun. (Pluto’s average orbital dis- distant planet may exist, with a mass of and many searches were conducted begin- tance is almost 3.7 billion miles.) about five Earths and an orbital period ning as early as 1877; Pluto was finally dis- In the 1950s, others hypothesized similar of 1,000 years. Conley Powell, also of JPL, covered by Clyde Tombaugh in 1930 by distant planets. Twenty years later, Tom van investigated the orbital data of Uranus and comparing pairs of photographic plates to Flandern of the U.S. Naval Observatory hypothesized a planet of three Earth masses help spot its motion. Oddly enough, the became convinced another planet existed some 5.6 billion miles perturbations weren’t really there, or at based on the orbital motions of Uranus and (9.0 billion km) from the DISTANT WANDERER. Discovered in 2003, Kuiper least Pluto wasn’t massive enough to cause Neptune. He and a colleague searched for Sun. When a search took Belt object Sedna glows them. In 2006, astronomers decided Pluto such a planet, but it was never found. place at Arizona’s Lowell faintly. Its discovery wasn’t up to planetary standards and so In 1987, John Anderson of JPL carefully Observatory using the intensified debate over the demoted it to a “dwarf planet.” examined the trajectory of the Pioneer 10 parameters from Powell’s definition of “planets.”

Nov. 14, 2003

1:32 A.M. EST 3:03 A.M. EST 4:38 A.M. EST NASA/CALTECH/MIKE BROWN NASA/CALTECH/MIKE

WWW.ASTRONOMY.COM 19 PLANET 10? In 2005, calculations, nothing of what led to Pluto’s demotion. And it leads Earth, Venus, and Mars, off and on. There astronomers discovered Eris the correct brightness to the most important question: What was “Nemesis,” a proposed distant compan- (illustrated here), which was in the right spot turned makes a planet, anyway? In 2006, the ion star to the Sun, that lurks about 1 light- announced as “the tenth up. Although a Planet X International Astronomical Union year away and occasionally kicks a new planet” before Pluto’s may exist, none has been decided that a planet is any set of comets sunward from the demotion. Lying 10 billion miles from the Sun, it’s the found. object orbiting a star that is Oort Cloud. most distant large body Still, intense interest neither a star itself nor It leads The latest alarm came at known in the solar system. in the outer solar system another planet’s moon; to the most the turn of the 21st cen- NASA/JPL-CALTECH has paid off handsomely. contains enough mass tury when researchers in With the first discovery for its gravity to have important the United Kingdom of a so-called Kuiper Belt object in 1992, forced it into a spheri- and at Louisiana State David Jewitt, Jane Luu, and other astrono- cal shape; and is big question: What University proposed the mers have uncovered a new element of the enough to have cleared existence of a gas giant solar system. As many as 100,000 bodies its orbital path of debris. makes a planet, planet named Tyche over 62 miles (100 km) in diameter — aster- Other hypotheses inside the distant Oort oids and burned-out comets — exist in a about solar system mon- anyway? Cloud. But none of these zone extending from the orbit of Neptune sters have come and gone. ideas has panned out or been outward, some 2.8 billion to 4.5 billion miles There was Vulcan, an intra-Mercurial verified by observation. At the least, we (4.5 to 7.2 billion km) from the Sun. planet, thought to exist in the late 19th cen- have a middleweight star with eight planets, The Kuiper Belt discoveries have infused tury (and briefly revived based on flawed at least five dwarf planets, and a vast collec- new sophistication into planetary scientists’ observations in 1970–1971). There were tion of small bodies orbiting it. But make no understanding of solar system dynamics. It’s phantom moons reported around Mercury, mistake: Astronomers will keep searching.

20 50 GREATEST MYSTERIES Are we alone? Astronomers as yet lack the technology to directly detect life on planets light-years away. But by looking for chemical signatures of life in the spectra of exoplanets, they can make educated guesses about the habitabil- environments. They maintain homeostasis, ity of other worlds. or internal balance. They evolve and adapt. The next generation of orbiting space Some living things even have evolved to telescopes is set to produce some astound- the point where they can walk and think PALE BLUE DOT. In 1990, Voyager 1 ing exoplanetary science over the next about the universe that surrounds them. We snapped this iconic portrait of Earth decade, adding onto the Kepler space tele- are literally products of the universe. Most suspended in a sunbeam, underscoring scope’s already impressive legacy. They of the atoms and molecules in our bodies the utter vastness of space. NASA include three powerful instruments that will were created in the engines of stars, and the heighten the ability to detect Earth-like energy we receive that enables life comes planets and perhaps the signatures of living from our star: the Sun. may be so incredibly rare when compared beings: the Transiting Exoplanet Survey But life on other planets may be very to such near-invisible organisms that we Satellite (TESS), the James Webb Space different. We can imagine a glimpse of what could explore 1,000 living planets and never Telescope (JWST), and the Exoplanet it might be like even by looking at bizarre see anything but Characterization Observatory (EChO). and different environments here on Earth. microbes. But consider IT’S A LIVIN’ THING. The whole issue about life on other For one thing, the vast majority of life on the numbers: 250 billion Complex hydrocarbons worlds begs the question: What is life, and our planet comes in the form of primitive stars in the Milky Way represent the building blocks how would we recognize it? Certainly, living bacteria, fungi, molds, and other squishy, and at least 125 billion of life in this illustration. NASA’s Spitzer Space things are made of cells (or a cell) and share incredibly tiny organisms. (Viruses are not galaxies in the universe. Telescope has detected three critical processes that make them considered alive because they require a host The numerical possibili- hydrocarbon chains 10 alive. They ingest energy, excrete waste to perform the functions of “life” — which, ties for extraterrestrial billion years back in time, energy, and pass on their genes through for them, amounts to cannibalizing cells.) life are astonishing, even suggesting life may have reproduction. But they also respond to their Life as complex as trees, rats, or insects if only a tiny fraction of started early in the universe. NASA/JPL-CALTECH planets with life have evolved any kind of sophisticated critters. “We have one planet, one example, one history, and we have intelligence,” Carl Sagan was fond of saying. “Intelligent species should be spread liberally throughout the universe.” Certainly a large number of Americans believe in the existence of extraterrestrial life. Indeed, probably a good portion of which believe UFOs have carried intelligent beings to our solar system, and possibly to Earth’s surface. But the debate over the existence of extraterrestrial life is not a democratic one, not something to be subjected to a popular vote. Distinguished scientists such as Harvard University anthropologist Irven DeVore have made detailed cases suggesting the evolution of intelligent life on Earth itself was unlikely, that it may have resulted from a whole series of improbable evolutionary coincidences. For example, DeVore asserts, EXOPLANET ZOO. “Evolution is history; it’s not a series of pre- Astronomers have found more Hydrocarbons in an dictions. Natural selection, which is the than 3,700 confirmed planets ultraluminous galaxy beyond our solar system. This engine driving evolution, is an uncaring, illustration depicts Kepler- blind process. From the fossil record, we can 186f, the first rocky planet judge that 99.9 percent of all species that discovered in the habitable Neon ever lived have gone extinct. There have gas zone of a star other than the Water lived as many as 50 billion species. Of those, Sun. NASA AMES/SETI INSTITUTE/JPL-CALTECH Carbon ice dioxide only one made civilizations.” ice But Seth Shostak of the SETI Institute in Hydrocarbons IS ANYONE OUT THERE? Mountain View, California, sees a different A spectrum made with the Silicates picture. In the 1920s and 1930s, we thought Carbon

Spitzer Space Telescope Brightness monoxide Hydrocarbons planetary systems were rare, he reminds us. shows the chemical gas and ice fingerprints of the building Now we see planetary systems around blocks of life. Seeing these Molecular thousands of nearby stars, and the count hydrogen compounds in a distant rises with each passing day. Until the 1970s, galaxy suggests the scientists believed cooking up DNA on a possibility of abundant life 5 10 20 in the cosmos. ASTRONOMY: Rest wavelength (microns) planetary surface was probably very spe- ROEN KELLY AFTER NASA/JPL-CALTECH cial. “But now we know that not only is physics universal, but probably biology, too,” says Shostak. On Earth, the first single-celled organisms arose soon after the period of heavy bombardment by comets and asteroids OTHER SUNS. The SPHERE slackened, some 3.8 billion years ago. This instrument on ESO’s Very Large Telescope suggests that life might get started else- suppresses starlight to where easily, too. image the planet-forming “Of course,” Shostak reminds us, “the debris disks that surround so-called Fermi paradox argues against this many young stars. So far, spreading out by simply asking, ‘So where is SPHERE has shown that the everybody?’ ” But if you woke up in the mid- dusty disks come in a wide dle of Nevada, you might wander about variety of bizarre shapes and sizes, indicating there and conclude you’re the only person on the may be more than one way continent. After all, absence of evidence is

to form a planet. COLLABORATIONS SHINE & ET AL./DARTT-S SISSA ET AL./E. AVENHAUS ESO/H. not evidence of absence.

22 50 GREATEST MYSTERIES What isthe fate “open” andwould expandforever; ifthe criticaldensity,certain theuniverse was contained. thecosmos hadlessthan a If based onhow muchmatter theuniverse end. There were basicallythree scenarios, all bang, buttheyhadlittlecluehow itwould convinced thattheuniverse beganwitha and sotheuniverse hasbeenevolving. cosmos wasmuchyounger thanitistoday, universe, whichmeanttheyexisted whenthe galaxies existed preferentially inthedistant Robert Wilson, andtherealization thatactive by radioastronomers ArnoPenzias and ofthecosmiccovery microwave background Bang. The two breakthroughs were thedis- theBig evidence supported observational — andcould have anend. the ideathatuniverse hadabeginning verse, astronomers were coming around to Hubble’s ofanexpandinguni- observations cept oftheBigBang. Coupled withEdwin mer Georges Lemaître developed the con- unchanging; itcanevolve. the universe doesnothave to bestaticand oped from Einstein’s equationsshowed that ofrelativity.theory The firstmodelsdevel- Einstein’sthe publicationofAlbert general or even itmightnotbetrue. suspected always will. Few peoplechallengedthedogma simple: The universe hasalways existed and For mostofrecorded history, theanswer was duringthepastcentury. about thesubject surprised mostastronomers whothought a clearoutcome. Andthat,inandofitself, would have there leaningstoward areunknown, strong observational Although theanswer to thisancientquestionisstill By the1980s, mostastronomers were wasn’tIt untilthe1960sthatstrong the1920s,In Belgian priestandastrono- to changeinthe1910swith That started of the universe? have thesamemass, theyallhave thesame thatcausesthestartoreaction explode. which triggersarunaway nuclearchain itself, cannolongersupport white dwarf above 1.4solarmasses. At thatstage, the matter from itscompanion starto pushit system pullsenough inabinary white dwarf ofexplodingstarariseswhena This variety ining dozens Iasupernovae. ofdistanttype large ground-based instrumentswere exam- the HubbleSpace Telescope andseveral came inthelate 1990s. Astronomers using also wasphilosophicallypleasing. the problems withtheBigBangmodeland accepted inflationbecauseitsolved someof the universe flat.Astronomers eagerly expansion intheuniverse’s firstsecond made hypothesis. This says abriefperiodofhyper- 1980s whenAlanGuthproposed hisinflation expansion? Nooneknew. existed. Would itbeenoughto stop the hasgravitationalnevertheless pull— of darkmatter —nonluminousmaterial that thatalot halt expansion.Butscientistsknew about 1percent ofthematter neededto universe, withastronomers findingonly rate would eventually slow to zero. expansion would continue forever, butthe were atthecriticaldensity, itwas “flat” and leading to a “Big Crunch”; iftheuniverse ultimately would stop andthenreverse, universe was “closed” andtheexpansion weredensity above thecriticalvalue, the Because all these exploding white dwarfs Because alltheseexplodingwhite dwarfs developmentBut themostremarkable Matters grew more interesting inthe seemedto favor anopen Observations galaxy. expected supernovainthat reveals afainter-than- the matchingtoppanel (captured byHubble),while shows ahostgalaxy cosmos. Eachbottompanel Dark energyrulesthe DISTANT SUPERNOVAE. NASA/ESA/A. RIESS (STS C I) most likelyfate oftheuniverse. ating asis, a “Big Freeze” seemsto bethe ahead. However, ifdarkenergy keepsoper- ally subside, a “Big Crunch” could stillbe outward push ofdarkenergy doeseventu- even exchangethey can't energy. Butifthe ecules are separated by such great distances will leadto unendingexpansionwhere mol- contentmass-energy ofthecosmos —likely — whichmakesup69percent ofthe similar effect. Buttheresults ofthisenergy stant, orsomeotherstrange namewitha energy, quintessence, thecosmological con- the expansionto accelerate. no match for themysterious force causing entered into anerawhere gravity became years. Butabout5billionyears ago, we had and itdidsosuccessfully for billionsof worksGravity to slow down theexpansion, expansion oftheuniverse isspeedingup. fainter thantheirdistances would imply. found thatthemostdistantsupernovae were Australia, were doing. Bothresearch teams in Stromlo andSidingSpringObservatory Team, ledby oftheMount BrianSchmidt Perlmutter, Supernova Search andtheHigh-Z of California, Berkeley, astronomer Saul Cosmology Project, headedby University nova appears, you cancalculate itsdistance. Iasuper- measuring how brightthetype approximate peakluminosity. So, by simply Bang Big expansion model. agreement withEinstein,istheindefinite- endgame answer. Thelikeliestscenario, in will allowastronomerstodeterminethe of supernovaethroughouttheuniverse BIG-BANG TESTS.Carefulobservations The force may taketheform ofdark The onlyway thismakessenseisifthe That’s whattheSupernova exactly Rate of cosmic expansion D ec years ago 10 billion

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h e ” ez e” 23 What will happen to life on Earth? There are few topics of greater interest and intrigue to anyone who’s ever contemplated the cosmos. Everyone alive on the planet today, all 7.6 billion of us, has an interest in the question of Earth’s habitabil- blue-green algae, are larger than ordinary MARCELO BASS/CTIO/NOAO/AURA/NSF MARCELO ity — and the approximately 100 billion bacteria and can leave behind fossils that people who have ever lived on the planet scientists are able to date radiometrically BIG BANG. Life on Earth still may be would have been interested in this as well. (by measuring radioactivity) to high preci- around a billion years from now, but there’s no guarantee. Besides the From an astronomer’s viewpoint, the sion. Moreover, these bacteria create stram- possibility of a human disaster, a nearby standard feeling about Earth’s habitability atolites, dome-shaped structures that grow supernova (Supernova 1987A appears near goes something like this: The Sun is nearly in aquatic environments and can leave the center of this image) could do us in. halfway through its main sequence life, behind fossilized remains. Dating these about 4.6 billion years old and with 5 billion colonies of microbes gives us our earliest years left, so life on Earth should be about view of life on Earth. (surprisingly) is a decrease in global carbon halfway through as well, right? So we know life on Earth has been dioxide, also a result of the Sun’s increasing Wrong. The earliest microfossils, primi- around for at least 3.5 billion years. Why luminosity, that could cut off the influx of tive, bacteria-like life, date to about 3.5 bil- shouldn’t it continue for another 3.5 or carbon into the planet’s biosphere. Third will lion years ago and come from the northern even 5 billion years, until the Sun becomes be the gradual loss of water on the planet Australian desert. Such cyanobacteria, or a red giant? In a paper titled “The Goldilocks and the inevitable depletion of the oceans. Problem” in Annual Review of Astronomy The evaporation of water into the space and Astrophysics in 1994, biologist Michael surrounding Earth will mark the final gasp Rampino of New York University and phys- of any life on the planet. This will occur icist Ken Caldeira of Lawrence Livermore about 2.5 billion years from now, but the National Laboratory described how future oceans themselves could be mostly gone by climatic changes on Earth will adversely 1 billion years into the future — a mere affect life. Three significant prob- blink of the eye in cosmic terms. The plan- lems will challenge future life on et’s surface temperature will increase dra- Earth. And human beings, we matically and be too hot for most life, also must remember, are among within a billion years. And the decrease in the more fragile types of carbon dioxide and a significant alteration life on the planet, not the of the atmosphere could take place well hardiest. before 1 billion years from now. First is the looming Considering that life has been on the rise in temperature planet for at least 3.5 billion years, the story brought to us by the of life on our planet could be some 80 per- Sun’s increasing radia- cent done — far more than the halfway tion output. This will mark we tend to think of as an analog to our happen long before Sun’s lifetime. And this simply looks at life’s the Sun swells into a endgame: It does not take into account a red giant. Second host of other mechanisms that could wipe out human civilization. BLUE PLANET. Earth Killer asteroid or comet impacts, a nearby has abundant life supernova or gamma-ray burst, global because water exists warming, and supervolcanoes are just some on its surface. When of the climate-changing events that could that ceases to be the case, perhaps as soon as have a catastrophic impact on our planet a billion years from now, and life. The universe can be a violent, life will perish or have to uncaring place indeed! find another home. NASA/GSFC/ R. STÖCKLI/N. SALEOUS/M. JENTOFT-NILSEN RED SUN RISING. Several billion years from now, a red-giant Sun will consume the inner planets as it expands. Earth might escape incineration, but the temperature increase will boil the oceans and scorch the land. ASTRONOMY: ROEN KELLY

WWW.ASTRONOMY.COM 25 What is dark energy?

In a 1998 research breakthrough, Saul Perlmutter of the cosmos consists of dark energy, which is the mysterious force responsible for the University of California, Berkeley, and his colleagues in the accelerating expansion of the universe. If dark energy is as prevalent as astronomers Supernova Cosmology Project found that the expansion believe it is, it will eventually force the uni- rate of the universe is accelerating. other is constant. Several implications fol- verse into a cold, dark, ever-expanding end Perlmutter and his team made the discovery lowed the new finding, the most significant — the universe will go out with a whimper, by observing distant type Ia supernovae, of which has turned cosmology on its head. not with a bang. whose intrinsic brightnesses are well- In May 1999, Perlmutter and his col- Dark energy, now that it is known known, enabling a straightforward deter- leagues published a paper in Science maga- to exist, has come to the fore as one of mination of their distances. His team made zine that outlined their ideas about a newly science’s greatest mysteries. Although observations in conjunction with a team understood force in the universe — dark astronomers don’t yet know exactly what led by Brian Warner of the Mount Stromlo energy. “The universe is made up mostly it is, a number of leading contenders and Siding Spring Observatories. This aston- of dark matter and dark energy,” wrote offer possible explanations. The first is the ishing finding contradicted conventional Perlmutter. Based on the Planck mission’s cosmological constant, or a static field of wisdom, which suggested that the universal 2015 results, astronomers now think 69 fixed energy, proposed by Albert Einstein expansion rate of galaxies away from each percent of the mass-energy content of the (and which he later declared his biggest

BIG BANG. In 2003, a SUPERNOVA SIGHTED. This distant supernova in the composite image shows the region of this Hubble supernova, some 8 billion Wide-Field and Planetary light-years distant, in red. Camera 2 image added The exploding star was key more weight to the dark to determining the nature energy argument. of the repulsive force NASA/J. BLAKESLEE known as dark energy. NASA/J. BLAKESLEE blunder). A second possibility is quintessence, a dynamic, scalar field of energy that varies through time and space. The third possibility (the theory of modified gravity) is that dark energy doesn’t exist at all; what astronomers observe with distant supernovae represents an unknown breakdown of Einsteinian gravitational physics. A fourth option is that dark energy is something we simply don’t yet understand. Determining what dark energy is will be far more complex than the discovery of the accelerating universe. Astronomers are still using the Hubble Space Telescope to “push back to higher redshifts to measure the onset of acceleration,” says Adam Riess of the Space Telescope Science Institute in Baltimore. One important moment to focus on is the so-called transition point, the time when dark-energy acceleration overtook the normal pull of gravity and became the dominant force in the universe. This probably happened about 5 billion years ago, according to Riess. Research on type Ia supernovae must continue to get a handle on this question. Could the reliability of type Ia supernovae come into question? The fact that not as many heavy metals existed in the very early ABELL 85. The Chandra universe could influence the brightnesses X-ray Observatory targeted of these objects and throw astronomers’ this galaxy cluster, located observations off the mark. 740 light-years from Earth, and nearly 100 others in Other techniques will also come into an effort to study dark play. As part of the Dark Energy Survey, for energy. The diffuse purple example, astronomers are closely investi- emission in this image is gating galaxy clusters to see how their den- multimillion-degree inter- sities vary with distance. This could shed galactic gas, heated by the light on whether gravity or dark energy galaxy cluster’s enormous dominated at certain times in the universe’s gravity. NASA/CXC/SAO/A. VIKHLININ ET AL. (X-RAY); SDSS (OPTICAL) past. Cosmologists also will continue to study the distortion patterns seen in gravitationally lensed objects to hunt for clues related to dark energy’s true nature. However, this powerful technique, developed by the University of Pennsylvania’s DARK SECRETS. Three of the most distant supernovae Gary Bernstein, is still in its formative years. known, imaged by the “Right now,” says Bernstein, “we’re just start- Hubble Space Telescope, ing to measure dark energy, but the more Before supernova Before supernova Before supernova help reveal secrets about galaxies we get, the better we’ll do. The dark energy. The stars goal is to see as much sky as possible.” exploded when the universe The era of dark energy has just begun. was half its current age. By Although no one yet knows what this elu- measuring the expansion rate of the cosmos carefully, sive force is, astronomers are fairly certain it astronomers can see that a represents an essential part of understand- mysterious dark force has ing the universe. So, without question, vast After supernova After supernova After supernova pushed space apart. amounts of research will focus on dark NASA/ADAM RIESS energy in the years to come.

WWW.ASTRONOMY.COM 27 5" 1"

DISTANT BOOM. On January 23, 1999, an intense gamma-ray burst exploded with the energy of 100 million billion stars. Hubble’s camera caught the interloper in a galaxy two-thirds of the way to What are the edge of the visible universe. ANDREW FRUCHTER/NASA gamma-ray bursts?

