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CONTENTS KELLY ROEN : ASTRONOMY FEATURES ON THE COVER 22 33 50 The past 45 years have brought COVER STORY Inflation leaves The weird mystery incredible explorations and discoveries in astronomy, The 10 its mark of dark energy planetary science, and cosmology. biggest things YEARS Alan Guth’s remarkable hough it dominates the OF ASTRONOMY theory provides a master universe, dark energy is the in astronomy MAGAZINE For nearly half a key to the universe we see biggest discovery we don’t century, the world’s today. MARA JOHNSONGROH understand. LIZ KRUESI leading title on astronomy and 53 space has reported mesmerizing 36 COLUMNS science. DAVID J. EICHER Sky This Month Exoplanets burst Prime time for the Perseids. onto the scene Strange Universe 12 BOB BERMAN 24 MARTIN RATCLIFFE AND Since their discovery in The Red Planet revealed ALISTER LING 1992, planets outside our solar For Your Consideration 16 More than a dozen spacecrat system have been found around JEFF HESTER have shown Mars to be a 38 thousands of stars in the galaxy. KOREY HAYNES Observing Basics 18 wonderfully diverse world StarDome and GLENN CHAPLE with hidden stores of water. Path of the Planets 56 JOHN WENZ RICHARD TALCOTT; Secret Sky 64 ILLUSTRATIONS BY ROEN KELLY Pluto finds its place STEPHEN JAMES O’MEARA 27 No longer a lone world at the Binocular Universe 66 edge of the solar system, Pluto Shining light 44 PHIL HARRINGTON on black holes is the brightest and best-studied Decoding the cosmic member of a vast throng of It took 200 years to ind the irst microwave background QUANTUM GRAVITY black hole. Now we know they’re Kuiper Belt objects. JOHN WENZ he Big Bang let behind a unique Snapshot 9 everywhere. FRANCIS REDDY signature on the sky. Probes 59 Astro News 10 30 such as COBE, WMAP, and Glimpsing Planck taught us how to read it. gravitational waves Voyager’s Grand Tour LIZ KRUESI IN EVERY ISSUE he twin probes explored more he screams of colliding black planets, discovered more moons, 47 holes allow astronomers to study From the Editor 6 and ofered more breaking the universe in a whole new way. Astro Letters 8 Hubble’s ROBERT NAEYE news than any other spacecrat. New Products 62 MICHAEL E. BAKICH astounding legacy NASA’s inest instrument has cap- 68 Advertiser Index 65 tured more images and generated Ask Astro Reader Gallery 70 more research than any telescope he expanding universe. in history. MICHAEL E. BAKICH Breakthrough 74

Astronomy (ISSN 0091-6358, USPS 531-350) ONLINE is published monthly by Kalmbach Media Co., 21027 Crossroads Circle, P. O. Box 1612, Waukesha, WI 53187–1612. Periodicals post- FAVORITES age paid at Waukesha, WI, and additional offices. POSTMASTER: Send address changes to Astronomy, P.O. Box 62320, Tampa, Fla. 33662-2320. Go to www.Astronomy.com Dave’s News Picture Venus Globe Canada Publication Mail Agreement #40010760. for info on the biggest news and Universe The latest of the Day Get the observing events, stunning photos, The inside updates from Gorgeous hottest globe scoop from the science photos from around. informative videos, and more. the editor. and the hobby. our readers.