One of the greatest mysteries of observational astronomy you could infer an energy release 1,000 times bigger than it really is. And if the beams are during the past half-century has been the nature of gamma- as narrow as we think they are, for every GRB I see, there are 1,000 I don’t see." ray bursts. The most powerful blasts in the cosmos, these A key moment in researching GRBs flashes of light randomly appear through- start of the 21st century proved that these occurred suddenly on March 29, 2003, when out the sky every day, giving astronomers blasts come from far away. a brilliant burst in the constellation Leo few clues about the origins of the The key to unraveling the nature appeared in the data collectors of NASA’s elusive bursts. of GRBs comes in part from the High Energy and Transient Explorer (HETE-2) Some gamma-ray bursts HETE-2 discovery that detectors are satellite. By immediately capturing the burst (GRBs) last for only a frac- seeing narrowly focused and its afterglow, HETE-2 showed GRB tion of a second — some showed beams. This realization 030329 to be 2.6 billion light-years away as long as minutes or allowed astronomers to and revealed its association with a bright even hours — and beam GRB 030329 to estimate energies for supernova that exploded at the same time. so much energy in a individual bursts and This led researchers to link the most com- focused searchlight that be 2.6 billion hypothesize the num- mon type of GRB, those lasting 20 seconds they make most superno- ber of total GRBs occur- or longer, with the collapse of massive stars vae appear weak in com- light-years ring over a given time about 30 or more times larger than the Sun. parison. For decades, away. interval. “If you didn’t know The stars go supernova and create powerful astronomers debated whether the geometry,” says Shri black holes in the process. GRBs are powerful events in our Kulkarni of the California Institute The next big step came in November Milky Way or super-powerful events of Technology in Pasadena, “and assumed 2004 when NASA launched the Swift beyond it, until observations around the spherical emission when it’s really conical, mission, a GRB-focused telescope. The Swift

28 50 GREATEST MYSTERIES satellite has orbited Earth ever since, observing these extreme bursts. Almost a year after its launch, in October 2005, astronomers using Swift solved a 35-year-old mystery of one class of GRBs known as short bursts, which last just a few milliseconds. What could produce enough radiation to equal that of a billion Suns in such a short period? On May 9, 2005, Swift detected a short burst, marking the first time astronomers detected a short burst and its afterglow — something more common with longer bursts. “We had a hunch that short gamma-ray bursts came from a neutron star crashing into a black hole or another neutron star, but these new detections leave no doubt,” says Derek Fox, an astronomer at Penn State. Had there been any doubt, it has since completely disappeared thanks to the 2017 detection of gravitational waves from two merging neutron stars. Just 1.7 seconds after the Laser Interferometer Gravitational-Wave — a star about 1,000 times wider than the MATCH GAME. Are gamma-ray bursts common Observatory (LIGO) detected the waves, Sun — collapsing into a black hole at the in normal galaxies? In astronomers witnessed a short GRB emanat- end of its life. However, more research is 1997, Hubble’s Wide Field ing from the same location as the merger, needed to confirm this scenario. and Planetary Camera 2 further confirming that merging neutron So, even though scientists have learned captured GRB 970228’s stars can produce short-duration GRBs. a great deal about GRBs, they still have visible glow, the first that Although the mystery surrounding the many areas of inquiry left to explore. They linked a gamma-ray burst origin of short GRBs now seems to be also do not yet know the full details of how with a specific host galaxy. Astronomers estimate that solved, observations in 2010, 2011, and 2012 these incredibly energetic objects work. the GRB’s host galaxy’s CLOSE BURST. In 2003, revealed a previously hidden piece of the Luckily, astronomers have many instru- redshift is 0.835, which the VLBA snapped this radio overall GRB puzzle. Swift detected three ments, including the Fermi Gamma-ray corresponds to a distance image of 2.6-billion-light- GRBs that spewed gamma rays for hours. Space Telescope, to help refine the picture of hundreds of millions of years-distant GRB 030329. Astronomers think such an “ultra-long dura- until it becomes crystal clear — an exciting light-years. K. SAHU/M. LIVIO/ NRAO/AUI/NSF tion” burst results from a blue supergiant moment in science. L. PETRO/D. MACCHETTO/NASA ELENA PIAN/ANDREW FRUCHTER/NASA PIAN/ANDREW ELENA LONE FLASH. On May 8, 1997, Hubble caught the visible fireball from a distant RECORD BLAST. NASA’s Fermi Gamma-ray Space Telescope spied the highest-energy light ever seen from a gamma-ray burst that doesn’t appear to gamma-ray burst, called GRB 130427A, on April 27, 2013. This image compares the sky in high-energy gamma be surrounded by a host galaxy. rays during a three-hour interval prior to the event (left) with a three-hour interval ranging from two and a half hours before the blast to 30 minutes after the blast. NASA/DOE/FERMI LAT COLLABORATION

WWW.ASTRONOMY.COM 29 Will asteroids threaten life on Earth?

The solar system’s history is riddled with violent impacts. planet,” says astronomer Bill Cooke of NASA’s Marshall Space Flight Center in One good look at the Moon through a small telescope Huntsville, Alabama, “and once every great while manages to score a hit.” shows that. Whereas the airless Moon preserves its ancient Of course, the population of small cratering record almost perfectly, planets debris ignited enormous fires and choked objects is much larger than that of big like Earth — with wind, water, and erosion out about 70 percent of life. ones, so small objects strike Earth more — slowly cover the ravages of time. Even a smaller air-blast over Siberia in frequently. To find out how often our planet Although a barrage of large impacts 1908, which occurred along the is struck, astronomers study the occurred early in the solar system’s his- Tunguska River, felled 60 mil- The record of mass extinctions, tory, a time referred to as the heavy bom- lion trees over an area of the orbits of NEOs, and bardment era, significant impacts rocked more than 1,330 square destructive records of explosions Earth’s terrain in geologically recent times. miles (2,150 square in Earth’s upper atmo- For example, the K-Pg km). If the explosion power a rock sphere. Satellites record BATTERED SPUD. On impact in the Yucatán had occurred near a carries to Earth the amount of heat October 29, 1991, the Peninsula some 66 populated city, the released from NEOs Galileo spacecraft imaged million years ago wit- results would have been is directly that explode in Earth’s the asteroid 951 Gaspra. nessed a 6-mile-wide catastrophic. atmosphere. Data from The potato-shaped body (10 kilometers) asteroid Although most inner proportional atmospheric explosions measures about 12 by 8 by striking Earth. That’s orbital debris was cleared during the past 40 years 7 miles (20 by 13 by 11 to its size. kilometers). More than 600 large enough to create a out during our solar system’s show meteoroids erupting in craters on its surface each firestorm of hot debris. early days, a huge population of the atmosphere produce at least span 300 to 1,500 feet (90 to Falling back through near-Earth objects (NEOs) is still out there. one 5-kiloton explosion each year. 460 meters). USGS Earth’s atmosphere, the “Nature is blindly throwing rocks at our Scientists expect about one hundred 300-foot-wide (100 meters) objects will strike Earth over the next million years, while during that same time interval, two 0.6-mile-wide (1 km) objects will hit. It’s these larger objects that, like the K-Pg impactor, pose a threat to civilization. The destructive power a rock carries to Earth is directly proportional to its size. A 0.6-mile-wide rock, which strikes Earth every 500,000 years or so, delivers a blow equivalent to 20,000 megatons of TNT. An asteroid only 300 feet across, however, can strike every couple hundred years and hit us with a 20-megaton explosion. “Put into more meaningful terms,” says Cooke, “a person living to the age of 100 has about a 1-in-10 chance of a 10-megaton Tunguska-like impact occurring somewhere on Earth at some time during his or her life. Of course, the odds of being hurt by such an event are vastly lower — by roughly a factor of 50,000 — because the asteroid would have to strike nearby to cause injury.”

30 50 GREATEST MYSTERIES NASA/DON DAVIS NASA/DON

DANGEROUS ROCKS. Even a small asteroid striking Earth would have catastrophic regional consequences. A half-mile-wide asteroid impact would have global consequences. A 6-mile-wide (10 km) or larger asteroid impact could wipe out civilization.

Given the potential danger to the human race, aggressive NEO discovery programs are underway, led in large part by NASA’s Near-Earth Object Observations Program and the Lincoln Near-Earth Aster- oid Research program. Together with other surveys, the collective “Spaceguard” program — a term borrowed from Arthur C. Clarke’s novel Rendezvous with Rama — has found an estimated 90 percent of the largest near-Earth asteroids, meeting a WANDERING STAR. As Congress-directed goal set in 1998. the Hubble Space Telescope After finding NEOs, the real challenge imaged the Sagittarius is to identify their orbits and project them Dwarf Elliptical Galaxy in forward to predict possible impacts. New August 2003, the wandering discoveries are forwarded to the Minor light trail of an asteroid Planet Center in Cambridge, Massachusetts, interrupted the exposure. Because the camera’s where planetary scientists calculate orbits shutter intermittently closed, and publish the results. a series of arcs appears Because humans now have the technol- rather than a continuous ogy to find and observe NEOs doesn’t mean line. STSCI the historical record of Earth impacts will ASTEROID CLOSE-UP. change. What should we do when we find Minor planet 433 Eros, shot an object that will strike our planet? Given by the NEAR-Shoemaker enough time, scientists could send a space- spacecraft February 14, craft carrying a charge that would detonate 2000, reveals details of the near the asteroid and nudge it out of its asteroid’s battered surface. Eros spans 20 by 8 by Earth-impacting orbit. But if a long-period 8 miles (32 by 13 by comet were headed toward us, we would 13 kilometers). The have little notice. We would have to duck, prominent crater at center cover, and hope for the best. is 4 miles (6 km) wide. NASA

WWW.ASTRONOMY.COM 31 Is water necessary for life?

Water drives NASA’s solar-system exploration program. By definition, living things have several properties that separate them from rocks Wherever there’s a chance water could exist, either as liquid and dirt. Living things on Earth are arranged into cells. They are also highly organized, on or ice — like on Mars, Jupiter’s moon Europa, and Saturn’s different levels and with different tasks. They moon Enceladus — future exploration will The seed for finding extrasolar life where take in energy from the environment and be a focus for scientists. there’s water goes back to Harvard biologist excrete waste products. They exhibit This works for planetary-exploration Lawrence Henderson, who wrote in his homeostasis, stable internal conditions that missions and for exoplanet telescopes like 1913 work, The Fitness of the Environment, are required to stay alive. They grow and Kepler and the recently launched Transiting “Life necessarily must be based on carbon change, showing differentiation and muta- Exoplanet Survey Satellite (TESS). For many and water, and have its higher forms tion. They also reproduce, passing genetic years, planetary scien- metabolizing free oxygen.” For decades, material to their descendants. Anything in LIFE FROM ORBIT. This tists and biologists have scientists took this statement as gospel, but the universe that exhibits these characteris- incredible image from held fast to one maxim: it was questioned in the 1970s. Carl Sagan tics would be considered a living being. space shows evidence of Water is essential for life. wrote he doubted it “because Lawrence The movement of molecules plays a key life in water. Taken July 6, Great interest focuses Henderson was made of carbon and water part in making a place hospitable for life. A 2016, with the MODIS on the possibility of and metabolized free oxygen. Henderson flowing solvent can trigger molecule move- instrument on the Terra extraterrestrial life, even had a vested interest.” ment and, thus, the energy reactions life satellite while in low Earth if it’s merely microbial. Life in the universe, astronomers and requires to take in energy and excrete waste. orbit, the image shows a phytoplankton bloom (blue- So, to find where the big biologists now admit, could be based on On Earth, that solvent is water. But water green) in the Barents Sea, planetary-exploration completely different chemical systems from may not be the molecule-moving solvent off the northern coasts of dollars will be spent, sim- ours. The first thing to do is to define what everywhere in the universe. Clues to alterna- Russia and Norway. ply follow the water. we mean by “life.” tive environments have come from strange NASA/JPL/MALIN SPACE SCIENCE SYSTEMS SCIENCE SPACE NASA/JPL/MALIN

LIQUID MARS. NASA’s Opportunity rover captured this image of Burns Cliff in November 2004. Located on the inner wall of Endurance Crater, the cliff displays the effects of groundwater infiltration caused by ancient water flows. NASA/GSFC/JEFF SCHMALTZ/JOSHUA STEVENS SCHMALTZ/JOSHUA NASA/GSFC/JEFF

32 50 GREATEST MYSTERIES LIFE’S BIG POOL. Earth’s places as well as from our own familiar Huygens probe discovered signs of abun- planets around other stars. watery surface is perfect planet. On Earth, microbes can remain alive dant liquid methane that acts as Titan’s Although the TESS tele- for sustaining life, even in the absence of water nearly indefinitely. “water.” It also found abundant ethane. There scope will soon identify despite such fearsome They go dormant, take in no energy, is no evidence of life on Titan, and promising exoplanetary recurring events as and do not reproduce or grow. Huygens was not designed to targets, the survey mission hurricanes. Astronauts Yet microbes can revive and On Earth, detect it; however, some cannot carry out more aboard the ISS imaged the become active again at a scientists speculate detailed analyses. Instead, massive storm Isabel on September 15, 2003. NASA much later date. microbes can self-replicating mol- it will rely on the upcom- If liquids other than ecules might exist in ing James Webb Space water might help create remain alive in Titan’s methane-rich Telescope to follow up and characterize indi- and sustain life, which environment or may vidual exoplanetary atmospheres. ones might they be? the absence of have even produced Is water needed for life, or could other Water does have an some of it. Furthermore, solvents do? That’s the key question, but it amazing number of prop- water nearly the Cassini mission gath- ties in to another, even larger one: Does erties that help support life. indefinitely. ered evidence that sug- extraterrestrial life exist in the universe? With But could another fluid — gests Titan has a subsurface around 200 billion stars in our galaxy and ammonia, methane, formamide, or ocean below its icy crust, most 125 billion galaxies in the universe, life has sulfuric acid — create a place where exotic likely filled with salty water. plenty of places to gain a foothold. There are life forms could flourish? The biggest challenge is to extend the also plenty of opportunities for life to flour- At Saturn’s large moon Titan, the hunt for life by searching the atmospheres of ish based on solvents other than water.

WWW.ASTRONOMY.COM 33 STERILE RED PLANET? Aside from a controversial meteorite from Mars, no evidence of life, microbial Is there life or otherwise, exists for the Red Planet. Only when spacecraft are able to dig into subsurface aquifers on Mars, Titan, will planetary scientists answer the question. or Europa? NASA/JPL-CALTECH/CORNELL Mars has been a target of speculation about extraterrestrial life for more than 200 years, ever since observers tracked the seasonal growth and decay of the planet’s polar ice caps. Fevered speculation about martian life raged When the Viking landers analyzed mar- in the late 19th century, when British astrono- tian soil in 1976, their results on finding mer William Whewell — and later, American traces of life were inconclusive. Observations astronomer Percival Lowell — proposed a in the 1990s by the Mars Global Surveyor civilization on Mars. Lowell based his claim demonstrated the planet’s lack of a cohesive on flawed observations of “canals” he made at magnetic field, which allows cosmic radia- Lowell Observatory in Arizona. The Victorian tion to bombard the planet’s surface. With momentum for a martian civilization led H.G. no protective magnetic field, much of Mars’ Wells to write The War of the Worlds, in which atmosphere dissipated into the solar wind, martians, faced with a planet that was drying leaving a cold, arid desert that would be HAZY SHADE OF METHANE. out, were forced to seize Earth. extremely hostile to life. Saturn’s largest moon, Titan, has a thick However, no evidence for life on Mars has A media frenzy over martian life occurred atmosphere where methane forms a hazy ever come to light. In 1965, the Mariner 4 in 1996 when scientists announced they had layer. Lower in the atmosphere, Titan has a spacecraft dashed the hopes of pro-martians found suspicious bacteria-like structures in thick smog of complex organic molecules, when it revealed an arid, stark landscape in martian meteorite Allan Hills 84001. This the building blocks of life. NASA/JPL-CALTECH the first close-up images. hand-sized meteorite, plucked from Antarctic

34 50 GREATEST MYSTERIES ice in 1984, showed wormlike structures a substantial atmosphere. Titan’s thick atmo- measuring only 100 nanometers in diameter. sphere abounds with organic compounds. Late NASA astrobiologist David McKay con- Although its atmosphere consists mainly jectured the structures were possible bacte- of nitrogen, it also contains vast ria from Mars. After years of debate, however, amounts of methane. the evidence seems inconclusive at best, and On Earth, methane is a byprod- most scientists believe the lines are chemical uct of living organisms. However, structures not indicative of living organisms. Titan is now a hostile place for liv- All this does not necessarily add up to a ing organisms, with temperatures lifeless Mars, however. Recent spacecraft mis- too cold for water to exist in liquid sions have shown evidence of abundant form (at least on its surface). But flowing water on Mars in its past. Planetary could it have been different in the scientists believe the Red Planet likely also past? Might a huge impact on Titan has subsurface aquifers that contain vast have delivered enough heat to liq- amounts of the liquid, as well as large depos- uefy water for a time and sustain the its of frozen water ice scattered in many development of primitive life? Or, as places just below the surface soil. recent Cassini data suggests, is Titan hid- The Spirit and Opportunity Mars rovers ing a global ocean of liquid water under its demonstrated in 2004 that Mars clearly had icy shell? Although planetary scientists are a wet past. And where there’s water, there not sure if Titan is the most likely place we’ll COOL REPOSITORY. Europa, could be life. The amounts of methane and find life, they do believe that its methane-rich field. This suggests Jupiter’s sizable moon, formaldehyde discovered on Mars are more atmosphere and possible subsurface ocean Europa harbors a subsur- has a slushy liquid ocean than planetary scientists would think could are ripe for further exploration. face ocean of salty water. beneath its icy crust. Could exist, given the planet’s atmosphere. Could What about life on Europa? The smallest Planetary scientists the watery vault hold extremophiles, microbial life forms existing of Jupiter’s four Galilean moons is an intrigu- believe microbes could microorganisms? NASA/JPL/SSI somewhere inside the planet, be producing possibility. The moon’s smooth icy sur- exist in this ocean the ing these gases? face is not at issue here — it’s the liquid same way they do around hydrothermal Another intriguing possibility for life ocean that scientists know exists under its vents in Earth’s oceans. Future explorations elsewhere in the solar system could be on surface. The Galileo spacecraft observed a missions, such as NASA’s cryobot concept, Titan, Saturn’s largest moon. Larger than the weak magnetic field that varies as Europa would bore through Europa’s thick crust to planet Mercury, Titan is the only moon with passes through Jupiter’s strong magnetic explore the ocean below for signs of life.

WWW.ASTRONOMY.COM 35 Why did Mars dry out?

Years ago, astronomers detected signs of Mars’ watery past. DRY GULLIES. These gullies in Mars’ Early evidence came from imaging large numbers of Terra Sirenum region suggest liquids once flowed on the Red Planet. NASA/JPL/UNIV. OF ARIZ. winding channels on the Red Planet. These images suggest abundant liquids of some type flowed on Many of the features Parker interpreted in Canada, Iceland, and Greenland, where the planet at some point in its history. In and some of the river channels are scarred gullies are formed by water saturating the 1972, the Mariner 9 spacecraft orbited Mars by craters, suggesting the Red Planet’s era ground and then flowing downward. If and took photos of what appear to be dry of water had concluded by about 3.5 bil- water formed these martian features, the riverbeds scattered over the planet’s surface. lion years ago. implication is rather amazing in that large More recently, Tim Parker of the Califor- The clear implication is that Mars had a flows must have happened recently — and nia Institute of Technol- watery past and for some reason dried out, maybe still go on today. The mysterious COLD AND DRY. This mosaic ogy suggested many dry but not completely. Using images from the source of such flowing water might be ice highlighting Mars’ Valles lakebeds also exist on Mars Global Surveyor satellite, planetary that suddenly melts. The flows would have Marineris, the solar system’s Mars — though this scientists Devon Burr, Alfred McEwen, and to be significant to leave surface features largest canyon, is composed interpretation is not as colleagues looked carefully at Marte Vallis, because the temperatures and pressures on of 102 Viking orbiter images. clear as that of the river- a river channel that extends from Elysium Mars’ surface today mean water quickly Three Tharsis-area volcanoes are visible in the beds. Parker also inter- Planitia into Amazonis Planitia. evaporates once it reaches the surface. picture (arrows). In this preted some features as Freshly sculpted features and a lack of So if Mars was wet and therefore also volcanic region and others, ancient shorelines, and craters indicate this river system harbored relatively warm in the past, why did it dry clear evidence of liquid even some as basins water in the last 40 million years. Astrono- out? Planetary scientists don’t yet know, but water on the martian once filled by lakes. mers believe a significant amount of water data from the Mars Exploration Rovers and surface appears. NASA/JPL/USGS lies locked inside underground martian Mars Express suggest a substantial climate aquifers. Future explorers will drill change occurred some 600 million years into the planet’s mantle to after the planet’s formation. One study explore the nature of this released in 2006 by French astrophysicist watery deposit. Jean-Pierre Bibring suggests volcanic erup- In 2000, Mars Global tions drove Mars’ changing climate. By Surveyor scientists studying the mineralogy of gypsum and Ken Edgett and hematite on the martian surface, Bibring Michael Malin and his colleagues found evidence of a explored heavy period of volcanism. another fea- If life did gain a toehold on Mars by ture that sug- this point, the climatic change may have gests a shifted the planet to a less hospitable water-laden place. Moreover, Mars cooled following past for Mars. this period of volcanism. During the cool- Edgett and ing, astronomers believe Mars’ magnetic Malin identi- field dissipated because the planet’s fied numer- molten-iron core solidified. Without the ous “gullies” Red Planet’s protective shield, the solar extending wind may have stripped away much of the from rocky out- martian atmosphere and sent a large por- crop highlands to tion of the planet’s water into space. After a lower portions of rather Earth-like start, Mars may have — on hills. They appear to a planetary timescale — suddenly turned be analogs to features cold, dry, and hostile to living things. CLOSE NEIGHBOR. The Moon looms large in this view of the lunar disk, shot in January 2003 during the final mission of space Columbia How did the shuttle . NASA Moon form?

Our planet is a strange one, judging by the standards of our orbit after its formation and a near-miss encounter; and “fission,” in which Earth’s solar system. It’s the only one with lots of liquid water. But interior belched out the Moon like the splitting of a cell. None of these ideas fully there’s a much weirder aspect to Earth — the Moon is huge convinced astronomers or matched up relative to its nearest neighbors. Look at For many years, planetary scientists with what planetary scientists knew about Venus with no moon, Mars with its two tiny struggled with ideas about the Moon’s the Earth-Moon system. potato-shaped moons, and Mercury with no origin. Their ideas included “co-accretion,” The Apollo missions to the Moon revolu- moon. Earth is special in the solar system in which Earth and the Moon formed tionized thinking about our nearest celestial because it has such a large moon. (Pluto independently and then came together neighbor. Apollo astronauts found that oxy- also has a moon, Charon, that's large com- gravitationally; “capture,” in which Earth gen isotopes in Moon rocks are similar to pared with its host planet.) gravitationally dragged the Moon into those on Earth, indicating that the Moon

WWW.ASTRONOMY.COM 37 SMACKDOWN. Evidence suggests the Moon formed when a giant object collided with the proto-Earth several billion years ago, liberating material that then coalesced into the Moon. The Clementine spacecraft shot this enhanced-color Moon image in 1992. NASA

separate body, resemble the material in Earth’s mantle? The clues leading to a likely solution LUNAR LANDSCAPE. An oblique view came from an unex- across Mare Imbrium shows the 58-mile- pected direction. Apollo 11 wide (93 kilometers) Copernicus Crater in astronauts returned samples the distance. Several small chains of craters, oriented toward Copernicus, containing strange white peb- show where splashes of material from the bles that suggested the lunar Copernicus impact landed. Apollo 17 highlands are composed of an igne- astronauts took this photo in 1972. NASA ous rock called anorthosite. The rock contains copious amounts of a particular mineral class called plagioclase About 4.6 billion years ago, two planets feldspars. These minerals, composed of floated in the space now occupied by the sodium and calcium aluminum silicates, Earth-Moon system, according to most are commonly found in Earth’s crust. The planetary scientists. Proto-Earth had 50 to samples also contain small amounts of 90 percent of its current size and mass, pyroxenes and olivine. The strangeness of while the second planet — a body about this finding comes from the relative purity the size of Mars — no longer exists. of the feldspars; most minerals in Planetary scientists believe the nature are all mixed up into smaller protoplanet struck rocks. But Apollo showed The Moon Earth at a grazing angle. that much of the lunar The collision sped up crust is composed of and Earth Earth’s rotation and anorthosite and sim- melted the mantles of ilar rocks, with high formed in the both planets and concentrations of reformed them, creat- plagioclase feldspars. same region ing the plagioclase STRING OF PLANETS. and Earth formed in the That’s not what scientists feldspar. Later on, the Planets strung along the of the solar ecliptic peek out behind the same region of the would have expected. majority of the impac- Moon’s disk in an image solar system. The solution had to be system. tor’s mass accreted onto captured by the Clementine The Moon is also something like this: Soon after Earth’s surface. For a time, spacecraft. The Sun’s devoid of volatile ele- its formation, the Moon may have then, after the impact, Earth would corona rises above the ments that melt at high been covered by an ocean of liquid rock have had Saturn-like rings. Moon’s limb, while Saturn, temperatures, suggest- that crystallized as plagioclase feldspar The Saturn-like disk would be unstable, Mars, and Mercury (left to ing it had a very hot over its surface. Another key piece of evi- however, and the Moon likely accreted right) lie next to the Sun. NASA birth. Oddly, the more dence centered on the solar system’s larg- from this huge mass of particles. This sce- scientists found out est impact basin, the South Pole-Aitken nario probably occurred in as little as a few about the Moon’s rocks, the more the rocks Basin on the Moon’s farside. The basin’s size years to as many as a few thousand years, appeared to resemble Earth’s mantle — — more than 1,500 miles (2,400 kilometers) either one a very short time span in plane- the outer shell of rock on our planet. But across — shows that huge catastrophic tary terms. This hypothesis, based on geol- Earth’s mantle emerged from the denser, collisions occurred. These two lines of evi- ogy and orbital mechanics, offers by far the more metallic core when our planet dence have led to the current thinking best and most logical way to understand formed. How could rocks from the Moon, a about where the Moon came from. how Earth’s Moon formed.

38 50 GREATEST MYSTERIES Where do

meteorites METEOR CRATER CHUNK. About 50,000 years ago, an iron asteroid fragment come from? the size of a football stadium struck the Arizona Every night, it’s possible to see brilliant flashes make their mineralogies, and tex- desert, producing the tures all match the sam- famous Barringer Meteor way across the sky. Meteors are beautiful, startling, and a ples brought back by the Crater. The Canyon Diablo meteorite was mostly Apollo astronauts. reminder that debris lies scattered throughout the solar vaporized, but scattered All meteorites except fragments like this one system — and some of it falls toward Earth. contain chondrules, millimeter-sized orbs of those from the Moon, remain. ASTRONOMY: DAVID J. EICHER Collectors prize meteorites — particles that minerals that were once molten droplets. Mars, and potentially survive to strike Earth’s surface — and These primitive meteorites give scientists a comets, however, originated from the aster- value holding a piece of the distant solar window to the early solar system; the chon- oid belt. Because of the numerous similarities system in their hands. drites formed from aggregates of dust flash- among meteorites and asteroids, their huge Altogether, about 60,000 meteorites have melted during the solar system’s earliest days, numbers, and the fact that many asteroids been recovered. The most common are avail- when the Sun and Earth were forming. have since fragmented and broken up, it’s able for a few tens of dollars for each small Meteorites, like the one that fell in Allende, difficult to match meteorites with their par- piece — the least common types go for sev- Mexico, or the inky-black, carbon-dominated ent bodies. Three subclasses of stone meteor- eral thousand dollars per gram. Tagish Lake specimen from British Columbia, ites — howardites, eucrites, and diogenites Nearly all meteorites are pieces of aster- provide an unfettered glimpse at the most — are high-temperature basaltic rocks similar oids, most of which lie in the main asteroid primitive matter we know about. in composition to the asteroid 4 Vesta, so belt between Mars and Jupiter, but others About 95 percent of meteorites that fall are Vesta is likely their parent body. The primitive were born in more distant reaches of the stones. Of those, around 130 are known to carbonaceous chondrite Tagish Lake has solar system. A few may be pieces of comets. have come from Mars. How is this possible? been spectroscopically tied to asteroid 368 Some are fragments of Mars or the Moon. Take the case of the largest and most cele- Haidea. Yet most associations are made sim- How do scientists know about the origins brated martian meteorite, Zagami, which fell ply by asteroid classes, as astronomers have of meteorites? First, they separate meteorites in Nigeria in 1962. Zagami is a basaltic sher- much more work to do to tie specific aster- into three classes — stones, irons, and stony- gottite, which means it’s similar to Shergotty, oids to meteorites that have fallen to Earth. irons. In general, irons originate in the metal- the second martian meteorite found. Zagami’s lic cores of asteroids, while stones are pieces solidified lava crystallized about 1.3 billion of the outer mantle or crust of asteroids. years ago in a magma chamber. The certainty Stony-irons, rare compared with the others, of its martian origin comes from its young are boundary pieces containing mostly met- age (much younger than the crystallization als but also silicate ages of most meteorites), the gas isotopic ANCIENT MESSENGER. minerals. composition that matches the martian atmo- Inclusions in the Allende Within the class sphere (compared to Viking lander data), and meteorite, which fell in of stone meteor- the high deuterium-to-hydrogen ratio. Mexico in 1969, date back ites exists an Similarly, around 300 individual meteorite 4.55 billion years. This carbonaceous chondrite important type fragments are known to have come from the stone contains solidified called carbona- Moon. As with the martian meteorites, these drops of minerals from the ceous chondrites. objects were knocked into space in antiquity ancient solar system. These meteorites by large impacts, and their orbits eventually brought them into Earth’s gravitational tug. CELESTIAL ART. A rare type of meteorite — As an example, Dar al Gani 400, one of the stony-iron pallasites — contains a metallic largest lunar meteorites, fell in the Libyan matrix included with the mineral olivine. Sahara in 1998. It is a feldspathic regolith Astronomers believe these seldom-found breccia, meaning it contains large amounts space rocks formed at the boundary between an asteroid’s core (metallic) and mantle of the mineral anorthite, a calcium-aluminum (stone). The most beautiful pallasite, the : DAVID J. EICHER J. DAVID : silicate mineral that is abundantly present Esquel meteorite shown here, was found in in the lunar highlands. The lunar meteor- Argentina in 1951. ASTRONOMY: DAVID J. EICHER

ASTRONOMY ites’ chemical compositions, isotope ratios,

WWW.ASTRONOMY.COM 39 STOP THE LEAK! If black holes are, by definition, regions from which nothing can escape, then how could light “leak” from them? Viewed in terms of quantum Can light escape mechanics, black holes are not quite black: They release small amounts of light called from black holes? Hawking radiation. NASA/G. BACON

Four decades after theories predicted the existence of black through their interactions with other nearby bodies subject to their influence. holes, astrophysicists held a simple view of them. They were But this straightforward view of black holes began to change in 1974. In that year, concentrations of mass so great that no light could escape British cosmologist Stephen Hawking their staggering gravitational pulls. John Michell. In the 1920s and 1930s, astro- worked out a complex set of equations Astronomers believed black holes are black physicists tightened theories about black about black holes using quantum field the- because they are bounded by a limit called holes, but it’s only been in the last 35 years ory. To the astonishment of physicists, the event horizon, which traps radiation in that astronomers have begun to collect good Hawking showed that black holes can actu- gravitational wells, out of which no electro- observational evidence of black holes’ exis- ally emit radiation. The notion that nothing magnetic radiation can escape. tence. The label held fast: Black holes were at all can escape a black hole now seemed The concept of such “dark stars” was put black because nothing could escape their wrong. The resulting emission, small as it forward as early as 1783 by British geologist gravitational grasps. They had to be observed may be, was dubbed Hawking radiation.

40 50 GREATEST MYSTERIES REAWAKENED GIANT. In the center of this Hubble image of nearby galaxy Centaurus A, a supermassive black hole lurks and feeds on a smaller colliding galaxy. When galaxies fuel dormant black holes that lie

within them, sparks fly. E. J. SCHREIER/STSCI/NASA

Hawking showed there are several ways to understand how energy can escape a black hole. First, separation of matter-antimatter pairs occurs just beyond the event horizon. This would produce radiation not from within the black hole itself, but from virtual particles that are boosted to higher energy states by the black hole’s gravitation. Another way to look at the pro- RED SKY RISING. The cess is that vacuum fluctuations could cause active galaxy NGC 4261 a particle-antiparticle pair to appear close harbors a supermassive black to the event horizon. One particle could fall hole. An observer on a into the black hole while the other would hypothetical planet orbiting a not. To fill the “hole” in energy left by the star in this galaxy might see lone particle, energy could tunnel its way the night sky as shown in this out of the black hole and across the event illustration. J. GITLIN/STSCI horizon, producing the observed radiation. This would cause the black hole to slowly DISK WARP. This artist’s lose mass, and an observer would see radia- concept shows how the tion being emitted from the black hole. accretion disk around a black If black holes can lose mass, then it logi- hole (orange) would appear to an observer looking at the cally follows that at least some of them disk nearly edge-on. Images could eventually disappear. This process is of the real disk reveal that called black-hole evaporation. When parti- only one side reflects light, cles escape from a black hole, the black hole creating its unusual warp. loses not only energy, but also mass, NASA GSFC/J. SCHNITTMAN because the two are interchangeable equals, as governed by Einstein’s famous Assuming a black hole is dormant — not and stars. A black hole with the mass of 1022 E=mc2 equation. For the simplest kind of a accumulating any more matter — it’s possi- pounds (1011 kilograms) would disappear in black hole, a non-charged, non-rotating ble to calculate how long it would take one only 3 billion years. Schwarzschild black hole, physicists can to evaporate. For a 1-solar-mass black hole, The classical concept of black holes being estimate the amount of Hawking radiation the answer is staggering: 1067 years, or more black is nearly right, but not exactly. Stephen that should be produced. A 1-solar-mass than a million times longer than the whole Hawking showed the world that black holes black hole would produce a tiny output of history of the universe to date. But tiny black can show themselves, though the evidence energy — only about 10–28 watts. That’s holes would evaporate more quickly, pro- may be subtle, and that — given enough pretty close to being absolutely black! vided they were not gorging on radiation time — they can even vanish.

WWW.ASTRONOMY.COM 41 Did stars, galaxies, or black holes come first?

Although astronomers understand certain aspects of the after certain gamma-ray bursts. Matter spi- raling into black holes at high velocities cre- universe clearly, others are more muddled. The Big Bang has ates quasar emissions. By observing quasars a mountain of evidence behind it, while the picture of how in so many galaxies in the early universe, astronomers have come to believe quasars protogalaxies evolved into normal galaxies nearby universe today. But others believe existed in virtually every galaxy near the in the first few billion years of the cosmos giant sheets of matter formed in galaxy beginning, save for tiny ones not massive is just coming together. But what hap- superclusters and then broke apart into enough to support a central black hole. So pened, exactly, at the end of the cosmic smaller units. Either way, no one yet knows an intriguing question has come about: Did Dark Ages, following the Big Bang, that whether the gas and dust that came galaxies form and develop black holes brought together the seeds of matter to together to make galaxies pre- in their centers, or did black form the first stars and galaxies? That ceded star formation or holes form first and act as question is still wrapped in a hazy cloud whether stars formed Black holes the gravitational “seeds” of mostly speculation. simultaneously as the that attracted matter to At the core of the issue is which came first units of matter fell start forming form galaxies? first: stars, galaxies, or black holes? With together to form the before galaxies The jury is still out respect to galaxy formation, many astrono- earliest protogalaxies. on this question. mers believe in the “bottom-up” model of A more intriguing do, or form at a However, astronomers how matter came question arises from have conducted some BLACK HOLE CANDIDATE. together. In this model, observations of numer- much faster rate, research that may Elliptical galaxy NGC 4261 small clumps merged ous quasars in the early provide clues. In 2003, may contain a black hole at repeatedly to form pro- universe. Quasars are the or both. Marianne Vestergaard, then a its core. The Hubble Space togalaxies, and further, energetic “central engines” found postdoctoral fellow at Ohio State Telescope snapped this many small protogalax- in infant galaxies that spew vast University, suggested black holes form image of a giant disk of cold gas and dust surrounding, ies clumped together to amounts of high-energy radiation into first as galactic seeds. and possibly fueling, what form the larger, normal space. They are the second most energetic Vestergaard studied a collection of qua- is likely a black hole. galaxies we see in the things in the universe (except the Big Bang) sars about 12 billion light-years distant. Rather than postulating that black holes form after matter builds up in the centers of galaxies, she “think[s] that black holes start forming before galaxies do, or form at a much faster rate, or both.” The evidence came from studying a set I)

C of quasars from Sloan Digital Sky Survey data and comparing their spectra to those of quasars that are closer to Earth. After looking at several hundred quasars, Vestergaard noticed a pattern: Even the smallest, most inactive galaxies studied have supermassive black holes at their centers. Presumably, those black holes should have taken a considerable amount of time to grow to their current sizes. However, the galaxies surrounding them showed signs of youthful vigor. This suggests the black holes may have come first. NASA/WALTER JAFFE (LEIDEN OBSERVATORY)/HOLLAND FORD (JHU/STS FORD OBSERVATORY)/HOLLAND (LEIDEN JAFFE NASA/WALTER

42 50 GREATEST MYSTERIES The first stars and their descendants

First star

Massive blue star (100 solar masses)

Blue giant

MASSIVE FIRST STARS. One of the first stars would have been extremely massive — 100 solar masses in this example — formed mostly from hydrogen, helium, and a tiny amount of lithium gas. After just a Brilliant few million years, the star burned its fuel and ended explosion in fantastic style: as a huge explosion (shown here). The star’s material, including heavy elements, was ejected. Either its core collapsed as the first black hole, or the explosion was powerful enough to blow the star up completely and scatter its material throughout space.

Sun

Sun today Black hole (1 solar mass)

Red giant

Protoplanetary nebula

THE SUN’S LIFE CYCLE. For 10 billion years, our Sun steadily burns, converting hydrogen to helium in its core. Then, the Sun will expand 100 times, and its outer layer will cool. It becomes a red giant. The Sun will fuse helium into heavier elements (carbon and oxygen), burning brighter and brighter for tens of millions of years. Finally, the Sun will shed its outer layers — initially as a protoplanetary nebula — releasing elements crucial to life. What was once the Sun‘s core will contract into a white dwarf. White dwarf ASTRONOMY: ROEN KELLY Planetary nebula COSMIC-RAY GUN. Supernovae — exploding massive stars — are one important source of cosmic rays. Astronomers combined images made with the Hubble Space Telescope, the Spitzer Space Telescope, and the Chandra X-ray Observatory to create this portrait of the supernova remnant Cassiopeia A. NASA/ASU/J. HESTER ET AL.

Where do cosmic rays come from? After the discovery of radiation by French physicist Henri altitude of about 15,000 feet (4,600 meters) during a total solar eclipse. Becquerel in 1896, scientists believed atmospheric ionization He found an ionization rate about four times greater than at ground level. Hess could (where an electron is stripped from an air molecule) explain the variant observations only if a pow- occurred only from radioactive elements an additional source in 1912, when he erful source of radiation were penetrating the found in ground rocks or from radioactive strapped three electrometers into a balloon atmosphere from above. Much later, in 1936, gases. Austrian physicist Victor Hess found and measured atmospheric radiation at an Hess received the Nobel Prize in physics for

44 50 GREATEST MYSTERIES HEAVY BOOM. A massive Super-Kamiokande supernova remnant, detector in Japan N63A, marks the site of found evidence of a blast of cosmic rays one type of neu- that occurred when trino changing into a star 50 times more massive than the Sun another “flavor.” And ended its life. NASA/ESA/ recently, the IceCube HEIC/THE HUBBLE HERITAGE TEAM detector, embedded in a cubic kilometer the discovery of what we of South Pole ice, found now call cosmic rays. evidence of extremely Physicists initially believed high-energy neutrinos com- cosmic rays were gamma rays, ing from outside the Milky Way. high-energy radiation produced by radioac- By studying cosmic rays for several tive decay. During the 1930s, however, decades now, researchers have begun to experiments revealed that cosmic rays are understand them. Astronomers categorize LOCAL STREAM. The Sun produces a stream of mostly charged particles. In 1937, French cosmic rays into four basic types. The first cosmic rays that constantly bombards our planet. physicist Pierre Auger found that extensive are those with low energies, called anoma- This Extreme Ultraviolet Imaging Telescope photo, particle showers (called air showers) occur lous cosmic rays. They probably originate in made with the SOHO spacecraft February 17, 2001, when cosmic rays collide with particles high the heliosheath, at the solar system’s edge, shows a huge loop prominence at upper right. SOHO in the atmosphere, producing a cascade of where the solar wind meets interstellar electrons, positrons, photons, muons (parti- space. Astronomers believe anomalous cos- cles similar to electrons but 200 times as mic rays occur when electrically neutral The intense magnetic fields at the Sun’s sur- massive), and other particles that reach atoms in the heliosheath are ionized and face energize them. Earth’s surface. accelerated. When the Voyager 1 spacecraft The last kind is ultra-high-energy cosmic In 1954, members of the Rossi Cosmic passed from the heliosheath into interstellar rays, the type being studied by the Auger X-ray Group at the Massachusetts Institute space in 2012, it detected significantly fewer Observatory and other projects. These include of Technology in Cambridge made the first anomalous cosmic rays and a huge increase the Oh-My-God particles. In 2017, researchers samplings of extensive air showers. The in those from outside the solar system. announced compelling evidence that sug- network of detectors at Harvard College The second type, galactic cosmic rays, gests these extremely powerful cosmic rays Observatory produced much data on cos- flow into the solar system from other parts of originate from outside the Milky Way; how- mic rays and their levels of energy. the Milky Way. In early 2013, astronomers ever, they were unable to Today in Argentina, the Pierre Auger announced confirmation that supernovae pinpoint specific sources. COSMIC SHOWER. High- Observatory is continuing the hunt for produce most cosmic rays of this type. In the Researchers hope that energy cosmic rays that extremely high-energy cosmic rays. The aftermath of a supernova explosion, parti- the future Cherenkov make it to Earth strike particles under detection have as much cles bounce repeatedly within the entangled Telescope Array, set to molecules in our upper energy as a tennis ball traveling at 340 mph magnetic fields of the gaseous remnant and begin operations in 2022, atmosphere, causing a cascading reaction that (550 km/h) packed into the space of a single accelerate into cosmic rays. At some critical will help them better percolates downward until proton. Physicists have nicknamed them point, they escape into the galaxy. understand the cataclys- thousands of particles reach “Oh-My-God particles.” Third are the abundant cosmic rays that mic events that forged the ground. The Pierre Auger Aside from the Auger Observatory, other originate from the Sun. Most of these are these extremely energetic Observatory detects these cosmic-ray research projects include the protons, particles at relatively low energies. cosmic rays. secondary particles. Telescope Array Project at the University of Utah; MARIACHI, a project of the National Science Foundation, Stony Brook University, and Brookhaven National Laboratory; and the HESS II detector in Namibia. When the particles produced in air showers decay, three kinds of ghostly neutral-charged particles called neutrinos OBSERVATORY AUGER BRET/PIERRE result. Because neutrinos so rarely inter-

act with other matter, most pass through STAFFI/L. E/S. Earth undetected. But ambitious detector projects are searching for neutrinos pro-

duced by cosmic-ray showers. In 1998, the CHANTELAUZ A. How are comets and asteroids related?

The solar system is a complex place. Aside from the Sun and major planets, it is filled with hundreds of thousands of smaller bits of debris left from the solar system’s formation some 4.6 billion years ago. In the 19th cen- heating up and putting on a show with their tury, astronomers were concerned primarily spectacular tails. with discovery. Whether moons, comets, or Asteroids, on the other hand, are rocky asteroids, finding and cataloging objects bodies — micro-planets, if you will — most was the primary means of employment. And of which reside in the asteroid belt, which it is still an important part of the scientific lies between Mars and Jupiter. Asteroids process because many unusual and import- show no traits similar to comets, such as the ant objects certainly remain to be found. outgassing of volatiles that form pretty tails But in recent times, planetary scientists in the sky. Moreover, their orbits are dis- have turned increasingly to interpreting tinctly different from those of comets. what they have found. Analyzing objects But a view developed during the past with a growing number of techniques has 25 years suggests comets and asteroids, allowed astronomers to paint a sharper pic- although distinct types of bodies, are more ture of the solar system’s formation and the alike than astronomers originally thought. RUBBER DUCKY. Comet 67P/ relationships between objects in it. Trillions of comets probably exist in our solar Comets with Churyumov-Gerasimenko 1 For many years, astronomers kept com- system; the long-period ones exist in a vast hyperbolic or para- is seen close-up in this ets and minor planets, or reservoir called the Oort Cloud, named after bolic orbits appear image taken January 31, SPACE SPUD. This set of asteroids, in separate Dutch astronomer Jan Oort, who hypothe- only once, after which 2015, by the Rosetta nine color images of the categories. After all, com- sized its existence in the 1950s. they are flung outside spacecraft. Comets and asteroid 243 Ida reveals ets are frozen balls of ice Periodically, a gravitational kick from a the solar system. asteroids are leftover debris the potato-shaped structure and gas that live in the passing star or a disturbance in the Oort Lastly, main-belt com- from the solar system’s of this space rock, which initial formation 4.6 billion solar system’s distant Cloud sends long-period comets inward ets, a group consisting measures 36 by 14 miles in years ago. ESA/ROSETTA/NAVCAM extent. The images were reaches. Occasionally, toward the Sun. Short-period comets, on of only about a dozen captured by the Galileo through a gravitational the other hand, reside closer in and have members, reside in spacecraft August 28, 1993. kick, they travel inward, orbits with periods less than 200 years. the main asteroid belt. Similar objects, with their circular orbits, may have deposited water on Earth billions of years ago. This class, which includes 133P/Elst-Pizarro, 118401 (1999 RE70), and P/2005 U1 (Read), exists at the crossroads between comets and asteroids. By contrast, according to the Minor Planet Center, scientists know of nearly 780,000 minor planets. Of these, about 520,000 have been studied well enough to warrant permanent numerical designations, and more than 21,000 asteroids have names. Astronomers believe the total number of asteroids in the solar system larger than 1 mile (1.6 km) across is over 1 million. The largest asteroid in the main belt is Ceres, which has a diameter of about 600 miles (970 km). Altogether, the mass of NASA/JPL

46 50 GREATEST MYSTERIES NASA/NOAO/NSF/T. RECTOR, Z. LEVAY, AND L. FRATTARE L. AND LEVAY, Z. RECTOR, NASA/NOAO/NSF/T.

DIRTY ICEBALL. HOLY ASTEROID. From main-belt asteroids adds up to only about found a faint cometary coma surrounding Comet NEAT (C/2001 Q4) July 2011 to September 2012, 4 percent of the Moon’s mass. this body. Finally, 4015 Wilson-Harrington, typifies the kind of NASA’s Dawn spacecraft During the 1980s and 1990s, scientists’ an object first observed as a comet in 1949 soft coma and glowing captured the images used to neat distinction between comets and aster- and later independently discovered as an tail that amateur create this beautiful mosaic of oids began to blur. With the discovery of the asteroid in 1979, shares both designations. astronomers hope to see the massive asteroid 4 Vesta. Kuiper Belt, it became clear that a vast pop- The 1949 observations showed cometary through their telescopes. Vesta has a towering ulation of icy bodies existed on the solar sys- characteristics, while the later ones did not. Astronomers using the mountain at its south pole WIYN Telescope on Kitt (bottom) that’s twice the tem’s fringe, much closer than the Oort Cloud. All this points to a complex solar system in Peak, Arizona, caught height of Mount Everest, as Discoveries of main-belt comets further which the orbits and behaviors of asteroids this view of NEAT on well as a set of three craters blurred the distinction. Certainly, some and comets are related more than astrono- May 7, 2004. (top left) commonly referred to main-belt asteroids could be ancient mers originally envisioned. as the “snowman.” ex-comets whose volatiles evaporated long ago, leaving only a rocky core. The discoveries of other bodies with peculiar properties led to the cross-listing of several objects as both asteroids and comets. These include 2060 Chiron, an object discovered in 1977 by American astronomer Charles T. Kowal, then at Palomar Observatory. It’s the first member of an asteroid class known as centaurs, with orbits between Saturn and Uranus. This unusual asteroid came under intense study, and in 1988, astronomers watched it surprisingly undergo an outburst in brightness, an act characteristic of comets, not asteroids. Additionally, 60558 Echeclus, a centaur discovered in 2000, appeared to be an inno- cent asteroid. But in late 2005, astronomers NASA/JPL-CALTECH/UCAL/MPS/DLR/IDA

WWW.ASTRONOMY.COM 47 OLD TIMER. As of July 2018, astronomers had found more than 2,500 extrasolar planetary systems totaling 3,772 exoplanets. In 2003, using the Hubble Space Telescope, they measured the mass of one of the galaxy’s oldest known planets, which formed nearly 13 billion years ago. The planet — PSR B1620-26 b — orbits a pulsar in the globular cluster M4 and is shown here in artwork. NASA/BRAD HANSEN/HARVEY RICHER/ STEINN SIGURDSSON/INGRID STAIRS/STEPHEN THORSETT

How many planets surround other star systems? Standing under a dark night sky, looking out at the Milky and Alrai (Gamma Cephei) in Cepheus — and attributed those observations to the Way, it’s hard to imagine countless planets don’t exist else- pull of exoplanets. However, follow-up data showed the cause was unknown, and it took where in the universe. After all, some 200 to 400 billion another 10 years to confirm exoplanets orbit stars inhabit our galaxy, and astronomers other planetary systems remained a these stars. Meanwhile, in 1992, another estimate at least 125 billion other galaxies supreme challenge for astronomers until the team of astronomers claimed to detect an exist. That’s one heck of a number of stars in past two decades. exoplanet orbiting the pulsar PSR 1257+12. the universe — at least 25,000 billion billion. The first suspected detections of extraso- Astronomers eventually confirmed this “pul- Looking at the nebular hypothesis, which lar planets, or exoplanets, as they are called, sar planet,” as it came to be known. describes how the solar system formed (see occurred in 1989. That year, astronomers During the mid-1990s, exoplanet “How did the solar system form?” p. 77), it reported variations in the radial velocities of detections picked up. Astronomers honed seems clear most stars would form planets two stars — HD 114762 in Coma Berenices their techniques and created improved as their progenitor clouds collapsed. Yet, due to the immense distances, seeing

MINI JOVE. In 2002, astronomers measured the mass of Gliese 876b, an exoplanet orbiting a red dwarf star 15 light-years away in the constellation Aquarius. They found the little planet, depicted here in artwork, has only half Jupiter’s mass.

48 50 GREATEST MYSTERIES NASA/G. BACON “transit,” their host stars, causing small dips The final method, direct observation, first in light intensity. During its primary mis- worked in 2005 when astronomers used the sion, the Kepler satellite stared at the same Spitzer Space Telescope to image infrared 160,000 stars for about four years, looking radiation from two exoplanets. Since then, for those brightness dips. Over the course researchers have directly observed some 50 of Kepler’s life, astronomers have detected other worlds using this method. over 2,300 worlds outside our solar system. With the number of exoplanets growing They also have another almost 2,300 candi- week by week, researchers plan new and date exoplanets; most of these are likely real ambitious projects to expand the number of worlds, but scientists first need additional discoveries dramatically. Before Kepler, most observations to confirm them. exoplanets found were large “hot Jupiters.” The fifth technique is called gravita- The satellite, though, found hoards of smaller tional microlensing, in which the gravita- worlds and even spied a few possible Earth LONG-RANGE WEATHER. In 2001, tional field of a star and its exoplanet bend cousins. Current and future missions such as astronomers first measured an atmosphere the light from a distant background object. the Transiting Exoplanet Survey Satellite around an exoplanet. In this artwork, the A lone star bends light differently from one (TESS), the CHaracterising ExOPlanet Satellite planet appears as a Jupiter-like world only with a planet orbiting it. The number of (CHEOPS), and the Planetary Transits and 4 million miles from its star. G. BACON/STSCI background stars seen with this technique Oscillations of stars (PLATO) will use the tran- is maximized when astronomers look at sit method to further expand the number of stars between Earth and the galactic cen- exoplanet discoveries. TESS, for example, will instruments, and by 1995, they found the ter. Though only a few dozen planets have soon look across the entire sky to find all first exoplanet around a Sun-like star — been detected using gravitational micro- exoplanets around dim red stars within a few 51 Pegasi. As of July 2018, astronomers have lensing, future missions such as WFIRST will hundred light-years from Earth, as well as confirmed the existence of nearly 3,800 offer some of the best opportunities to give astronomers targets for future missions planets orbiting stars other than the Sun. detect low-mass exoplanets in relatively that will search for gases produced by life, Detecting exoplanets is tricky because wide orbits. like methane and carbon dioxide. the feeble light from the planets appears next to the bright glare of the host stars. At EVAPORATING PLANET. present, astronomers use about half a dozen In 2003, astronomers primary techniques to find exoplanets. The discovered the planet first method is pulsar timing, which was used designated HD 209458b. It to find the planet orbiting PSR 1257+12. This lies so close to its parent technique measures anomalies in the regu- star, it’s scorched like a larity of the pulsar’s pulses. moth that flies too close Second, astronomers have employed to a flame. Eventually, all that will be left of the astrometry — precise positional measure- planet will be its metallic ments — to attempt to detect exoplanets core. ESA/A. VIDAL-MADJAR/NASA since 1943. Although it took until 2010 to uncover the first exoplanet using this method, the recent second release of high-precision observations from the Gaia mission may mean thousands more astrometry-detected exoplanets are on the way. Third is the radial velocity method, which measures the speed at which an object (in this case, a star) moves toward or away from Earth. An exoplanet’s gravitational tug creates anomalies in the star’s speed. This was the most successful method astronomers used in the exoplanet hunt until 2009, when NASA launched its Kepler telescope. Radial velocity searches have nabbed astronomers about 670 exoplanets thus far. In the fourth method, scientists detect exoplanets as they pass in front of, or How many asteroids are locked up in the Kuiper Belt?

In 1930, American astronomer Frederick C. Leonard WANDERING ROCK. Sedna tracks its way across the proposed the existence of a band of small sky in an image captured by the Hubble Space bodies in the solar system that extends from Telescope March 16, 2004. NASA/ESA/MIKE BROWN Neptune’s orbit to a diameter of about 50 for Leonard’s outer astronomical units (AU). Leonard based solar system belt but this hypothesis on models of solar-system found nothing. The belt exists formation and on orbital dynamics of All that changed in between about the outer planets. Nearly a decade later, 1992. With the discov- 30 AU, the outer Dutch-American astron- ery of asteroid 15760, edge of Neptune’s DISTANT SUN. An artist’s omer Gerard Kuiper pub- designated 1992 QB1, orbit, and 50 AU, where impression of noontime lished research on the astronomers finally found Neptune’s orbital reso- on Sedna, the most distant proposed belt and sug- a trans-Neptunian object nance causes the number minor planet from the Sun, gested it was the source (TNO), the first real member of the of objects to drop off rapidly. shows feeble rays from our of many short-period hypothetical belt. Astronomers believed Farther out from the Kuiper Belt are the star. The temperature is –400 degrees Fahrenheit on comets. During the many other such objects lay in the region, and so-called scattered disk and the Oort Cloud. this airless, frozen world. 1960s, 1970s, and 1980s, they named it the Edgeworth-Kuiper Belt, or Over the past 26 years, the number of NASA/ESA/ADOLF SCHALLER researchers searched alternatively, the Kuiper Belt. Kuiper Belt objects (KBOs) found has grown most notablecubewanos is20000 Varuna, Amongthe 1992QB1(Q–B–1–ohs). after neptunian orbitalresonance cubewanos 1,475 miles(2,375km). showedspacecraft Pluto hasadiameter of For comparison, in 2015,theNewHorizons across.about 1,000miles(1,600kilometers) Telescope in2008,thisbodymeasures madeusingtheSpitzer Space observations 90482 Orcus, wasfound in2004.Basedon Pluto), plutino(againafter largest known was1993RO. The ered Pluto (after itself) are calledplutinos. The firstplutinodiscov- cent longerthanNeptune’s), likePluto, orbital periodmeasures 50per- Neptune (thatis, whose orbital resonance with Belt thathave a3:2 sidered aplanet?” p. 64.) “Should Pluto becon- an asteroid. (See classified asaplanetor whether Pluto shouldbe led to the debate over largest moon, Charon, which KBOsareworthy Pluto andits to more than1,000.Amongthenote- Astronomers callKBOswithoutthe3:2 intheKuiper Objects of comets. larger, sphericalOortCloud that floatswithinthefar asteroids (insetdiagram) shaped collectionof Kuiper Beltisadisk- ROCKS ANDICEBALLS.The Kuiper Belt NASA/ANN FEILD diameters larger TNOs existwith than 60miles least 70,000 (100 km). At At total of icy bodies that inhabittheKuipertotal oficy grow, astronomers don’t thegrand know across.1,600 km) where from 800to 1,000miles(1,300to thatspansany- Cloudobject an innerOort in2003,uncoveredSedna whatisperhaps the fringeofsolarsystem, thatof90377 asteroid on discovery Another important itslightlysmallerthanPluto.making has adiameter of1,450miles(2,325km), madein2010,showedlater observations, it thought to bethelargest known TNO, but named Xena. wasinitiallyThis object Although KBOnumbers continue to disk object) andEris, formerlydisk object) nick- discovered in2000.Anotherlarger object catalogedasascatteredobject discovery occurreddiscovery in2002 include 1996 TL66 (thefirst — thatof50000Quaoar. At in thescattered disk the timeofitsdiscovery, Quaoar, at780miles 1930. across,(1,260 km) was tem since Pluto in found inthesolarsys- the largest object Major objects found objects Major how itsouter reaches work today. ofhowpicture thesolarsystem formed and scientiststo gleanaclearerallow planetary cataloging for years to come. alsowill It astronomers busywithdiscoveries and orbits asclose50AU from theSun. andwithlarger than60miles(100km) least 70,000 TNOs existwithdiameters co-discoverer ofthefirst TNO, believes at Belt. Americanastronomer David Jewett, This oficeballs vastreservoir willkeep asteroid 1998WW TWO FORONE.ThedistantKuiperBelt asteroid orbitstheSunevery 301years. system inthisartwork.This binary icy bodiesonthefringeofsolar Oort Cloud Oort 31 appearsasapairof WWW.ASTRONOMY.COM

NASA/G. BACON 51 Does string theory control the universe?

One of the strangest ideas about the nature of the universe The current incarnation of the theory suggests cosmic strings arose after the infla- could be one of the most important. Do long, thin, and tionary period. The strings researchers currently propose are less massive and more incredibly dense strands of matter called cosmic strings stable than the ones originally thought up wind their way throughout the universe? This clumpy things like galaxies could have in the 1980s. Because of these changes, they theoretical idea took off with a bang in the formed suddenly from it. The answer could would have less effect on the cosmos than 1980s, received a torrent of skepticism in the be cosmic strings. astronomers originally thought, so they 1990s, underwent a resur- Some cosmologists are embracing the would not necessarily be ruled in or out of STRINGY MESS. When gence of credibility in possibilities. Edward Witten of Princeton existence by recent observations. cosmologists model the 2000s, and has since University, one of the world’s foremost theo- With the reformulation of what astrono- cosmic-string evolution, remained a tantalizing, retical physicists, says, “Strings of different mers think strings might be, the question of their goal is to predict yet unproven, model. sizes and kinds probably exist.” Thirty years whether they can be detected still hangs how string properties One of the universe’s ago, Witten opposed string theory. He now out there. At least two research teams have such as speed and separation change over strangest conundrums is believes these tiny stringlike loops of energy time. This simulation the smoothness of the could be the universe’s basic form of matter shows cosmic strings early cosmos following and energy and that some strings could when the universe was the Big Bang and how reach enormous sizes. young and dominated by radiation.

LIKE CRACKED ICE. Cosmic strings may have formed as defects in space-time when the universe cooled. The process is analogous to cracks that form as water freezes to ice. ASTRONOMY: JAY SMITH B. ALLEN AND E. P. SHELLARD (UNIVERSITY CAMBRIDGE) SHELLARD OF P. E. AND ALLEN B.

52 50 GREATEST MYSTERIES 1 2 3

4 5 6 R. BATTYE AND E. P. SHELLARD (UNIVERSITY CAMBRIDGE) SHELLARD OF P. E. AND BATTYE R.

COSMIC BONES. When reported evidence of cosmic strings in differ- arose, and these cracks created thin, super- incredibly dense, much strings collide, they can ent parts of the sky, but these observations dense strings of matter and energy. These denser than the matter at exchange pieces and form remain unconfirmed. However, according to features might have formed like fissures in a neutron star’s center. a free-floating loop. In this Alexander Vilenkin of Tufts University, who ice, along faults between transition zones. With such density, cosmic computer simulation, two pioneered cosmic string theory by suggest- These sinewy filaments of matter might for- strings would act as grav- strings approach one ing strings could have triggered the forma- ever be frozen in a primordial state, having itational lenses if they another at half the speed tion of galaxies, the discoveries have avoided the cosmic inflation the rest of the floated in front of bright of light. Both strings emit radiation — usually “breathed new life into this field.” universe experienced. background objects, and gravitational waves. A The current thinking on cosmic strings If they exist, cosmic strings are almost this could be one way to new loop forms in the goes as follows: When inflation occurred, unimaginably thin, yet they possess nearly find one. Yet spotting a collision’s aftermath. “cracks” in the universe’s phase transition unlimited length. Strings also may be long cosmic string could be incredibly difficult: Computer simulations suggest they would be spaced about 325 million light-years apart. The nearest long cosmic string might be 10,000 light-years away. The possibility of detecting a cosmic string by lensing exists, but another avenue may offer better odds. Since 2015, astronomers have been looking for bursts of gravitational waves associated with cosmic strings using gravitational wave observatories such as LIGO and Virgo, though they have not yet found any evidence. “Cosmic strings might actually provide the best observational window into fundamental string theory,” says Thomas Kibble of Imperial College London. With a rebirth of study and credibility, cosmic strings will carry on as a hot topic.

DEEP MIRAGE. Lensed quasars may reveal the presence of cosmic strings. NOAO/AURA/NSF

WWW.ASTRONOMY.COM 53 What creates gravitational waves?

In 1916, Albert Einstein revolutionized our understanding of the universe when he published his general theory of relativity. In it, the German-born physicist described the complex relationship between the fabric sheet downward and curves the fabric. The of space-time and the mass of celestial bod- same thing happens in the four-dimensional ies. Space-time is the combination of three universe. Near massive objects, which have spatial directions (height, width, and depth) large gravitational pulls, the “fabric” of with the time dimension. space-time curves and stretches. The easiest way to interpret gravitational Massive objects also cause another effect interactions, Einstein said, is to think of the in the fabric of space-time. Just as a boat space-time continuum as a stretchable mate- creates waves on a lake as it slices forward rial that bends as massive objects “sit” inside through the water, stars and other bodies in it. While this two-dimensional analogy does the universe create ripples in the fabric of not represent what is hap- space-time as they move. Astronomers call pening in four-dimensional these ripples gravitational waves. GRAVITATIONAL-WAVE space-time, it serves as a Immense objects like black holes create DETECTOR. The Laser Interferometer Space capable model. larger gravitational waves than less massive Antenna (LISA), planned When you stretch a objects. Likewise, objects moving rapidly for launch in 2034, will pliable plastic sheet tautly through space create more sustained gravi- detect low-frequency and place a bowling ball tational waves than slower moving ones. massive bodies usu- WAVY BANG. Supernova gravitational waves. It on it, the gravity well When these gravitational-wave signals ally produces gravita- explosions, like the one that will thus open up a new around the ball pushes the finally reach Earth, however, they are tional waves. The produced the Crab Nebula window on the universe. (M1) in the constellation extremely weak. Like waves in water, gravi- interactions can be Taurus in 1054, are tational waves diminish as they move out- between binary black significant producers ward from their origin. So gravitational holes or neutron of gravitational waves. waves prove difficult to detect and inter- stars, but they also NASA/THE HUBBLE HERITAGE TEAM pret once they reach us from a variety of may be between distant locations. merging galaxies or normal stars simply But after years of searching, in late encountering each other. 2015, researchers finally detected the first To help detect these faint gravitation- clear signal of a gravitational wave pass- al-wave signals, astronomers use a tech- ing through Earth. This signal, dubbed nique called interferometry. Two large test GW150914, originated from the merger of masses placed a great distance apart serve two black holes with a combined mass of as detectors. The masses are free to move in roughly 60 suns. Since then, scientists have all directions, and lasers continuously mea-

ESA/NASA confirmed gravitational-wave signals from sure the exact distance between them. four additional merging pairs of black holes, When a gravitational wave passes through as well as a merging pair of neutron stars for them, the cosmic ripple causes their dis- good measure. tance to fluctuate slightly. It’s an ingenious With these detections, astronomers now technique, and scientists have used such know the interaction of two compact and devices in several places around the world to hunt for gravitational waves. CENTRAL ENGINE. Black The Advanced Laser Interferometer holes are important creators Gravitational-wave Observatory (LIGO), a of gravitational waves. This joint project between MIT and Caltech, has artwork shows the central supermassive black hole of two locations: one in Hanford, Washington, an active galaxy as a swirling and the other in Livingston, Louisiana.

NASA/JPL-CALTECH disk of material falls onto it. Teaming up with LIGO is the French- and

54 50 GREATEST MYSTERIES What happens when black holes collide? For more than 30 years, astronomers have considered an intriguing question: What happens when two black holes meet? Inside a galaxy, black holes that formed from dead massive stars sometimes encounter each other, especially in double or multiple star systems. And for years, though no one had witnessed such a collision take place, the subject remained a hot topic of theoretical astrophysics. Studying the possibilities took a big step forward in 2004 when a team of astronomers — Marc Favata, then of Cornell University, Scott Hughes of MIT, and Daniel Holz of

I the University of Chicago — authored a C study that appeared in The Astrophysical Journal Letters; this kicked off a number of other studies that ultimately led to a DANA BERRY/STS DANA burgeoning field. SPINNING STAR. Gravitational waves It turns out a funny thing happens when originate from a variety of objects, black holes collide. They spiral toward each including pulsars, rapidly rotating neutron stars. Using NASA’s Rossi X-ray other and merge into a single entity. In Timing Explorer, astronomers found in some cases, a gravitational “slingshot” effect 2003 the upper limit to a pulsar’s spin, then violently whips them outside their based on an outburst on a pulsar, shown host galaxies into intergalactic space. The BABY BLACK HOLE. in this series of illustrations. ejection mechanism results from a byprod- holes,” says Hughes, The feeble light from a uct of the merger: gravitational waves. The “and galaxies merge redshift 6.4 quasar (arrow) gravitational waves can actually kick the like mad — especially took roughly 13 billion Italian-led Virgo collaboration, which oper- merged black hole far away from the site of a couple billion years years to reach our eyes. ates the Advanced Virgo interferometer, its merger. In 2017, astronomers found com- ago.” However, detecting This image shows the quasar when the universe a third gravitational-wave detector that pelling evidence of such a mechanism when them is still difficult. was just 800 million enables researchers to better pinpoint the they used Hubble to image quasar 3C 186. Black holes escaping years old. Physicists sources of gravitational-wave events like Observations of the offset quasar suggested their parent galaxies are learning how the those found in recent years. that it was jettisoned from the core of its gal- would be shot out at supermassive black hole Above all these Earth-based projects, the axy by gravitational waves produced by the high velocities, probably that feeds the quasar European Space Agency (ESA) plans to merger of two supermassive black holes. 685,000 mph (1.1 million got so big early in the launch the Laser Interferometer Space But what role does this process play in km/h). Such high-speed universe. SDSS Antenna (LISA) in 2034. To test the space- building black holes? Could large numbers of objects eventually might based technologies needed for such an black holes exist outside galaxies, where their join other nomadic black holes in deep enormous mission, ESA, with the help of presence would be extremely difficult to space. These freeform black holes would NASA, launched the LISA Pathfinder satellite detect? These questions and others are still prove elusive. “If they’re not shining [from in 2016. Since Pathfinder far exceeded on the table. Although scientists have already radiation produced by swallowing nearby expectations, the LISA project will likely pro- detected gravitational waves from five merg- bright material], it’s hard to know where to vide the best observatory for detecting ing pairs of stellar-mass black holes, research- look,” according to Piero Madau of the gravitational waves and will give astrono- ers are tuning up their detectors for another University of California, Santa Cruz. One way mers significant clues about the interaction observing run in early 2019, hoping more to detect intergalactic black holes would be of matter and space-time, and how the uni- data will further illuminate the subject. from gravitational-lensing effects. Another verse came to be. “Almost all large galaxies contain black method, first outlined by University of

WWW.ASTRONOMY.COM 55 TRAIN WRECK. Two supermassive black holes sit at the center of the galaxy NGC 6240 after two small galaxies collided. Both black holes are active, shown by their X-ray radiation (inset) imaged by the Chandra X-ray Observatory. While they are roughly 3,000 light-years apart now, in a few hundred million years, the two black holes will merge. NASA/CXC/MPE/S. KOMOSSA ET AL.

Cambridge researchers in 2016, would be to black holes inside galaxies.” Further study of black-hole mergers could measure Doppler shifts in gravitational-wave The evidence for small black holes gone pay off big dividends when it comes to under- signals, which would reveal how hard a black missing from normal galaxies does hold some standing large black holes in the early uni- hole is kicked. potential. According to David Merritt of the verse. Do they form by the process of mergers Intergalactic black holes could absorb Rochester Institute of Technology, “As we look or by gradual accretion? Observational tests material without radiating and so continue at ever smaller galaxies, there’s a point where for determining how big black holes formed in along below the radar. “For that reason,” says you stop seeing black holes.” Merritt and other the early cosmos are lacking; perhaps further Mitch Begelman of the University of Colorado, astronomers wonder if smaller galaxies may study of how smaller black holes behave in “we can’t rule out the possibility that black have had their small stellar black holes shot more recent times will continue to shed light holes outside galaxies contain more mass than out into intergalactic space. on this question.

SIMULATING SMASH-UPS. Astronomers can model black-hole kicks (which may eject them from galaxies) with computer simulations. In these, hundreds of primordial black holes live within dark-matter halos. Each image shows black- hole trajectories over billions of years — from redshift 8.16, when the universe was 600 million years old, to redshift 1, at an age of 6.5 billion years — based on different initial velocities. Yellow corresponds to high- density areas, red to low- density. TOM ABEL/MIROSLAV MICIC/ STEINN SIGURDSSON Why does antimatter exist?

In the earliest days of the universe, shortly after the Big JONES

Bang, the cosmos was awash in particles. Not all of them N/H. were normal particles of matter, however. Corresponding with each type of particle is an antiparticle Twenty years later, physicists discovered BLAND-HAWTHOR with the same mass and spin. antiprotons and antineutrons while con- O/J. The nature of our universe results from ducting experiments with the Bevatron par- the fact that matter exists in slightly more ticle accelerator at the University of

quantities than antimatter. The difference is California, Berkeley. In 1968, scientists first ET AL./AA STAPPERS, minuscule, however: For every billion parti- produced anti-atoms at the Stanford Linear cles of antimatter, there must have been a Accelerator Center, and in 1995, near billion and one particles of matter in the Geneva, Switzerland, physicists created anti-

early universe. Everything that exists — gal- hydrogen atoms that lasted long enough for NASA/CXC/ASTRON/B. axies, stars, planets, trees, people — owes its scientists to study their behavior. ANTIMATTER JET. existence to the slight surplus of matter. These experiments still do not enable Anywhere high-energy The Chandra X-ray The question of why antimatter exists and astronomers to explain the asymmetry of collisions take place, anti- Observatory imaged the why matter is slightly more abundant dates matter to antimatter. Other important phys- matter is sure to be there. “Black Widow Pulsar” back to 1928, when British physicist Paul ics experiments with particle colliders and The powerful black hole in in 2003. The photo Dirac described the behavior of electrons. other methods are planned for the future. the center of the Milky Way reveals a rejuvenated Dirac worked with quantum mechanics and How could matter now be so dominant? produces an antimatter jet. pulsar emitting a high- relativity and worked out an equation gov- One idea is that primordial black holes, The boundary where the speed beam of matter and antimatter particles. erning how electrons should interact with which formed in the infant universe, may antimatter collides with other particles. Dirac’s equation predicted have evaporated and thus thrown the sym- normal matter produces that for every electron, there should be a cor- metry askew. Another group of physicists gamma rays. responding antiparticle with the same mass believes the asymmetry lies in the category Antiparticles get made POWER BLAST. Explosions on the Sun but otherwise a mirror image of the original. of particles called leptons, which includes where the temperature is blast antimatter from the In 1932, American physicist Carl D. neutrinos and muons. extremely high — for exam- solar surface in this Anderson observed a particle track, caused Neutrinos seem to be the leading culprit. ple, the event horizon of a illustration. The largest by a cosmic ray, that appeared to be “some- Austrian physicist Wolfgang Pauli developed black hole. Should we ever eruptions on the Sun can thing positively charged, and with the same the idea of neutrinos in 1930 when he was get to the point of traveling unleash as much energy mass as an electron.” Following a year’s desperately searching for an explanation of deep into space, hazards as a billion 1-megaton worth of experiments, Anderson deter- the process called beta decay. from antimatter, which anni- bombs. In July 2002, a solar flare released about mined anti-electrons exist, dubbed them In beta decay, a neutron disintegrates hilates matter when the two 1 pound of antimatter, positrons, and won the Nobel Prize in phys- into a proton and an electron plus some- collide, would pose a real enough energy to power ics for the effort. thing else — a neutrino. Because neutrinos and somewhat unpredict- the United States for two interact minimally with other matter, they able hazard. days. NASA are very difficult to diagnose. Countless numbers of neutrinos produced within the Sun’s core pass through us every second. A number of physicists believe heavy neutrinos existed in the early universe that might have decayed, tipping the scales far toward matter. BIZARRO MILKY WAY. The Milky Way’s Whatever the reason for such a small center contains a glowing cloud of gamma quantity of antimatter in today’s universe, it rays. Antimatter particles annihilate matter is out there. Tiny quantities of antimatter to create the energetic jet. rain down from cosmic rays and are quickly

W. PURCELL ET AL./OSSE/COMPTON OBSERVATORY/NASA ET AL./OSSE/COMPTON PURCELL W. evaporated by interactions with matter.

WWW.ASTRONOMY.COM 57 Are there other planets like Earth?

One of the astonishing breakthroughs in observational astronomy of the 1990s was detecting the first planets outside our solar system. As of July 2018, astronomers have found 3,772 exoplanets in more than astronomers detected a planet circling the 2,500 different planetary systems, and they star 51 Pegasi. The planet is a gas giant more have more than 2,300 candidate planets massive than Saturn that orbits its star every awaiting confirmation. The first exoplanets 4.2 days. (Mercury orbits the Sun every 88 discovered were very dissimilar to Earth — days.) The planet is so close to its star that for example, massive Jupiter-sized worlds. astronomers termed it — and many other GOOD ECOBALANCE. But NASA’s Kepler space telescope changed massive, close-orbiting planets they began the puzzling results on Earth affords life all the that. With Kepler data, astronomers have to find — “hot Jupiters.” large planets, astrono- right luxuries: moderate discovered hundreds of worlds just slightly The discoveries of 51 Pegasi’s planet and mers found no smaller temperatures, lush larger than Earth and even dozens the same others like it perplexed astronomers at first planets similar to vegetation, and abundant size or smaller. They have not, however, because, as they understood solar system Earth in their searches. water, like that in the found Earth’s identical twin … yet. formation, gas giants formed on the outer But at the turn of United States’ Great Lakes region. SEAWIFS PROJECT Despite the fact that truly Earth-like plan- fringes, not close in to stars. How could hot the 21st century, the ets have not yet been found, astronomers Jupiters form so close to their stars when trend began to reverse feel there’s good reason to think many exist. the star’s heat would presumably destroy as technology and planet-searching tech- The history of exoplanet discoveries around the planet’s gases before they came niques advanced so that astronomers could ordinary stars reaches back to 1995, when together to make a planet? In addition to find planets smaller than the hot Jupiters. In fact, more than three-quarters of Kepler’s discoveries are planets smaller than 1 2 3 Neptune and larger than Earth. It’s becom- ing clear that hot Jupiters, once thought to be the norm, are actually the exception. As the list of exoplanets grows, astronomers are discovering a lot about what it takes to host a planetary system. For example, relatively low-mass stars are more likely to have planets around them than heavier ones. Out of the more than 3,700 exoplanets found so far, almost all orbit stars less than 1.5 times the Sun’s mass and cooler than 4 5 6 7,500 kelvins. (Our star has a surface temperature of about 5,800 K.) Additionally, single stars (a minority in the universe) are the best suited for planetary systems. Relatively few binary systems seem to have planets orbiting them, though scientists have found some worlds in multiple-star systems. And the chemistry of stars matters, too. Stars rich in metals — elements heavier than hydrogen and helium — FAMILY PORTRAIT. In these six portraits taken by the outbound Voyager 1 (launched are better hosts for planetary systems than September 5, 1977) spacecraft in 1990, six planets are visible. Starting at top left, they are: less evolved stars. 1) Venus, 2) Earth, 3) Jupiter, 4) Saturn, 5) Uranus, and 6) Neptune. NASA The search for exoplanets has sharp- ened astronomers’ views of how other star

58 50 GREATEST MYSTERIES BIG GLOW. Earth’s systems form. The leading model, called Planets are most often found in the galaxy’s exoplanets and look for nighttime city lights glow core accretion, suggests that a rotating arms — particularly in the less chaotic additional worlds in those as recorded from space planetary disk settles around a young star, regions of them. systems; and PLAnetary by a U.S. Department of and then billions of collisions in the disk The best stars to look at are those of F, Transits and Oscillations of Defense Meteorological cause particles to clump together. As more G, K, and M spectral types. Perhaps the best stars (PLATO), scheduled Satellite. Could and more particles stick together, proto- tip for exoplanet searchers is to keep look- for a 2026 launch, will civilizations on planets planets several miles across eventually form, ing — as they certainly plan to do. Although search nearly a million surrounding other stars likewise be casting light and they become planetary cores. the wildly successful Kepler mission is solar-type stars across half into the darkness? It soon The process happens relatively quickly in concluding, NASA launched the Transiting of the sky looking for tran- may be possible to detect the inner part of a solar system, where Exoplanet Survey Satellite (TESS) mission in siting planets. such light originating on rocky-planet building takes place. Terrestrial April 2018. The instruments aboard TESS will Furthermore, the James other worlds. NASA planets like those in our solar system could monitor stars with spectral types ranging Webb Space Telescope largely form in as little as 100,000 years. But from F to M across the entire sky, looking (JWST) will go beyond just finding exoplan- the creation of gas giants is not well for drops in brightness that signal planets ets — it will study the atmospheres of about explained in the core-accretion model. The transiting those stars. The European Space a dozen such worlds once it launches in gravitational-instability model is an alterna- Agency also has two exoplanet missions 2021. Astronomers are looking for signs of tive, less popular, idea. This says instabilities coming down the pipe: the CHaracterising life in that planetary air, like carbon dioxide form as a disk collapses, leading to the ExOPlanet Satellite (CHEOPS), set for an early and ozone. If the past decade is any indica- clumping together of matter into planets. 2019 launch, will stare at bright and nearby tion, the next one promises many exciting Neither idea yet addresses one surprise from stars that astronomers already know host surprises in the hunt for Earth’s twin. the exoplanet discovery program: the fact that most exoplanets have orbits that are far more eccentric than those of the planets in our solar system. HEAVY SMOKER. Despite the Popocatépetl, or Popo, mysteries, explana- sends a plume of tions of exoplanets smoke skyward are getting better January 23, 2001, as by the month as imaged from the space discoveries shuttle. The Aztecs increase. The best gave the mountain the name Popocatépetl, places to find them which means “smoking are in the younger mountain.” Other Earth- metal-rich areas of like planets may spiral galaxies like produce similar smoke the Milky Way. plumes. NASA Does every big galaxy have a central black hole?

In the early 1960s, astronomer Maarten Schmidt of the California Institute of Technology made a breakthrough discovery. Looking at several stars that were strangely bright at radio wavelengths, Schmidt what could be causing such an amazing obtained a spectrum of the “star” 3C 273 and outpouring of energy, apparently so early in found its distance to be extremely large. It the cosmos’ history? wasn’t a star at all, but a remote, extraordi- The first quasar identified, 3C 273, lies in narily energetic object that looked like a star the constellation Virgo some 2 billion light- — a quasi-stellar object, or quasar. years away. But at this tremendous distance, COSMIC BELCH. Centaurus A is an For many years, the mystery of exactly it still glows brightly enough to be viewed active galaxy currently “snacking” on what quasars were remained unsolved. They with amateur telescopes in the backyard. a meal of gas and dust. As it eats, the baffled astronomers at every turn. Their How could an object that looks like a star be galaxy’s supermassive black hole shoots spectral lines were shifted by an incredible producing several thousand times the entire out jets of highly energetic material. ESO/ amount toward the red end of the spec- energy output of the Milky Way Galaxy? WFI (OPTICAL); MPIFR/ESO/APEX/A. WEISS ET AL. (SUBMILLIMETER); NASA/CXC/CFA/R. KRAFT ET AL. (X-RAY) trum; because of their great brightnesses, Oddly, astronomers found that these they must represent the brightest objects in objects vary their light outputs over months the universe, astronomers deduced. But or even days, so they had to be relatively small objects, too. As searches for quasars picked up steam, astronomers found many more of them — starlike objects with colors different from those of stars and point sources associated with X-ray emission or strong radio output. Now, almost 60 years after the discovery of 3C 273, the latest catalogs contain about 200,000 quasars, thanks to systematic surveys like the Sloan Digital Sky Survey. Years into the quasar puzzle, in the late 1970s and 1980s, astronomers began to get a handle on what these objects might be. Quasars fit into a classification of a large number of seemingly related objects called active galactic nuclei, or AGN. Now astronomers realize that, clearly, all types of active galaxies — quasars, Seyfert galaxies, TEAM

HIDDEN MONSTERS. Distant galaxies imaged in the Hubble Space Telescope’s GOODS field (a DICKINSON/THE GOODS special deep field) reveal R/M. R/M. massive galaxies in the two left images; the right-hand OEKEMOE counterparts show the glow of powerful black holes as revealed by the Spitzer

NASA/ESA/A. M. K M. NASA/ESA/A. Space Telescope.

60 50 GREATEST MYSTERIES AWAITING LUNCH. A 300-million-solar- mass black hole lies within the elliptical galaxy NGC 7052. This Hubble image shows the BL Lacertae objects, radio galaxies, and orientations to galaxy’s thick, other weird entities — have one thing in our line of sight dark dust band, common: They are all driven by powerful in addition to which enshrouds the black hole. central black holes. Material falling into other geomet- In several billion a central-engine black hole — stars, gas, rical differences. years, the black and dust — gets spun quickly and creates Moreover, Hubble hole will swallow this high-powered jets of radiation that produce Space Telescope data feature. ROELAND P. VAN DER MAREL/ the amazing output we see as a quasar. continued turning up huge FRANK C. VAN DEN BOSCH More recently, in the 1990s, astronomers numbers of monster black holes realized AGN are really different shades in the centers of many galaxies — even picture emerged of how quasars and black of the same creature, some appearing “normal” galaxies like the Andromeda holes fit into the cosmos. like different animals because of different Galaxy and the Milky Way. Soon, a general The idea came forward that most galaxies other than dwarfs have central black Black-hole mass scales with galaxy size holes. The idea is that black-hole “seeds” either attracted matter into forming galaxies or formed within young galaxies and From ground From HST Black-hole mass acted as powerful, hungry engines in the early universe. This action produced quasars and explains why most quasars are extremely distant. As the black hole “ate” more and more matter from galaxies’ cen- NGC 4649 2 billion Suns ters, less and less fuel remained for them to feast upon nearby, so they slowly quieted down. Most galaxies in the recent universe have slumbering giants in their centers. Sleeping giants can awake, however. When interactions with other galaxies, star- bursts, or gas clouds falling into the centers of galaxies “wake up” central black 200 million Suns NGC 4291 holes, they can erupt again with an outburst of energy. This explains AGN in the nearby universe. The prevail- ing notion, then, is that most large galaxies contain big black holes, the majority of which are asleep after a wild youth. NGC 2778 20 million Suns ONE SIZE DOESN’T FIT ALL. The hearts of four massive elliptical galaxies show that the more massive a galaxy’s central bulge, the more massive its black hole. The black-and-white images at left show the galaxies; close-ups taken with the Hubble Space Telescope NGC 7457 3 million Suns fill the middle column. The right- hand column illustrates the corresponding black-hole masses. NASA/KARL GEBHARDT

75,000 light-years 3,000 light-years Diameter of Earth’s orbit (186 million miles or 301 million km) WWW.ASTRONOMY.COM 61 Does inflation theory govern the universe?

Ever since the Big Bang theory of the origin of the universe developed in the 1920s and 1930s, the idea has had its skeptics. The radical concept stated that the universe is expanding; run time 1970s the Big Bang backward, and all matter and energy inter- seemed to outweigh sect at a point in time some 13.8 billion competing ideas years ago, the moment of the “bang.” about the forma- Big Bang cosmology has received enor- tion of the cos- mous support from observational tests. In mos, it left some 1929, American astronomer Edwin Hubble problems observed galaxies generally recede from us; unsolved. that was the linchpin of the evidence. The first was In 1964, Bell Laboratories physicists Arno causality. To Penzias and Robert Wilson discovered the astronomers’ faint echo of the Big Bang — the cosmic amazement, they microwave background radiation (CMB). found the CMB’s Along with the expan- temperature was MINUSCULE TIME. How sion of the universe and uniform everywhere much is 10–34 second? To get the CMB, the Big Bang they looked, to a high an idea, compare 1 second to model also explains level of precision. If the the 13.7 billion-year age of nucleosynthesis, the universe began as a hot, pri- the universe. Next, divide process by which light meval fireball, why would tem- that 1 second into an elements formed in the peratures everywhere be so uniform? equivalent number (13.7 billion) of parts to get 10–17 early universe. Leaving this enigma unsolved would second. Repeat that step one But the Big Bang threaten the Big Bang idea. more time, and you’ll get does not yet explain it Second was the flatness problem. The both the universe’s TINY COSMOS. This back to inflation’s beginning. all. Although by the cosmological number Omega (Ω) describes shape and fate. artist’s conception shows Astronomers found the universe at actual size, immediately after the Omega's value period of hyperexpansion How quickly 13.7 billion years equals 1, which 0 known as inflation. ASTRONOMY: did inflation seemed highly coin- ROEN KELLY AND CHUCK BRAASCH take place? cidental. A number less than 1 would mean space is open and will continue to expand forever; greater than 1 would mean a closed universe and 1 second an eventual “Big Crunch,” with the cosmos 0 1 second falling back in on itself. (See "What is the fate of the universe?" p. 23.) Third was the magnetic monopole problem. The cosmos is filled with electric monopoles, particles like electrons and protons. –17 10 second But astronomers have not observed any –34 –17 10 second 10 second magnetic monopoles. The lack of these particles bothered particle physicists. In 1981, to solve these problems, scien-

: ROEN KELLY ROEN : tists presented a new idea that expanded ASTRONOMY

62 50 GREATEST MYSTERIES BIG BANG ECHO. The cosmic microwave background radiation is a the Big Bang theory and added weight to it. space and blow it up by a factor of 1050.” decay. The released relic of hot radiation emitted when atoms first formed and Alan Guth of the Massachusetts Institute of Inflation resolves some questions surround- energy acted as an anti- light could travel freely, Technology wrote a paper that described ing the Big Bang. gravity force, giving the 380,000 years after the Big the “inflationary” model of cosmology, Guth and his colleagues imagined a universe a kick. The uni- Bang. The smoothness of developed with his colleagues Andrei Linde, huge energy field in the early universe, verse exploded by many this radiation suggests the Paul Steinhardt, and Andy Albrecht. which they called the false vacuum. As it factors in an instant. universe expanded at Inflation proposes a short period of expanded, the false vacuum was in a peril- Viewed as a radical breakneck speed in the expansion — 10–34 second — in the early ous state of equilibrium, and it had to decay concept when Guth first fraction of a second. (Different colors show universe. As astrophysicist Mario Livio says , into a real vacuum, leading to an enormous published his paper, the temperature differences of a “All inflation theories … grab a speck of release of energy. Inflation resulted from the idea has received sub- few millionths of a degree.) stantial support from a ESA/PLANCK COLLABORATION vast number of astro- 12physicists since. And observational tests of the Big Bang all have supported the theory. In 1992, the Cosmic Background Explorer (COBE) satellite discovered temperature fluctuations in the CMB, further evidence of the Big Bang. Later, the Wilkinson Microwave Anisotropy Probe (WMAP) and the Planck satellite refined this picture, adding weight to it. Other verifications of the Big Bang have helped convince skeptical astronomers that it occurred. In the first decades of a new century, 34evidence suggests the Big Bang occurred and that inflation was a key component of the cosmos’ earliest moments.

INFLATIONARY STEP. Inflation made the universe flat. Here, a curved surface expands by a factor of 3 in each panel, appearing nearly flat by the end. To relive cosmic inflation, repeat this expansion 87 more times. ASTRONOMY: ROEN KELLY

WWW.ASTRONOMY.COM 63 Should Pluto be considered a planet?

Ever since Neptune’s 1846 discovery by German astronomer

Johann Galle, discrepancies in its orbit suggested another Water ice planet existed far beyond it. Thus, the search for Planet X began, with “X” representing “unknown.” called Pluto (after the Roman god of the Astronomers at many institutions underworld). However, after analyzing the responded, but none with the vigor of new planet’s orbit, astronomers declared American philanthropist Percival Pluto had too little mass to signifi- Core Lowell, who determined his cantly affect Neptune’s orbit. It (iron-nickel alloy, rock) observatory should be Pluto’s seems the discrepancies in responsible for finding Neptune’s orbit arose from FROZEN WORLD. Pluto’s internal structure the next new world. In planetary poor estimates of its is not well understood, but it probably 1930, an extensive mass. But the solar sys- consists of an iron-nickel core, water ice, search at Lowell deathblow came tem then contained nine and large amounts of methane ice. LPI Observatory in planets, and everyone Flagstaff, Arizona, by August 24, 2006, was happy. the young American In 1978, James W. 1,475 miles (2,375 kilometers) across, while astronomer Clyde at the IAU Christy, an astronomer at Charon is about 750 miles (1,200 km) across. Tombaugh paid off when meeting. the U.S. Naval Observatory, Like Earth’s Moon, Charon has a large diame- he found images of the new discovered a “bulge” on a ter relative to its parent planet. planet on two separate photo- high-resolution image of Pluto and The happy, nine-planet solar system graphic plates, revealing its motion against identified it as a large moon, which he began to unravel in the early 1990s, how- the background stars. named Charon. Pluto was now a double ever. Planetary scientists found large num- Astronomers had found Planet X, now planet: Pluto itself measures some bers of objects in the Kuiper Belt, a region of the solar system beyond Neptune. These Kuiper Belt objects (KBOs) are part of a group of bodies called trans-Neptunian objects (TNOs), which are prevalent throughout the outer solar system. Many thousands of TNOs exist (more than 2,500 have been discovered as of July 2018), and some are large bodies, including Pluto. Pluto has a 3:2 orbital resonance with Neptune (meaning Pluto’s orbital period is 50 percent longer than Neptune’s), which prompted scientists to create a new category of TNOs with the same resonances called “plutinos.” The following firestorm of TNO discoveries fueled a heated debate over whether Pluto itself should be considered a planet. The “controversy” grew in the mid- to late 1990s as media picked up on the story, and

DYNAMIC DUO. New Horizons took these images of Pluto (lower right) and Charon (upper left) as the probe flew by the system on July 14, 2015. NASA/JHUAPL/SWRI PLANET HAVEN. The Kuiper Belt begins at Neptune’s orbit and extends 50 times the Earth-Sun distance from the Sun. Here, thousands of potential Pluto-like objects formed early in our solar system’s history. The largest may surpass Pluto in size. ADOLF SCHALLER editorials, television, the internet, and other venues joined in the fracas. In 1999, Brian G. Marsden, then director of the International Astronomical Union’s Minor Planet Center, the official naming body, joked that with the 10,000th asteroid naming, Pluto should be awarded that number and given “dual citizenship” as a planet and an asteroid. The issue seemed to culminate when, in 2000, New York’s newly opened Rose Center for Earth and Space, home of the Hayden Planetarium, categorized Pluto in one of its exhibits as a sub-planet. “There is no scientific insight to be gained by counting planets,” said Hayden Planetarium’s director, Neil deGrasse Tyson, “eight or nine — the numbers don’t matter.” Amazingly, the volatile response from schoolchildren across the United States indi- cated the number did matter. Upset by Pluto’s apparent demotion, they campaigned hard against any such nonsensical thoughts. The whole issue raises the question of what exactly makes a planet? The word comes from the Greek for “wanderer,” and the modern definition depends somewhat on the emphasis placed on an object’s size, orbit, or origin. Under the argument that any body large enough to be spherical under its own gravity should be a planet (which occurs at a diameter of about 200 miles [320 km]), Pluto would be counted as a planet — as well as moons like Ganymede, Titan, Earth’s Moon, and the largest asteroids. The discovery that Pluto is a wandering TNO and that many similar bodies exist argues against its status as a major planet. Since 2002, large TNOs named Quaoar, NEW MOON. In 1978, Orcus, Sedna, and Eris have been found — astronomer James W. and at a diameter of 1,450 miles (2,330 km), Christy at the U.S. Naval Eris is just slightly smaller than Pluto. Observatory discovered Charon Pluto’s planetary deathblow finally came Pluto’s largest moon, on August 24, 2006, during the final day of Charon. As the moon the International Astronomical Union’s revolves around Pluto, it reveals itself as a “bump” meeting in Prague. The remaining 424 mem- on Pluto’s dark disk (left). bers voted to strip Pluto of major-planet No bump appears (right) status and place it in a new category — when Charon is in front dwarf planet. of, or behind, Pluto. USNO

WWW.ASTRONOMY.COM 65 Why did Venus turn itself inside out?

Years ago, planetary scientists thought of Venus as Earth’s Magellan became computerized, 3D images of regions with altitude effects exaggerated sister planet. Similar in size, both close to the Sun, both by the image processors. For the first time, humans had a good look at what Venus is rocky bodies, Earth and Venus were practically considered really like. two of a kind. That abruptly changed, how- living things of any sort. There were surprises. The most ever, when astronomers got their first Scientists’ understanding of amazing was the relative lack of close-up look at Venus. The moment arrived Venus and its geology cata- craters compared with other in 1962, when Mariner 2 flew by the planet, pulted forward with the inner solar system bodies and far more forcefully in 1970, when Venera most significant mission Surface like the Moon, Mars, and 7 touched down on the hellishly hot surface. thus far, the Magellan Mercury. Water, wind, Not only do surface temperatures on our spacecraft. It arrived in temperatures on volcanoes, and tectonic sister planet exceed 850 degrees Fahrenheit venusian orbit in 1990 our sister planet shifts constantly resur- (450 degrees Celsius), and continued to col- face our planet. Venus VOLCANIC VENUS. but Venus’ thick car- lect data through 1994. exceed 850° F must be hiding many old Volcanoes in a region on bon-dioxide atmosphere Magellan mapped 98 craters, too. Astronomers Venus called Guinevere produces a greenhouse percent of the planet’s wondered what resurfacing Planitia lowland suggest (450° C). thick, sticky lava oozed effect that hosts sul- surface and returned thou- forces could keep Venus’ sur- from a point at the surface fur-dioxide and sulfu- sands of spectacular views of face looking so young. here. The center volcano ric-acid clouds. It’s not a Venus’ geological features. Almost Astronomers observed other weird spans 31 miles (50 km). friendly environment for one-quarter of the images returned by artifacts on the planet’s surface, in addition NASA/JPL VENUSIAN 3D. A portion of western Eistla Regio on Venus appears in this dramatic 3D reconstruction of the terrain made using Magellan orbiter data. NASA/JPL

WWW.ASTRONOMY.COM 67 HELLISH WORLD. Scientists created this hemispheric view of Venus using Magellan spacecraft data. The image is color-coded to elevation and reveals features as small as 2 miles across. NASA/JPL

Planetary scientists believe strongly in the gradual, slow, methodical workings of nature. Venus’ geology placed them in an awkward position because a huge catastrophic event apparently attacked the planet suddenly. Nonetheless, in 1992, Gerald Schaber of the U.S. Geological Survey wrote that what was observed on the planet may have resulted from a “global resurfacing event or events.” Don Turcotte of Cornell University followed a year later, proposing the venusian crust may have grown so thick over time that it trapped the planet’s heat inside, which eventually flooded the planet with molten lava. Turcotte described the process as cyclical, suggesting that the event of several hundred million years ago may have been just one in a series. Others have suggested that low-level volcanism may be responsible for coating the Color shows radius (km) planet’s surface over time without a need for 6,048 6,050 6,052 6,054 6,058 6,060 6,062 any global catastrophes. The European Venus Express spacecraft, which orbited the planet between 2006 and 2014, found the best evi- to many substantial volcanoes that suggest Dating various features on the planet’s sur- dence to date that Venus has been volcani- an active geology in the recent past. These face subsequently revealed Venus must cally active in the recent geological past. “All include coronae (crown-shaped surface have undergone a cataclysmic upheaval the geologists agree,” says Schaber, “some- features), tesserae (crunched features where about 750 million years ago, very recently thing very strange happened.” However scien- the planet’s crust is pushed together and in geologic terms. At about that time, tists still have work to do before they can buckles), and arachnoids (circular or oval Venus’ surface seems to have been com- pinpoint the exact mechanisms that caused features filled with concentric rings) — so pletely wiped clean. Venus to undergo a makeover. named because they are spider-like in appearance. Moreover, scientists found trace signs of erosion and tectonic shifts on our sister planet. As scientists looked more carefully at the body of data returned from Magellan, it became clearer that this was a planet that had, somehow, turned itself inside out.

HILLY LAVA. Three thick domelike hills dominate the center of this 3D Venus image made using Magellan data. The hills, on the eastern edge of Alpha Regio, were formed by thick eruptions of lava that

solidified on level ground. NASA/JPL

68 50 GREATEST MYSTERIES How could we recognize life elsewhere in the cosmos? Living things could permeate the universe. With at least 25 thousand billion billion star systems out there, it’s an incredible conceit to think Earth is the only planet in the whole universe hosting life. Yet over the right way — that’s why the search for life on history of astronomy, we know of only one planets in the solar system abides by the planet that hosts life — ours. If we were to mantra, “Follow the water.” find life elsewhere, whether microbes in our Astronomers already know of more than solar system or more complex beings far- 2,500 planetary systems scattered through- ther away, how would we recognize it? out the Milky Way, plenty of which contain It might not be easy. But there are start- rocky terrestrial worlds like Earth and Mars. BURIED WHISPERS? ing points. “Astrobiologists argue some But even with many rocky planets out they undergo while they Jupiter’s moon Europa has properties must be universal to life wher- there, how easy is it to make life? Again, the are alive. These reactions an icy and cracked surface. ever it occurs,” said late astronomer and only example we have is right here on Earth. can manifest themselves Could microbial life exist chemist Alan Longstaff. First, life is defined After our planet’s formation 4.54 billion in planetary spectra as under the ice? Recognizing as a complex chemical system that uses years ago, life left traces enriched with car- molecules that should not it may be difficult for future energy, generates waste, reproduces, and bon isotopes in sedimentary rocks. The earli- exist unless living crea- space probes. NASA/JPL takes part in evolution over time. The suc- est yet examined, from Labrador, Canada, tures are producing them. cessful critters on Earth, and presumably in date to 3.95 billion years ago. That’s the ear- Scientists know that Earth’s atmosphere other places too, exist in huge numbers. liest evidence of life on Earth. Considering a contains oxygen only because photosynthe- They also can replicate themselves success- rain of rocks periodically battered Earth’s sis creates it faster than it is depleted by fully enough to survive the rigors of a some- surface, life probably started and was other processes, such as iron rocks oxidizing. times hostile environment. snuffed out several times before finally tak- So detecting an abundance of ozone and All known life is carbon-based, Longstaff ing hold. This suggests life started quickly methane in a planet’s atmosphere, for exam- reminded us. No other chemical element is on Earth, and, therefore, the odds it could ple, would be strongly suggestive of living as adaptive to the variety of reactions car- start elsewhere, under challenging condi- beings on it. bon can undergo. And, crucially, all life we tions, are good. But life will not be found virtually any- are familiar with needs liquid water. Water Life can exist in hellish places — and where. Temperatures have to be just right, allows biological molecules to interact the likely on frozen worlds, too. In recent times, and liquid water probably has to be present. scientists have discovered hydrothermo- Equilibrium is good: Life doesn’t like wild philes that thrive in high-pressure ocean swings in its environment. Alien life-forms, water as warm as 242 degrees Fahrenheit although they could be radically different (117 degrees Celsius). Many species of bac- from what we know on Earth, will likely teria exist several miles underneath Earth’s share common characteristics and chemis- crust, one place that would have been safe try. If astronomers do succeed in detecting during the heavy bombardment period. life elsewhere in the cosmos, one thing is Conversely, microbial life is known to exist certain: It will be a huge in the pack ice of the Arctic and Antarctic. moment for the history It, therefore, might also thrive in similar of human civilization. conditions on Jupiter’s moon Europa or LIFE ON MARS? Arguments over life on Saturn’s moon Enceladus. BLACK SMOKER. A Mars erupted in 1996 when scientists hydrothermal vent deep in proposed that chainlike structures in As astronomers discover more and more the Atlantic Ocean shows a martian meteorite, ALH84001, might exoplanets, how will they know which ones the kind of environment be fossilized microbial life. Now the to focus on as possible habitats for life? where life might have first consensus is the structures are Planetary spectra will give astronomers sig- taken hold on Earth. The chemical, not biological. NASA/JSC nificant clues. Living things alter their envi- surprising ability to adapt ronments through the chemical reactions to harsh environments suggests life could be

plentiful in the cosmos. NOAA

WWW.ASTRONOMY.COM 69 What created Saturn’s rings?

A glance through a small telescope at the planet Saturn is often the experience that turns people on to astronomy. Simply walking outside, setting up a little scope, and enjoying a spectacular view of a distant scientific operations up close. Saturn’s globe planet, colorful and beautiful, surrounded measures 72,400 miles (116,500 kilometers) by razor-sharp rings, is deeply satisfying. across; its rings span about 175,000 miles Saturn’s rings were also one of the first tar- (280,000 km). gets of Galileo Galilei’s new telescope some The rings were first divided into three RINGED PORTRAIT. Saturn’s magnificent 400 years ago, when he revolutionized groups, designated C, B, and A, working ring system appears incredibly detailed human observations of the cosmos. Among outward from the planet. Visible in a small in this image created by the Cassini the most identifiable and familiar symbols telescope is the black band that separates spacecraft. The rings are comprised of of astronomy, Saturn’s rings remain almost rings B and A called the Cassini Division, particles from tiny sizes up to large as mysterious today as they were to that after Giovanni Cassini, who discovered the boulders measuring meters across. NASA/JPL Italian explorer. Scientists don’t yet know gap in 1675. the origin of the rings. Astronomers continue to discover fainter In 1980 and 1981, scientists got their first rings. They are designated D (closest to the The early view of Saturn’s rings as a continu- great view of Saturn’s rings when the planet), F (a narrow feature just outside the ous flat disk has changed, courtesy of the Voyager 1 and 2 spacecraft conducted A ring), and two distant rings called G and E. Voyagers. Today, astronomers realize the rings comprise countless thousands of particles of dirty water ice ranging from microns to meters in size. The Voyagers found surprising structures in the rings. Unresolved ringlets and gaps might be caused by tiny satellites orbiting within the rings, astronomers reasoned. By observing stars behind Saturn’s rings, Voyager resolved objects as small as 1,000 feet (305 meters) across and showed that few gaps exist in certain rings. Instead, density waves create the effect. Some of the rings contain clumps and spokes. In the wake of the Voyagers, many unanswered questions remained. The next chapter in Saturn exploration began in 2004 when the Cassini-Huygens spacecraft entered orbit around the planet. Over the next 13 years, Cassini took vast amount of data and thousands of images, and the European Huygens probe touched down on Saturn’s largest moon, Titan.

PSYCH-OUT. Saturn’s C ring appears psychedelic in a false-color view shot by a camera aboard the Voyager 2 spacecraft in 1981. At the time, the spacecraft was 1.7 million miles (2.7 million km) from the planet. NASA/JPL HOLIDAY COLORS. *_SHADOW SHOT. The Red dirt blends with green Cassini spacecraft took this ice in this false-color, stunning image of Saturn ultraviolet shot of the A eclipsing the Sun in 2006, as ring. The Cassini Division, the probe traveled through which is the thick red line Saturn's shadow. NASA/JPL near the right side of the image, contains lots of dirt. NASA/JPL/UNIV. OF COLORADO

WAVY MOON. A new moon In April 2006, Cassini findings shed light pieces. Saturn’s titanic gravity then discovered by the Cassini on the origin of the ring system. Strangely smoothed out the pieces into a flattened spacecraft May 1, 2005, shaped gaps in some of the rings suggest disk around the planet. Data collected orbits inside a gap in Saturn’s elusive moonlets exist and support the during five of Cassini’s final 22 close passes rings and creates waves notion that the rings are debris from an icy by Saturn before it plummeted into the inside the rings, visible as moon that broke up eons ago as a result of a ringed planet in 2017 suggests that scalloped edges. NASA/JPL violent collision. However, careful analysis of Saturn’s rings are just 150 to 300 million Saturn’s rings remains an ongoing project. years old. Though these results are not The scenario may be that, a few hun- quite conclusive, after hundreds of years, dred million years ago, a comet or asteroid the origin of the Saturn's mysterious rings slammed into an icy moon, breaking it into is finally beginning to come into focus.

WWW.ASTRONOMY.COM 71

Could a distant, dark body end life on Earth?

Although we live in relative quiet within the cosmos, going about our lives and seeing the stars as a distant backdrop, we are very much part of the universe that surrounds us. Dangers lurk in space, as any glance at the well-known stars will occur far on down the Moon’s cratered surface confirms. line: For example, in less than a million and Not only must our planet avoid collisions a half years, Gliese 710, a red dwarf now with Earth-crossing asteroids, but more 60 light-years away, will slide within a light- remote threats exist. If a nearby star went year of the Sun. This will unleash a torrent of supernova, a gamma-ray burst erupted comets from the Oort Cloud into orbits that nearby, or a black hole or stream of antimat- could intersect Earth’s, and they will arrive ter somehow wandered into our neighbor- near our planet within a liberal span of hood, it could spell disaster. While about 2 million years. astronomers say those events are unlikely, But bombings from comet nuclei could UNSEEN COMPANION. A small dim another dark, distant interloper could create result from other sources, too. A number of object called a brown dwarf could orbit havoc on Earth by its mere presence. astronomers suggest the Sun may have a the Sun in our solar system’s distant Where could such trouble come from? hidden, dark companion that periodically regions. Brown dwarfs fall somewhere The Sun hasn’t always been a solitary star. It sends comets sunward, raining them down between the smallest star and the largest was born in a group of suns, as all stars are, on the inner solar system. planet. This artist’s conception shows a and its native companions have been scat- In 1984, University of Chicago paleontol- pair of brown dwarfs. NASA/ESA/A. FEILD (STSCI) tered by the gravitational tug created by ogists David Raup and J. John Sepkoski orbiting the galaxy’s center. revealed their finding that Earth’s extinction Yet some 5 billion years after the Sun’s events were periodic. At the time, they sug- Not all scientists are unconcerned about birth, a few of its associates still gested the Sun’s orbit about the the idea of a dark threat. Michael Rampino, a linger near the old neighbor- Milky Way’s center was respon- geologist at New York University, searches hood. Among them are the sible, unleashing comets at for an astronomical object he believes may Sun-like star Alpha (α) In a million regular intervals of about be responsible for recurring extinction Centauri, the yellowish and a half years, 26 million years. events every 25 to 35 million years. dwarf Tau (τ) Ceti, and In the same year, As suggestive evidence, Rampino cites the the cool red dwarf Wolf Gliese 710 will University of California, large-impact events that produced craters 359. Is the Sun truly Berkeley, physicist under the Chesapeake Bay between Virginia single, or could a cool, slide within a Richard Muller proposed and Maryland and in Popigai, Siberia, about dark companion loom in the responsible mecha- 35 million years ago and the K-Pg impact in the background, periodi- light-year of nism was “Nemesis,” an the Yucatán Peninsula, the “dinosaur killer” cally nudging comets unseen, distant stellar com- that occurred about 66 million years ago. toward Earth? the Sun. panion to the Sun. Muller Rampino believes several smaller lines of The discovery of Sedna, a thought an M dwarf — a small, cool evidence suggest another catastrophic trans-Neptunian object found in 2003, and star — could lurk unnoticed in the distance impact 95 million years ago. the subsequent discovery of Eris, fueled the yet have a huge effect on the Oort Cloud. If a dark monster is out there, it could be idea that large, dark bodies float in the solar With the advent of the Two Micron All- a small brown dwarf. If such a starlet exists, system’s distant reaches. Those bodies exist Sky Survey (2MASS), however, astronomers it might weigh less than 40 Jupiter masses, apart from the numerous comets that popu- scoured the whole sky at near-infrared making it slip under the radar of 2MASS and late the Oort Cloud. Close passages of wavelengths, producing millions of images similar surveys. It would have a highly ellip- that would have uncovered Nemesis, had it tical orbit that would make it hard to spot DANGER ZONE. Many dark existed. So the mystery of what lurks out in because most of its time would be spent far objects lie too far from the Sun to be observed. An the darkness beyond the Oort Cloud, if any- from us. Still, most astronomers remain unseen threat might come thing, continues. skeptical. Only time will tell. from deep space in the future. ASTRONOMY: ROEN KELLY

WWW.ASTRONOMY.COM 73 Do we live in a multiple universe?

For as long as humans have gazed skyward, a question has early history of the universe. (See “Does inflation theory govern the universe?” p. 62.) loomed in the back of our collective mind: How do we Third, ideas about inflation suggest many Big Bangs may have occurred. Fourth, know everything that we see is everything there is? notions about string theory imply universes Decades of astrophysical research begin- from the existence of the famous cosmolog- of very different types may have formed. ning in the late 19th century established the ical constant. Some of the notions that (See “Does string theory control the uni- universe as we see it, culminating with the come out of these lines of evidence are verse?” p. 52.) Altogether, these notions sug- Big Bang theory. We now know the universe pretty counterintuitive. Yet that doesn’t gest it was possible, if not probable, that is about 13.8 billion years old and at least worry astronomers. “I fully expect the true 150 billion trillion miles across. But in recent nature of reality to be weird and counterin- years, astronomers have begun to address a tuitive,” says cosmologist Max Tegmark of Universe in the balance

staggering possibility — the universe we the Massachusetts Institute of Technology, st fa o can observe, and in which we live, may be “which is why I believe these crazy things.” o t g one of many that make up the cosmos. The idea of multiple universes, or multi- in d n a The suggestive evidence for this comes verses, poses the notion of the universe p ht x t rig E jus from several directions, from the idea of existing like a giant sponge. Each bubble is a ing and cosmic inflation, from string theory, and distinct universe, like ours, but others could Exp exist separated by giant voids.

What’s the evidence for all this? First, Scale of universe Other Exp andi universes measurements of distant supernovae sug- ng too gest the expansion of the cosmos is acceler- sl ow KELLY ROEN : ating. Second, more and more evidence ly supports the inflationary scenario of the ASTRONOMY

10 Big Bang Time 10 years Multiverse BIG PICTURE. Light’s finite speed determines how far we can see, but as the visible universe (small gray sphere) grows, it is an ever EXPANSION RATES. FORTUNATE smaller part of a larger universe Our cosmic fate hangs UNIVERSE. Physical expanding even faster. on the universe getting constants took on ASTRONOMY: ROEN KELLY the parameters right, values given by chance or nearly so. How the processes when the universe expands, and universe was born. But at what rate, determines if they were any differ- its ultimate fate. Our universe ent, we couldn’t exist.

Part visible to us Inﬁnite Gravity A lucky beginning dominates

Stars explode All atoms radioactive TO THE EDGE AND BEYOND. 1 With current instruments, we can see galaxies out to We are “technology’s horizon.” We here can’t observe galaxies past Technology’s horizon 0.1 Carbon unstable the “speed-of-light horizon,” Speed-of-light horizon TEGMARK M. AFTER KELLY, ROEN : but they may become visible Deuterium unstable in the future if the universe’s Relative strength of strong nuclear force expansion decelerates. 0 ASTRONOMY ASTRONOMY: ROEN KELLY 0 0.1 1 10 Inﬁnite Relative strength of electromagnetism 74 50 GREATEST MYSTERIES Here, there, and 4 Atoms everywhere How did the unstable

3 Fields Events unpredictable unstable Milky Way 2 Galaxy form?

Atoms Too simple We are 1 here unstable Number of time dimensions Taking a telescope out on a clear springtime night and : ROEN KELLY, AFTER M. TEGMARK M. AFTER KELLY, ROEN : 0 Events unpredictable scanning the area of the constellation Virgo reveals ASTRONOMY 01 234 an amazing sight: Large areas of the constellation are Number of spatial dimensions peppered with faint smudges, the light from STRANGE BREWS. Space distant galaxies bound up in a huge swarm and time might have other called the Virgo cluster. This area of sky gives values in other universes, us our best look at the closest large concen- but their fates would be quite different. tration of galaxies in the universe. Astronomers have understood the basic multiple universes of different types formed properties of galaxies — at least that in the past, and they coexist with the famil- they’re large congregations of stars, gas, iar cosmos we can see. and dust far beyond the Milky Way — since Even without other universes, astrono- the 1920s. But really understanding galax- mers know the universe is larger than what ies, the story of their formation in the early GALACTIC KEYHOLE. The Carina Nebula (NGC 3372) contains an we see with our telescopes. The view hori- universe and how they have evolved over extremely young, hot star (Eta Carinae) zon now spans over 13 billion light-years the past 13 billion years, is a tricky struggle that is unusual in the Milky Way. from Earth, and if you count the knowledge that challenges the best researchers. Forming only 3 million years ago, it that distant objects have expanded far We know our own galaxy, the Milky is as massive as 100 Suns. 2MASS beyond what we now see, the “currently” Way, best. Astronomers have had more existing horizon is at closer to 40 billion than a century to conduct astrophysical light-years away. research on countless objects within the raise others. With the Hubble Space Tele- Accepting an inflation-based universe of galaxy. Cosmologists have made great scope, for example, in its Ultra Deep Field, the size we see, Jaume Garriga of the strides over the past few decades in under- astronomers can see nearly back to the University of Barcelona and Alexander standing how the universe came to be. Yet universe’s infancy. Yet no one has seen the Vilenkin of Tufts University have proposed a how galaxies, including our Milky Way, so-called cosmic Dark Ages, the period cosmos peppered with numerous were born out of the early cosmos is just before stars and gal- MILKY DISK. The Milky “O-regions,” observable universes like ours. beginning to come into focus. axies existed. When Way’s edge appears ghostly Part of the idea goes that inflation, the Larger and larger telescopes being used they do, we may see in this infrared image hyperexpansion in the early universe, never over the past few years have helped the formation of the captured by the Infrared completely stopped. It stopped where we astronomers solve some questions and first stars and the first Astronomy Satellite (IRAS). are, producing our O-region, and many others. But in other areas of the universe at large, it continues. This creates a concept called eternal inflation — a universe unlike a simple sphere, instead rather like a sponge, pocked with holes that are bubble universes. How convincing is this to astrophysicists? Tegmark remains open. “As scientists,” he says, “we’re not testing the general idea of a multiverse. We’re testing inflation — a mathematical theory that predicts a multiverse and all kinds of other stuff.” Vilenkin looks ahead to an exciting future of learning more about multiverses. “By doing measurements in our own region,” he says, “we can test our predictions for what lies beyond.” NASA/GSFC

WWW.ASTRONOMY.COM 75 ESA/NASA/FELIX MIRABEL ESA/NASA/FELIX

SELF-PORTRAIT. An oblique view of the Milky Way, galaxies. (See "Did stars, in which huge clouds of material formed astronomers see vast numbers of protogal- created in an artist’s galaxies, or black holes giant, sheetlike structures, such as superclu- axies thought to have combined over time impression, shows a black- come first?" p. 42). sters of galaxies, that broke apart into into normal galaxies. If this is correct, the hole system, GRO J1655–40, For now, astronomers smaller components. Milky Way probably first formed when star streaking through space have to study the numer- For the time being, the momentum clusters came together to form the galaxy’s four times faster than the ous strange galaxies they appears to be with the first scenario core. As the gas clouds rotated faster, the stars in the galactic neighborhood around it. see in exposures like the because, in images of the early universe, galaxy flattened out into a disk. Ultra Deep Field for clues about how matter clumped in the cosmos’ early days. After the Big Bang, immense heat followed, so much that matter could not form. Only a soup of subatomic particles and radiation existed. When the universe cooled sufficiently, it became transparent to the radiation, and hydrogen atoms began to form. Ripples in the cosmic microwave background radiation — imaged by the COBE, WMAP, and Planck satellites — indicate the first seeds may have grown into black holes or galaxies. But exactly how this happened is unknown. Many astronomers believe structures in the universe grew from many tiny pieces, the so-called bottom-up scenario. In this view, small gas clouds, star clusters, and protogalaxies merged time and time again to form ever-larger structures. Other researchers believe in the top-down model,

GALACTIC CENTER. The Milky Way’s center, draped by thick dust clouds, stands out majestically in this infrared picture. The galaxy’s core lies 25,000 light-years away and contains a

supermassive black hole. 2MASS

76 50 GREATEST MYSTERIES How did the solar system form?

Astronomers and geologists have several techniques for dating Earth, and, therefore, the age of the solar system. From the radiometric dating of rocks, which measures the known decay rates of radioactive elements, had about two or three times the Sun’s mass. we know Earth and the solar system are The cloud’s gravitational collapse may have approximately 4.6 billion years old. The commenced by the flash of a nearby knowledge does not come from Earth rocks, supernova and the resulting pressure wave. however, the oldest of which are about 4 As the cloud fell in, several processes billion years old. (Earth rocks are constantly accelerated the collapse. The cloud’s involved in vigorous erosion by plate temperature rose, it began to rotate, and the tectonics and volcanism, making the oldest rotation settled material into a relatively flat

rocks on Earth extremely hard to find.) disk. The gravitational potential energy PYLE NASA/JPL-CALTECH/T. Instead, meteorites — chunks of increasingly transformed into heat, and the ALIEN SOLAR SYSTEM. asteroids, the Moon, and Mars — make density rose quickly. accumulated gaseous How would our solar dating the solar system more accurate. These Due to the conservation of angular clouds around their dense system appear from afar? bodies were left in more pristine form. The momentum, the flattening disk rotated more cores. Extensive sets of This artist’s view reveals oldest radiometrically dated thus far are 4.6 quickly as it decreased in size. As more and moons around the gas dust and debris left from billion years old, and so the solar system itself more pockets of gas and dust collided and giants grew as an analog a disk of material that must have formed near this time. stuck together, a protoplanetary disk formed, to the solar system’s formed planets when the While many ideas in astronomy have resembling a spinning pancake. planets themselves. Each solar system was young. changed radically over time, the notion of The greatest action took place at the disk’s gas giant helped sweep how the solar system formed has changed center. There, the infant protostar that the disk clean by its gravity and flung little in the last 250 years. In 1755, German became the Sun rapidly accumulated matter. many planetesimals into the distant Oort philosopher Immanuel Kant first proposed After some 50 million years, the protosun Cloud of comets. Then, a period of heavy the nebular hypothesis, in which a great gathered enough mass to commence nuclear bombardment of numerous objects cloud of material, the solar nebula, preceded fusion and it “turned on” as a star. impacting the inner planets began. A giant the Sun and planets. Out in the disk, meanwhile, matter body impacted Earth and created the Moon, In 1796, French astronomer Pierre Simon continued to clump together haphazardly, and other smaller bodies became satellites, Laplace put forth a similar theory. Although making planets, thousands of minor planets, as with the moons of Mars. he was unable to draw on supporting and smaller rocky balls. After the Sun’s The formation of the solar system offers evidence from observations of deep space, ignition, it produced a blazing solar wind that astronomers a rare model of an early Kant proposed the solar nebula was part of a blew minor debris and dust out of the disk. hypothesis being dead right. All the much larger cloud of gas and dust that fell in At this point, the gas giant planets stopped subsequent facts uncovered later in history by the weight of its own gravity and began to accreting into larger bodies. The gas fell right into place with Kant’s original idea. rotate. This gravitational contraction led to remaining in the disk, meanwhile, cooled and the formation of planets, both gaseous and condensed dust (silicates and metals) and ice rocky. Although the scope of knowledge from the cloud. Grains of dust and ice built about how this happened has grown other planetesimals, and more and more of considerably since Kant’s time, the basic idea them stuck together to build bigger bodies. is the same, and it has been borne out by Distant bodies in the outer solar system repeated bits of evidence. built up as ice worlds, and the gas giants Astronomers now know that when the solar system’s molecular cloud began to NEWBORN PLANET. collapse, it measured 100 astronomical units Researchers recently captured the first clear image across (one astronomical unit is the average of a planet forming (center distance between the Sun and Earth) and right) within the dusty disk of material that surrounds

the dwarf star PDS 70. NASA/ESA

WWW.ASTRONOMY.COM 77 What happens when galaxies collide? The vastness of space astounds us. Everywhere we look in the night sky, darkness abounds. The distances even to the nearest stars are so vast that caverns of emptiness exist between most objects in the cosmos. And Coma Berenices, NGC 4676A and B, the the voids between the majority of galaxies “Mice,” show a beautiful interaction, with a are millions of times larger. long arm shooting out one side of the merg- Despite the bigness of space, things do ing pair of galaxies. go bump in the night. Even on large cosmic In the violent world of stars, gas, and scales, in galaxy clusters and groups, whole interstellar dust, most galaxies that collide galaxies slam into each other in ornate merge fully into a single chaotic object. dances that last tens of millions of years. “Minor mergers, those between a large and Even with relatively small telescopes, a small galaxy,” says former Yale University examples of merging galaxies are visible to astronomer Daniel Christlein, “may well be a I)/J. TRAUGER TRAUGER (JPL) I)/J. backyard astronomers on Earth. In Canes normal part of galaxy life.” C Venatici, the Whirlpool Galaxy’s small com- In order for galaxies to merge, only two panion, NGC 5195, is a separate island uni- basic conditions need to be met: They must

verse passing it in the night. be relatively near each other, and they must (STS DONAHUE M. Centaurus A, the great high-energy gal- be traveling at relatively slow speeds with BLAZING COLLISION. In this axy in the southern sky, is the merged debris respect to each other. between two galax- Hubble Space Telescope from a head-on collision of If galaxies are too far apart, their gravita- ies of different sizes, image, a dusty galaxy GALAXY KISS. The two galaxies. NGC 4038 and tional attraction would be too weak to draw the larger object appears to be sliding on its Whirlpool Galaxy (M51) NGC 4039 in Corvus, known them together. If they are moving too normally draws the edge as it passes through and a little interloper, the larger, brighter galaxy NGC 5195 (top right), as the “Antennae” galaxies, quickly relative to each other, they might small one into a NGC 1275 in the Perseus are colliding as the provide a beautiful view of pass like ships in the night. long arc, extending galaxy cluster. smaller galaxy whizzes two highly disrupted galaxies So what happens when galaxies it like taffy being past the larger one. with an adjoined arm. In approach? If the interaction takes place pulled apart. The large object is relatively unaffected. More interesting, however, are mergers between galaxies of similar sizes. Then, the fireworks start in earnest. Enormous tails of matter can be ejected; huge regions of new star formation take place as gravity com- presses gas clouds; and chaotic disruption deep inside the galaxies can reorder the matter within them in wholesale fashion.

TEAM The basic elemental building block of

galaxies, hydrogen gas, is the fuel that gets twisted around in galaxy mergers. “Gas in the inner disk responds to the change in gravitational potential,” says astronomer I)/THE HUBBLE HERITAGE HUBBLE I)/THE C Daisuke Iono of the National Astronomical Observatory of Japan. “[It] loses energy and angular momentum, and flows toward the central regions of the galaxy. Numerical simulations predict radial gas inflow in the early

NASA/ESA/S. BECKWITH (STS NASA/ESA/S. stages, when the two galaxies collided for COSMIC TANGO. MEETING HEAD-ON. the first time, as well as during the final Although mergers occur over vastly long NGC 4676, a pair of After a collision and merger coalescence.” timescales, galaxies routinely come together galaxies known as the of two galaxies, a ring of In a study of galaxy mergers conducted and become one. In fact, even the Milky Way “Mice,” dances through blue star clusters encircles by Iono and his colleagues, the astronomers will collide and merge with another massive space as the two begin the yellowish nucleus of found the flow takes place rapidly and then galaxy — Andromeda. (See “Will the Milky merging into one giant what was a normal galaxy. slows down after more than half the gas Way merge with another galaxy?” p. 86.) One galaxy. STSCI/G. HARTIG/THE ACS The ring is larger than the SCIENCE TEAM/ESA Milky Way. reaches a galaxy’s center. The gas then forms day, this will rock our galaxy to its core. a ring around the galaxy’s center that can trigger the bursts of star formation often seen in galaxy interactions. Dormant central black holes in galaxies can also get an injection of gas that “wakes up” the black-hole engine. This produces violent activity observed in the cores of active galaxies undergoing mergers. The vastness of space also holds true on stellar scales. The distances between stars are vast enough that as galaxies merge, their stars rarely collide. The fact that galaxies are mostly empty space holds true even as mat- I) ter is compressed and galaxies, on the C whole, are rocked apart. If the Sun’s neighborhood of stars equaled the density of galaxies in the Local Group, our sky would be illuminated by a few stars brilliantly shining inside the orbit of Pluto! Inside clusters and groups, galaxies collide all the time. Mergers are commonplace. J. HIGDON (CORNELL UNIVERSITY)/I. JORDAN (STS JORDAN UNIVERSITY)/I. (CORNELL HIGDON J.

WWW.ASTRONOMY.COM 79 How do massive stars explode?

Just as people do, stars have a finite life. Born in dusty gas clouds of a galaxy’s spiral arms, stars fuse hydrogen into heavier elements during their energy-producing lifetimes.

For stars, mass translates into destiny. The behind a remnant that’s changing as smallest can glow like embers for trillions astronomers watch. A shock wave traveling of years. A middleweight star like our Sun at 10 million mph (16 million km/h) is plow- burns steadily for 10 billion years; eventu- ing into a ring of gas ejected before the star ally, it puffs off its outer layers as expand- died. This heats knots of gas in the ring to ing gaseous shells known as a planetary more than 18 million degrees Fahrenheit nebula. The most massive stars — furiously (10 million degrees Celsius) — so hot that hot, blue-white orbs — shine brightly for a the knots emit X-rays. few million years and end their lives in Stars fuse hydrogen and helium into spectacular explosions. heavier elements such as oxygen, carbon, Supernova explosions are rare, but and iron, but the remaining elements are incredible. In a mere second, a supernova forged in the heart of supernova explo- unleashes as much energy as the sum of all sions. The blasts cast these heavy elements other stars in the observable universe. For into the universe, enriching the galaxy for weeks, the shattered star may rival the light the next stellar generation. Atoms in our output of its entire host galaxy. bodies, including the iron in our blood and The brightest recent supernova the calcium in our teeth, were scattered occurred in 1987 in the Large Magellanic into space during the deaths of massive Cloud, a satellite galaxy of the Milky Way stars. As Carl Sagan was fond of saying, we about 160,000 light-years away. The explo- are made of star stuff. sion, known as Supernova 1987A, left Supernovae are not created equal, however. In cataloging these beasts, astronomers have found significant spectral

differences. The current classification MAUND R. NASA/ESA/JUSTYN scheme, devised in 1941 by American astronomer Rudolph Minkowski of supernovae, while those lacking both silicon California’s Mount Wilson Observatory, and helium were called type Ic supernovae. focuses on hydrogen, which is easy to trace. They reclassified all others as type Ia. A type I supernova does not show By the 1990s, astronomers had amassed broad absorption or emission lines corre- a wide enough set of observations to say sponding to hydrogen. If the supernova with some confidence what kinds of stars does show hydrogen either in absorption are exploding. or emission, astronomers class the explod- Generally, a type Ia supernova is the ing star as type II. result of a remnant white dwarf stealing Type I supernovae are remarkably con- material from a companion star. If the rate sistent; it’s easy to recognize them through- at which the stolen gas falls onto the dwarf LIGHT SHOW. The blast wave from Supernova 1987A in the Large Magellanic Cloud produced out the universe. However, later, some is slow enough, the material accumulates a ring of bright X-ray-glowing spots, imaged in peculiarities arose. Astronomers found that on the dwarf’s surface rather than immedi- 2003. The fast-moving shock wave slammed into some type I supernovae lack the presence ately fusing. The white dwarf slowly gains a ring-shaped cloud of nearby gas at more than of silicon, and others also appear deficient mass, and as it approaches a critical mass of 1 million mph. NASA/P. CHALLIS, R. KIRSHNER, AND B. SUGERMAN in helium. Scientists dubbed those lack- around 1.4 times the mass of the Sun, it ing silicon but containing helium type Ib explodes with fury.

80 50 GREATEST MYSTERIES For years, scientists believed all type Ia of the Sun. As a star this size exhausts its that shatters the star, DETONATION. A white explosions were produced when a white hydrogen fuel, it fuses ever-heavier ele- spewing gas light-years dwarf in a binary system dwarf collects gas from a larger ments, ultimately leaving a dense into space. However, the accumulates matter from its giant companion in this companion. But around iron core surrounded by finer physical details still illustration. The dwarf 2004, researchers found As a shells of silicon, oxygen, elude astronomers. accretes matter until its evidence that suggests carbon, and helium. Without supernovae, own carbon ignites. type Ia may be the white dwarf But iron fusion the heaviest elements The result is a type Ia result of this method requires more energy forged inside stars would supernova explosion. as well as another — approaches a than it produces, and never be scattered into when two white critical mass, this causes the star’s space. Type Ia supernovae show such lit- dwarfs collide in explo- iron core to collapse. tle variation in their energy outputs that sive fashion. it explodes At densities exceeding they’ve become important tools for explor- On the other hand, that of an atomic nucleus, ing the distant cosmos. Study of these Supernova 1987A was a with fury. the inner core stiffens, explosions has revealed the universe’s type II supernova. It began its rebounds, and expands out- expansion is accelerating, a finding that life as a brightly glowing blue- ward against the still-collapsing won the researchers who discovered it the white star more than eight times the mass star. This creates a violent shock wave 2011 Nobel Prize in Physics.

WWW.ASTRONOMY.COM 81 STELLAR CORPSE. After the Sun’s death, much of its matter will dissipate as a planetary nebula, a slowly expanding bubble of gas. The Helix Nebula in Aquarius represents one of the sky’s most beautiful such objects. NASA/ESA/C. R. O’DELL, M. MEIXNER,

AND P. MCCULLOUGH

What will happen to the Sun?

The Sun is an ordinary star. It bathes the solar system with At that point, its main sequence phase is over. In one of the most peculiar transfor- light and heat, making life possible on Earth. It’s as regular mations we know of, the Sun’s helium core, about the size of a giant planet, will contract as clockwork, and it sets our daily life cycles in conjunction and heat up. And in response, the Sun will with Earth’s spin. Little wonder ancient peo- At around 4.6 billion years old, the Sun is expand by 100 times. ples revered the Sun as a god. Yet the Sun roughly halfway through its life. Its adult- The swollen Sun will consume Mercury will not always be steady and reliable. hood, called the main sequence phase, lasts and Venus — and possibly Earth as well. Billions of years from now, the Sun’s finale around 10 billion years. When the Sun runs Astronomers watching from another solar will turn Earth — and the entire inner solar out of hydrogen fuel, it must generate system would classify this bloated version of system — into a very nasty place. energy by fusing heavier elements. our Sun as a red giant.

82 50 GREATEST MYSTERIES BIG STORM. An enormous, handle- shaped prominence juts from the Sun’s disk in this SOLAR PORTRAIT. March 19, 2011, Solar An image of the Sun from Dynamics Observatory the NASA/ESA Solar and image. Prominences are Heliospheric Observatory huge clouds of cool, dense (SOHO) reveals complex plasma suspended in the features visible uniquely Sun’s outermost at each wavelength. atmosphere. NASA/GSFC/SDO NASA/ESA/SOHO

With the Sun’s transformation into a red for tens or hundreds of millions of years. the Milky Way Galaxy. Among all of these giant comes new types of fusion reactions. “Our solar system will then harbor not one aged stars, might some have spawned new An outer shell will fuse hydrogen as the world with surface oceans,” says astronomer life on worlds that remained frozen during byproducts fall inward, further compress- S. Alan Stern of NASA's Science MIssion the stars’ main sequence phases? It’s possi- ing and heating the core. When the Directorate, “but hundreds — all the icy ble, say astronomers, but only time — and core reaches about 180 million moons of the gas giants, as well a whole lot more research — will tell. degrees Fahrenheit (100 as the icy dwarf planets of the million degrees Celsius), The Sun Kuiper Belt.” Pluto’s tempera- its helium will ignite and ture, says Stern, will resem- begin to fuse into car- will consume ble that of Miami Beach. bon and oxygen. A question Stern and The Sun will shrink Mercury and other planetary scientists somewhat, but after are asking: Will the outer a time, and for 100 Venus — and worlds with newfound million years, it will possibly Earth water evolve life in the again expand. It will then relatively brief intervals brighten significantly as it as well. they have to do so? The liquid plunges toward the end of its water on these worlds might helium-burning phase, when vigor- exist for only a few hundred million ous outflows called stellar winds will strip the years. After that, the Sun’s luminosity will dim CLOSE-UP. In 2017, researchers used the Sun’s outer layers. This will lead to the Sun’s to the point where these new water worlds ESO’s Very Large Telescope to capture this final phase — a cyclical shedding of gas into will permanently refreeze. Hydrocarbons unprecedented image showing the what astronomers call a planetary nebula. that could contribute to life’s emergence are convection bubbles present on the surface As the swollen Sun incinerates the solar already there, though. So, it’s possible that, in of a star other than the Sun. The star, Pi1 system’s inner planets, its outer icy worlds its death throes, our Sun may seed new life. Gruis, is an aging red giant that is about the same mass as the Sun, but 750 times will melt and transform into oases of water Some 10 billion red giants blaze today in the diameter. ESO

WWW.ASTRONOMY.COM 83 OLD WOUND. Evidence Did comets of ancient impacts still scar Earth’s surface. Quebec’s bring life Manicouagan Reservoir, some 60 miles (96 km) across, marks the to Earth? site of an ancient impact crater. Space shuttle Understanding how life began on Earth engages many astronauts took this orbital image in 1983. NASA fields of science. It’s a complex question involving related bits of physics, chemistry, astronomy, and biology. Things have come a long way since the fourth cen- structures spontaneously. Chemical com- system’s early days, Earth underwent a tury B.C., when Aristotle taught that life pounds called nucleotides, linked up during period of heavy bombardment. Hundreds of arose on its own from inanimate objects. chemical reactions, could have formed thousands of small bodies crashed into the Critical findings of the past seven decades self-replicating RNA molecules. planets and their moons. Vast numbers of all point to a picture of how complex, The ribosomes in cells, where RNA trans- comets and asteroids struck Earth, and they self-replicating cells could have commenced lates genetic code into proteins, could have left behind incredible amounts of water. In in Earth’s early days. In the formed out of precursor molecules and fact, impacts may have delivered much of BRINGER OF LIFE? In 1950s, chemists Harold Urey begun to synthesize proteins. And proteins the water now contained in Earth’s oceans. May 2004, Comet NEAT and Stanley Miller demon- themselves likely would have But comets likely left other (C/2001 Q4) blazed strated that small life-related become dominant life-re- important chemicals behind, across the night sky. molecules, such as amino lated large molecules, Complex too. Complex organic mole- In the early history of acids, could have formed leaving RNA and other cules like amino acids exist the solar system, comet under conditions likely pres- nucleic acids to carry on asteroids and comets collisions may have organic ent on the young Earth. genetic “blueprints” alike. The Murchison brought a great deal of water to Earth — and Phospholipids, compo- down subsequent molecules exist meteorite, a carbona- perhaps the building nents of biological mem- generations. ceous chondrite that fell blocks of life, too. branes, form cell-like Based on the reali- on asteroids in Australia in 1969, was zation that life could found to contain two get going on early Earth and comets types of amino acids. despite hostile conditions, alike. Impacting comets could also scientists have developed have deposited these protein several theories about how it first building blocks, which are crucial for emerged. One idea is the so-called RNA living organisms, into Earth’s oceans before world hypothesis. This suggests that RNA complex cells developed. molecules could have formed sponta- Experiments show complex organic mol- neously and catalyzed their own replication. ecules can survive the crash of a comet or So-called metabolism-first models come asteroid. How significant were cometary next. These ideas suggest a primitive contributions in seeding the young Earth metabolism arose and led to the develop- with organic chemicals? No one knows. ment of RNA. The so-called bubble theory SHOOTING GALLERY? suggests organic molecules concentrated In 1994, fragments of on ancient ocean shores just as bubbles Comet Shoemaker- concentrate in breaking waves. When Levy 9 slammed into enough prebiotic material came together to Jupiter. The impacts form the right chemical reactions, the created brownish, development of living systems began. Earth-sized dust Other models include physicist Thomas clouds in the planet’s atmosphere. Gold’s “deep-hot biosphere,” which posits NASA/R. EVANS, J. TRAUGER, that biomolecules formed several miles H. HAMMEL, AND THE HST COMET below Earth’s surface. SCIENCE TEAM Some suggest comets may have deliv-

WIYN/NOAO/AURA/NSF/T. A. RECTOR, Z. LEVAY, L. FRATTARE L. RECTOR, A. LEVAY, WIYN/NOAO/AURA/NSF/T. Z. ered Earth’s organic materials. In the solar NASA/ESA/ESO/FRÉDÉRIC COURBIN AND PIERRE MAGAIN PIERRE AND COURBIN NASA/ESA/ESO/FRÉDÉRIC

DEEP VIEW. Astronomers YOUNG GUNS. Quasars discovered water vapor in the are energetic galaxy cores. torus of gas and dust around The double quasar at left is the central black hole of 5 billion light-years away. quasar APM 08279+5255, The one at right, just shown here in this artist’s 1.5 billion light-years away, concept. NASA/ESA clearly lies within a galaxy.

were linked to the formation of galaxies How did was key to understanding how everything in the early universe formed. They’ve confirmed that quasars are energetic young quasars form? galaxies. And supermassive black holes appear to be typical in all but the smallest Quasars, short for quasi-stellar objects, were first identified galaxies — including our own. Many nearby galaxies have dormant in 1962 by Maarten Schmidt at the California Institute of black holes that are now noticeable only by their gravitational effects on nearby Technology. They appear as starlike points, but they lie at objects. In the Milky Way, the motions of enormous distances, which means they’re light. In recent years, Hubble has revealed stars near the galactic center reveal the emitting incredible amounts of energy. faint galactic forms around quasars. This presence of an invisible object several mil- By the 1980s, quasars’ prodigious X-ray confirms these distant beacons are, indeed, lion times the Sun’s mass. Studies of the and radio emissions led most astronomers young galactic cores. giant elliptical galaxy M87 reveal the pres- to believe these objects contain black holes Some quasars observed in the 1990s ence of a 5-billion-solar-mass object. in their centers. In the 1990s, scientists appeared “naked” — they seemed to have Since their discovery, quasars’ distant increasingly viewed quasars as young no host galaxy. But subsequent research beacons have shone a guiding light on galactic cores where gas, dust, and stars fed with better instruments revealed fuzz some of cosmology’s biggest questions. a central black hole. around many of these objects, too. They promise to give astronomers addi- One byproduct of the infalling matter is Another striking discovery came with tional answers, too — especially with the a high-energy jet erupting from near the Hubble observations showing more than help of Hubble and its descendants. black hole and hurling material into space. a third of quasar host galaxies Thanks to far-flung quasars, Quasars became part of a spectrum of ener- have a small companion. astronomers’ view of the getic galaxies called active galactic nuclei Perhaps encounters early universe will con- (AGN), which also includes Seyfert galaxies, between galaxies trig- tinue to improve. BL Lacertae objects, and radio galaxies. ger activity by send- Perhaps these diverse objects, astronomers ing extra fuel INSIDE VIEW. The thought, were similar creatures viewed toward the central nearby quasar 3C from different angles. black hole. 273 glows brightly Slowly, the question of what quasars For years, qua- enough to be are morphed into how quasars formed. sars provided the observed with backyard telescopes. Observational clues from the Hubble Space only way astrono- Astronomers imaged Telescope and other instruments able to mers could get a its host galaxy using observe the far reaches of the cosmos are glimpse of the early a special technique that giving astronomers leads. Some quasars cosmos. Astronomers’ suppresses the quasar’s seen at high resolution exhibit a “fuzz” of assumption that quasars brilliance. NASA/J. BAHCALL

WWW.ASTRONOMY.COM 85 INCOMING. Some 3 billion years from now, the Andromeda Galaxy (M31), the nearest large spiral to the Milky Way, will drift close enough to begin merging with our home galaxy. TONY AND DAPHNE HALLAS

Will the Milky Way merge

Galaxies in groups and clusters frequently pass close to each How do astronomers know about the Milky Way’s ancient galaxy mergers? The other. They sometimes collide and merge in spectacular evidence lies scattered in the record of globular star clusters and old stars orbiting fashion. The Milky Way is a dominant member of a tribe in the galaxy’s halo, which extends far of galaxies called the Local Group. galaxy has shredded and cannibalized as above and below its disk. Astronomers recognize over 50 members, many as 100 protogalaxies. Astronomer Dougal Mackey, a research most of which are quite small. Although But the galaxy’s merger-mania contin- fellow at the Australian National there’s a great deal of space between the ues. Astronomers see evidence that the Observatory, has studied these objects galaxies in the Local Group, the question Milky Way has gobbled up as many as a extensively. He finds that older clusters are arises: Will the Milky Way someday merge dozen small galaxies in the past few hun- remnants from the Milky Way’s formation. with one of its neighbors? dred million years. These mergers don’t Younger ones, however, may be imports In fact, our galaxy formed from past result in the massive bursts of star forma- carried into the Milky Way from dwarf galax- mergers, and it will be the scene of many tion observed when two large galaxies ies it has absorbed. Cataloging these to come. The likeliest scenario of galaxy come together. (See “What happens when objects, their positions, velocities, and formation and evolution suggests that gal- galaxies collide?” p. 78.) Instead, these nature can help astronomers reconstruct axies grew in the early universe by merging mergers occur as the Milky Way rips apart our galaxy’s merger history. with many small protogalaxies. Scientists and slowly absorbs small dwarf galaxies At least one merger is underway think that over the course of its lifetime, our that have strayed too close. now. The Milky Way is tearing apart and

86 50 GREATEST MYSTERIES CLOUDY LUNCH. Bursts of star formation explode in N11B, a cloud of gas and dust in the Large Magellanic Cloud (LMC), a satellite galaxy to the Milky Way. Although it lies about 160,000 light-years away, the LMC will be drawn in and “eaten” by the Milky Way over time. YOU-HUA CHU AND YÄLE NAZÉ with another galaxy?

absorbing a small dwarf galaxy called the Eating dwarf spheroidal galaxies is one intergalactic space, never to return. The first Sagittarius Dwarf Spheroidal. The galaxy thing. Major mergers are far more explosive, close pass will excite tidal tails and will also lies in Sagittarius, and a stream of stars, gas, far more traumatic. The Milky Way is due for probably result in a ‘bridge’ of stars between and debris over other parts of the sky show one several billion years from now with the Milky Way and Andromeda.” it is currently being shredded. Along with none other than the most famous galaxy in Mackey further explains: “After the first astronomer Gerry Gilmore, Mackey suspects the sky, the Andromeda Galaxy (M31). This close pass, the galaxies will move apart and the Milky Way has experienced at least seven largest member of the Local Group, a favor- then fall back together a second time, send- recent mergers with dwarf galaxies like this ite of backyard observers, is moving toward ing out more stellar streams and further one, and possibly many more. the Milky Way at about 250,000 mph disrupting each galaxy. This cycle will occur “A handful of mergers is not inconsistent (400,000 km/h). At this speed, 3 to 4 billion several additional times, producing ever with the remnants that we see,” Mackey years from now, the two giant spirals will more complex patterns of ejected stars.” says. “In my opinion, there’s no doubt more begin to merge. The result may resemble The interaction will hurl stars, dust clouds, remnants are to be discovered yet, probably the Antennae or Mice galaxies we now see gas, and planets into space, but each pass in the form of stellar streams like those locked in embrace. will bring the two distorted galaxies closer. observed from Sagittarius.” In fact, at the “It’s quite likely to be a rather messy affair,” “Eventually, the densest regions of the two beginning of 2018, astronomers announced says Mackey of the future Andromeda galaxies will merge, surrounded by a compli- the discovery of 11 new stellar streams encounter. “The gravitational forces between cated mixed halo of stars,” Mackey says. within the Milky Way, bringing the total the two galaxies will distort and disrupt them Those will be exciting times for inhabi- number of streams up to about 35. both, sending vast plumes of stars out into tants of the Milky Way.

WWW.ASTRONOMY.COM 87 How many brown dwarfs exist?

In 1975, Jill Tarter, then at NASA’s Ames Research Center, BROWN DWARF #1. In 1995, astronomers coined the term “brown dwarf.” Before that time, astronomers discovered their first brown dwarf, called hypothesized the existence of so-called black dwarfs, Gliese 229B, a small companion to the cool dark objects that were free-floating and extremely low-mass object. The star’s spectral red star Gliese 229, lacked the mass to “turn on” as stars. Back signature showed characteristics Becklin and located 19 light-years then, ideas about low-mass starlike objects Zuckerman expected from a red dwarf, but from Earth in the

suggested those with masses less than 9 with significant differences. Suddenly, astron- GOLIMOWSKI KULKARNI/D. NASA/S. constellation Lepus. percent of the Sun’s wouldn’t undergo nor- omers had a leading brown-dwarf candidate. mal stellar evolution. Instead, they would In 1995, astronomers found three more. but brown dwarfs cannot, so they show sig- become “stellar degenerates” heavily laden One, Gliese 229B, has a temperature and nificantly more lithium in their spectra. with dust and characterized by cool luminosity well below that of the But the galaxy’s true brown-dwarf popula- outer atmospheres. coolest star. It is now the tion remained ambiguous until the Two Various ideas about star Our galaxy prototype of a class of still Micron All-Sky Survey (2MASS) began in 1997. formation suggested there cooler objects called T Conducted at a wavelength of two microme- should be many brown may hold as dwarfs. With discover- ters, 2MASS revolutionized the search. Quickly dwarfs in the galaxy. But ies of many objects thereafter, J. Davy Kirkpatrick of the California being nearly dark, they’d many brown like Gliese 229B, one Institute of Technology and other astronomers be hard to find. The best of the most nagging found many objects like GD 165B. strategy would be to dwarfs as all mysteries found reso- The population of brown dwarfs bal- look in the infrared part lution in the 1990s. looned. In 2005, based on surveys using of the spectrum. other types Classifying such the Hubble Space Telescope, astronomers Lack of success in identi- of stars. objects is tricky, however. at Arizona State University estimated our fying brown dwarfs, which cer- When brown dwarfs are galaxy may hold as many brown dwarfs as tainly should have existed, stymied young, it’s extraordinarily difficult all other types of stars combined. Although astronomers. They turned to various meth- to distinguish them from very-low-mass a 2012 study suggested there may be only ods in vain attempts to find them. These stars. The best test is to measure the amount one brown dwarf for every six stars in the included careful imaging around main-se- of lithium in the object’s spectrum. Stars fuse Milky Way, astronomers more recently used quence stars and white dwarfs, hoping to lithium over their first 100 million years or so, the European Southern Observatory’s Very find companion brown dwarfs; surveys of Large Telescope to carry out an young open star clusters, in which brown extensive survey of the massive dwarfs could be floating freely; stellar radi- star-forming cluster RCW 38, al-velocity measurements; and multiwave- and their findings agree with length imaging surveys. five other surveys going back to The result? Nothing. 2006: Our galaxy contains about Then, in 1988, Eric Becklin and Ben 100 billion brown dwarfs. Zuckerman of the University of California, Los Angeles, identified a faint companion object WISE UP. Luhman 16, to a white-dwarf star designated GD 165. a brown-dwarf binary, is The white dwarf’s companion, GD 165B, at the center of the larger exhibited an unusually red spectrum. Astron- image, which was taken by omers classed it as the first L-type dwarf, an NASA’s Wide-field Infrared Survey Explorer (WISE). This is the closest star system discovered since 1916, and the third-closest to our Sun. NASA/JPL-CALTECH/GEMINI OBSERVATORY/AURA/NSF

88 50 GREATEST MYSTERIES WELL-KEPT SECRET. A planet-sized brown dwarf (upper right) designated Cha 110913–773444, depicted in this illustration, has less than a hundredth of the Sun’s mass. Yet it appears to harbor a planetary system. NASA/JPL-CALTECH BAD GALAXY DAY. Galaxy C153, illustrated here, is disintegrating as it plows through space. As the galaxy speeds through the gas in a large galaxy cluster, it loses much of its own gas. NASA/ADOLF SCHALLER What happens at the cores of galaxy clusters?

The centers of rich clusters of galaxies contain the densest Until recently, astronomers thought they understood how galaxy clusters form. As concentrations of matter in the universe. They’re also among matter collapses inward, pulled by gravity, the most violent places we know of. As time rolls on and groups of galaxies and clumps of matter crush together. The monsters of the scene, large galaxies swarm around meeker ones, throes of star formation. We live in a rela- the big galaxies, fall toward the center, mergers take place. Big galaxies grow larger tively quiet corner of the Milky Way Galaxy. where the most mass resides. by eating small ones. As this happens, By contrast, the centers of rich galaxy clus- Hot gas in the cluster core loses energy worlds are torn apart, stars shredded, and ters are the universe’s most chaotic loca- and cools by emitting X-rays. As the gas gas clouds compressed into reckless new tions, constantly bustling with activity. inside the cluster cools, it also contracts.

90 50 GREATEST MYSTERIES LOGJAM. The center of Astronomers dubbed this contracting gas a So what could be hiding in the cool gas? This was the largest single galaxy cluster Abell 1689 cooling flow. Up until 2006, the idea had Near the turn of the century, McNamara eruption astronomers appears chaotic, thanks been gospel since first proposed in 1977. uncovered a clue in the distant galaxy cluster have ever recorded. to a dense thicket of stars However, galaxy clusters have Hydra A, some 840 million light-years away. So, it appears the and dust shed by its multi- thrown astronomers a few surprises. Using NASA’s Chandra X-ray Observatory, he mysterious heat source tude of whirling galaxies. One of the theorists who came up with showed that powerful jets heated the sur- inside galaxy clusters are NASA/ESA/ACS SCIENCE TEAM the cooling-flow model, Paul Nulsen rounding gas to tens of millions of degrees. jets from active galax- of the Harvard-Smithsonian Center for In 2005, McNamara and collaborators ies powered by supermassive black holes. Astrophysics, says, “We now think it’s almost again used Chandra, this time to image But the mystery lingers — jet luminosities completely wrong.” Instead, researchers are X-ray emission from a very distant cluster, don’t exactly match the clusters’ X-ray cool- focusing on a model where more complex MS 0735.6+7421, which lies 2.6 billion light- ing rates. So, while the whole picture of flows, which contain copious amounts of years away in Camelopardalis. The team galaxy-cluster heating and cooling is becom- dark matter, drive the formation and evolu- found two gigantic cavities within the clus- ing clearer, it’s a long way from being solved. tion of galaxy clusters. ter. Each of these voids What astronomers do But gas cooling remains an important was roomy enough know is that massive feature of the latest models. The trouble is to house 600 Milky galaxy clusters remain that astronomers just don’t know what’s Ways. The cavities were among the cosmos’ most heating the gas. X-ray observations suggest expanding away from energetic spots. that a vast amount of cool gas should be a supermassive black produced in the cores of galaxy clusters hole. The team calcu- GRAVITATIONAL WALTZ. each year. This should lead to massive lated that the energy Engaging in a dance of episodes of star formation. “But when we required to displace destruction, galaxies in measured rates of star formation,” says Brian this gas was some 1061 the group called Seyfert’s McNamara, an astronomer at the University ergs — equivalent to Sextet flirt with mergers. NASA/J. ENGLISH, S. HUNSBERGER, of Waterloo, “we were getting 10 to 20 solar the energy released by S. ZONAK, J. CHAARLTON, S. GALLAGHER, masses a year or less.” 10 billion supernovae. AND L. FRATTARE

WWW.ASTRONOMY.COM 91 Is Jupiter a failed star?

The brilliant planet Jupiter dazzles anyone with a clear sky. Roman observers named Jupiter after the patron deity of the Roman state, following Greek mythology, which associated it with the supreme god, Zeus. has led to many spacecraft missions, begin- But when Galileo turned his telescope sky- ning with Pioneer 10’s 1973 flyby. A year ward in 1610, Jupiter took on new signifi- later, Pioneer 11 passed the great planet. cance. Galileo discovered the planet’s four But sophisticated, close-up study of the principal moons — and witnessed the first giant planet began with the twin flybys of clear observation of celestial motions cen- NASA’s Voyagers in 1979. tered on a body other than Earth. Voyager 1 and 2 increased our jovian Astronomers recognized Jupiter as the knowledge immensely. They mapped the largest planet in the solar system long planet’s moons, took detailed images of before any spacecraft provided detailed Jupiter’s complex atmosphere, and even exploration. The planet’s mammoth size discovered a faint set of rings. — 88,846 miles (142,984 kilometers) at the NASA’s Galileo mission, which entered JOVIAN TURBULENCE. True-color (left) and false- equator — holds 2.5 times the mass of all jovian orbit in 1995, gave scientists another color mosaics show how eastward and westward the other planets combined. This makes windfall. Even as it approached Jupiter in bands of air between the planet’s equator and polar Jupiter the most dominant body in the solar 1994, Galileo witnessed one of the greatest regions control Jupiter’s atmosphere. NASA/JPL system after the Sun. The planet’s volume events in solar system history — Comet is so great that over 1,300 Earths could fit Shoemaker-Levy 9’s spectacular crash mission was recently extended and is now neatly inside it. into the giant planet. Galileo sent a probe expected to run through July 2021. Jupiter is a magnificent example of a gas plummeting into Jupiter’s atmosphere. The Jupiter’s size and compositional similar- giant planet. It likely is composed of a small probe sampled the atmosphere directly and ity to brown dwarfs and small stars have led rocky core enclosed in a shell of metallic returned much information before immense some to label it a “failed star.” Had the planet hydrogen, which is surrounded by liquid pressure deep below Jupiter’s clouds formed with more mass, they claim, Jupiter hydrogen, which, in turn, is blanketed by crushed it. In 2003, at mission end, Galileo would have ignited nuclear fusion and the hydrogen gas. By count of atoms, the atmo- itself met the same fate. solar system would have been a double-star sphere is roughly 90 percent hydrogen and In July 2016, NASA’s Juno spacecraft system. Life might never have evolved on 10 percent helium, with a smattering of entered orbit around Jupiter to begin a new Earth because the temperature would have other trace elements. round of scientific observations. With Juno, been too high and its atmospheric charac- Jupiter’s dominance of the solar system researchers have precisely mapped Jupiter’s teristics all wrong. gravitational and However, although Jupiter is large for a magnetic fields, planet, it would need to be about 75 times learned a great deal its current mass to ignite nuclear fusion in its about the planet’s core and become a star. Astronomers have cluster of polar found other stars orbited by planets with cyclones, and dis- masses far greater than Jupiter’s. covered that the gas What about substellar brown dwarfs? giant surprisingly Our largest planet still doesn’t come close to rotates like a solid these “almost stars.” Astronomers define body just below brown dwarfs as bodies with at least 13 its chaotic cloud times Jupiter’s mass. At this point, a hydro- tops. Although gen isotope called deuterium can undergo SOUTHERN STORMS. This mosaic of infrared images taken by the Juno Juno was initially fusion early in a brown dwarf’s life. spacecraft shows Jupiter’s south pole, where five powerful cyclones encircle a sixth. Yellow areas are where the cloud cover is thin, while red areas slated to cease sci- So, while Jupiter is a planetary giant, its correspond to thicker clouds. NASA/JPL/SWRI entific operations in mass falls far short of the mark for consider- February 2018, the ing it a failed star.

92 50 GREATEST MYSTERIES JUPITER UP CLOSE. NASA’s Juno spacecraft snapped this enhanced- color image of Jupiter’s south polar region when it flew just 30,000 miles (47,000 km) above the planet’s cloud tops in May 2017. NASA/JPL/SWRI/ G. EICHSTÄDT/S. DORAN Leo A Distance: 2.57 million light-years Diameter: 4,000 light-years Top view 180° Side view

The Local Sextans A Sextans B

Group Sextans B Pinwheel (M33) NGC 3109 Sextans A Andromeda MOST OF THE nearby galaxies Antlia Dwarf that make up the Local Group (M31) NGC 3109 IC 1613 are dwarfs that cluster around Milky Way Cetus Pegasus Dwarf 270° the two large spirals: M31 and the Dwarf 270° 90° Milky Way. This illustration gives WLM GR 8 distances for established UKS 2323-326 NGC 55 members, and sizes for those NGC 6822 at least 4,000 light-years across. Tucana Dwarf Aquarius Dwarf Galaxy distances are shown to scale, but sizes are enlarged three times. ASTRONOMY: ROEN KELLY Leo II IC 5152 Sagittarius Dwarf Irregular Distance: 690,000 light-years Leo I Distance: 820,000 light-years Diameter: 2,000 light-years 1 million light-years 0° 1 million light-years

180$

Ursa Minor Dwarf Milky Way Distance: 225,000 light-years Sextans Dwarf Distance: Distance: 280,000 light-years 26,000 light-years Draco Dwarf Diameter: 4,000 light-years Diameter: Distance: 260,000 light-years 120,000 light-years 270° 90$

Carina Dwarf Sagittarius Dwarf Spheroidal Distance: 330,000 light-years Distance: 65,000 light-years Diameter: 10,000 light-years Large Magellanic Cloud Distance: 163,000 light-years Sculptor Dwarf Diameter: 14,000 light-years Distance: 290,000 light-years Small Magellanic Cloud NGC 185 Distance: 197,000 light-years Fornax Dwarf Distance: 2.05 million light-years Diameter: 7,000 light-years Distance: 490,000 light-years Diameter: 11,000 light-years Diameter: 6,000 light-years

NGC 147 Distance: 2.53 million light-years 0$ NGC 6822 Diameter: 12,000 light-years Distance: 1.63 million light-years And II Diameter: 7,000 light-years Distance: 2.22 million light-years Andromeda Galaxy (M31) Distance: 2.59 million light-years Diameter: 220,000 light-years

Pinwheel Galaxy (M33) Distance: 2.64 million light-years Diameter: 60,000 light-years Phoenix Dwarf Distance: 1.44 million light-years

Sagittarius Dwarf Irregular Distance: 3.39 million light-years LGS 3 Distance: 50,000 light-years 2.51 million Aquaris Dwarf light-years Distance: 3.49 million light-years GR 8 How many Leo A galaxies are in Leo II Leo I Milky Way 90° our Local Group?

NGC 6822 Andromeda (M31) Pinwheel (M33) Our Milky Way Galaxy wheels within the Local Group of IC 1613 Tucana Pegasus Dwarf Dwarf Cetus Dwarf galaxies in a relatively quiet corner of the cosmos. The Virgo WLM cluster of galaxies, some 60 million light-years away, plays city center to our boondocks. The Virgo the first clumps of matter accreted into pro- UKS 2323-326 Cluster holds as many as 2,000 “island uni- togalaxies. As these clumps compressed, IC 5152 NGC 55 verses.” Our little Local Group, by contrast, stars formed and ignited their nuclear-fusion contains around 54 galaxies, most of them fires. When the first stars and clusters unimpressive dwarfs. emerged from the billion-year-long Dark Many, perhaps most, galaxies exist in Ages that followed the Big Bang, the Local such small groups scattered throughout the Group stretched only 600,000 light-years cosmos. The Local Group spans nearly 10 across. Being so close together, galaxies million light-years, and only three large gal- merged more often back then. Such mergers axies lie within it. The most significant, the may have built the Milky Way out of 100 or Andromeda Galaxy (M31), is an expansive more protogalaxies. spiral whose magnificent disk This process continues: Our gal- extends roughly 220,000 light- axy is currently shredding and IC 10 years. Next in size is the Our devouring the Sagittarius Distance: 2.15 million light-years Milky Way, with a disk Dwarf Spheroidal Galaxy, Diameter: 8,000 light-years spanning approximately Local Group and it eventually will 120,000 light-years. The absorb both Magellanic And VII third spiral in the group, contains around Clouds. Furthermore, Distance: 2.49 million light-years the Pinwheel Galaxy some 4 billion years (M33), measures about 54 galaxies, from now, the 60,000 light-years across. most of them Andromeda Galaxy and And V The Local Group’s the Milky Way will collide Distance: 2.52 million light-years remaining members dwarfs. in a clash of fireworks that include irregulars, ellipticals, ultimately will create a single and dwarf galaxies of various messy galaxy that eventually will NGC 205 Distance: 2.69 million shapes, most of which are quite small. The settle down as a giant elliptical. light-years two big guys on the block, Andromeda and Observing the Local Group’s galaxies Diameter: 17,000 light-years the Milky Way, each have an entourage of gives astronomers a microcosm — a labora- satellite galaxies. Andromeda hosts ellipti- tory close at hand that represents the uni- M32 Distance: 2.49 million light-years cals M32 and NGC 205, dwarf elliptical NGC verse at large. A substance astronomers call Diameter: 6,500 light-years 147, and a slew of dwarf spheroidal galaxies dark matter accounts for 26 percent of the including NGC 185 and Andromeda I, II, III, V, universe’s content, but, as yet, no one knows And I Distance: 2.43 million light-years VI, VII, IX, and X — just to name a few. what the stuff is. Using a technique called The Milky Way holds the Large and Small gravitational lensing, astronomers have And III Magellanic Clouds, both irregulars, plus scoured the Milky Way’s halo and ruled out Distance: 2.44 million light-years many dwarf galaxies. Prominent ones lie in several suspected candidates. Boötes, Canis Major, Carina, Draco, Fornax, Likewise, astronomers use the nearest And IV Sagittarius, Sculptor, Sextans, Ursa Major, galaxies to study where black holes form. Distance: 2.55 million light-years and Ursa Minor. What they’ve found in our galactic neigh- The galaxies of the Local Group likely borhood matches up well with observations originated over 13 billion years ago when in more distant galaxies.

WWW.ASTRONOMY.COM 95 Do neutrinos hold secrets to the cosmos?

For a quarter of a century, Wolfgang Pauli’s prediction electron, muon, and tau. But these elusive particles don’t interact much with other remained an educated guess. In 1930, the Austrian physicist matter. Neutrinos can pass almost unfet- predicted the existence of a ghostly new subatomic particle. tered through us, Earth, the Sun, or even the superdense heart of an exploding star. While After observing beta decay in a radioactive nuclei and caused particle transformations. they exist in tremendous numbers, the chal- nucleus, Pauli noted that an undiscovered He called Pauli’s ghostly particle the neu- lenge of neutrinos is detecting them. particle must exist to explain the resulting trino, Italian for “little neutral one.” In the 1950s, physicists Fred Reines and spectrum. During beta decay, a proton German physicist Hans Bethe, mean- Clyde Cowan began a series of experiments becomes a neutron by emitting a positron. while, was attacking the question of how to try. By the mid-1950s, their Project Polter- But Pauli argued that the nucleus also emit- stars shine. While investigating this question, geist showed that it could be done. Their ted an unknown electrically neutral particle. Bethe realized that neutrinos played a key experiment picked up neutrinos by using a He thought this hypothetical particle had role. Fusion reactions in the Sun’s core create nuclear reactor as a source and a water tank less than 1 percent of a proton’s mass. a torrent of neutrinos, a fraction of which as a detector, both sunk deep in a mine. During the 1930s, Italian physicist Enrico pass straight through Earth eight minutes Although Bethe outlined the processes Fermi investigated the problem and com- later. These evanescent particles carry with by which stars obtain energy through hydro- pleted the work Pauli began. Fermi thought them a record of what happens inside a star. gen fusion, many neutrino mysteries remain. the weak nuclear force destabilized atomic Neutrinos come in three types — For a long time, astronomers have known

BOTTOM LINES. Like strands of high-tech kelp, the Antares detector array rises from the seafloor off Marseille, France. Precisely timing a neutrino’s light flash is the key for tracking a particle’s path through the array. F. MONTANET/ANTARES

96 50 GREATEST MYSTERIES Matter’s fundamentals: The particle zoo Leptons Quarks

e Ne d u

Electron Electron neutrino Down quark Up quark

M NM s c

Muon Muon neutrino Strange quark Charm quark

T NT b t

Tau particle Tau neutrino Bottom quark Top quark

Force carriers W Z g G

W boson Z boson Gluon Photon

COSMIC BASICS. Normal matter comprises electrons and neutrinos, plus particles built : ROEN KELLY from combinations of three quarks, like protons and neutrons. Exchanging force-carrying entities, like photons and gluons, gives rise to electromagnetism and nuclear forces. ASTRONOMY

that the universe contains much more mat- Independent of neutrinos’ possible role ter than the bright stuff we can see. They as dark matter, the hard-to-catch particles know this because they track galaxies mov- may also help astronomers decipher how ing in response to the gravitational pull of matter itself came to be. When the Big Bang large amounts of material that nei- occurred, matter and antimatter ther emits nor blocks light — should have been created in dark matter. Neutrinos equal amounts. And when Could untold varieties matter and antimatter of neutrinos account for don’t have meet, they annihilate much — or even all — each other. If the of the dark matter enough mass to amounts had been astronomers believe is equal, then only radia- out there? Unfortu- account for tion would have filled nately, scientists now the universe. think the answer is no. all the dark Why is there so much SUNKEN SENSORS. Recent research suggests matter in the cosmos? matter. Japan’s Super that while neutrinos do have Maybe neutrinos played a key Kamiokande neutrino mass, they do not have nearly role in the universe’s early asym- detector is a cylinder enough to account for all the dark matter in metry. If so, we owe our existence to them. 130 feet wide and high. the cosmos. Furthermore, neutrinos move at Neutrinos surface in other cosmic mys- Light sensors lining nearly the speed of light, meaning they teries, too. So, expect to hear a lot more the water-filled tank won’t easily clump together like dark matter about these strange particles as scientists hunt for neutrinos.

is observed doing in galaxies. continue to probe matter’s secrets. BARSZCZAK TOKYO/TOMASZ OF KAMIOKA OBSERVATORY/UNIVERSITY

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