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PRESENTS

WWW.ASTRONOMY.COM 5 FROM THE EDITOR BY DAVID J. EICHER Editor David J. Eicher Art Director LuAnn Williams Belter EDITORIAL Senior Editors Michael E. Bakich, Richard Talcott Production Editor Elisa R. Neckar Associate Editors Alison Klesman, Jake Parks Copy Editor Dave Lee Astronomy Editorial Assistant Amber Jorgenson ART Graphic Designer Kelly Katlaps Illustrator Roen Kelly Production Specialist Jodi Jeranek marks 45 CONTRIBUTING EDITORS Bob Berman, Adam Block, Glenn F. Chaple, Jr., Martin George, Tony Hallas, Phil Harrington, Korey Haynes, Jeff Hester, Liz Kruesi, Ray Jayawardhana, Alister Ling, Steve Nadis, Stephen James O’Meara, Tom Polakis, Martin Ratcliffe, Mike D. years Reynolds, Sheldon Reynolds, Erika Rix, Raymond Shubinski SCIENCE GROUP Executive Editor Becky Lang Design Director Dan Bishop ith this issue, the scientific results, in EDITORIAL ADVISORY BOARD Buzz Aldrin, Marcia Bartusiak, Timothy Ferris, Alex Filippenko, Astronomy detail, that streamed back Adam Frank, John S. Gallagher lll, Daniel W. E. Green, William K. Hartmann, Paul Hodge, Edward Kolb, Stephen P. Maran, Brian magazine cel- from the New Horizons May, S. Alan Stern, James Trefil Webrates its 45th mission to Pluto and its anniversary. family of oddball moons. Kalmbach Media A young college graduate If you’re more about CEO Dan Hickey Vice President, Content Stephen C. George named Stephen Walther the nuts and bolts of Vice President, Consumer Marketing Nicole McGuire took his passion for astron- astronomy in our gal- Art and Production Manager Michael Soliday First issue, August 1973 Advertising Sales Director Scott Redmond omy and, in the summer axy and beyond, check out Circulation Director Liz Runyon New Business Manager Cathy Daniels of 1973, founded a popular the explosive history of exo- Retention Manager Kathy Steele magazine on the subject. planet discoveries. 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Subscription rate: single copy: $5.99; U.S.: 1 year (12 issues) $42.95; 2 years (24 issues) $79.95; 3 years (36 issues) $114.95. Canadian: with the search for water on If cosmology is your Mara Johnson-Groh and Add $12.00 postage per year. Canadian price includes GST, payable in U.S. funds. All other international subscriptions: Add $16.00 postage per Mars. You can relive the game, I invite you to read current editor Michael year, payable in U.S. funds, drawn on a U.S. bank. BN 12271 3209 RT. Not responsible for unsolicited materials. excitement and break- about the latest thoughts on Bakich. through discoveries of the inflation theory, an idea that My hat’s off to them for a Voyager “Grand Tour,” neatly explains a lot about job well done. which painted the initial the universe we now Follow Astronomy detailed picture of the outer observe. You’ll also want to Yours truly, solar system. You can read know the details about the www.twitter.com/ www.facebook.com/ plus.google.com/ AstronomyMag AstronomyMagazine +astronomymagazine Follow the Dave’s Universe blog: www.Astronomy.com/davesuniverse David J. Eicher Follow Dave Eicher on Twitter: @deicherstar Editor

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WWW.ASTRONOMY.COM 7 ASTROLETTERS

Memories of McDonald Physics, along with Martin Ryle. Bell I really enjoyed the article “In pursuit Burnell did not. of exoplanets,” by Robert Reeves, in I do want to note that both Narkovic your March issue. The photos of the and Bell Burnell rightly appear as co- Harlan J. Smith Telescope at McDonald authors on the discovery papers associated Observatory brought back many happy with their finds. Academically speaking, memories of visits to Mount Locke. authorship on a paper is one identifiable I grew up about 170 miles away in and acceptable way of receiving credit for Odessa, Texas, and McDonald (pictured) one’s work. However, not all advisers was a frequent weekend destination for credit student contributions in this way, our family. I remember going when their and academic theft does occur. I think it’s only telescope was the 82-inch (now important to recognize this as a problem renamed the Otto Struve Telescope), and and hold advisers accountable to make

when the Harlan J. Smith Telescope was REEVES ROBERT sure credit is given where it is due, par- constructed in the early 1960s. The obser- ticularly in academic publications. vatory has become a great destination, Recognition is due — Alison Klesman, Associate Editor and I highly recommend it to your read- I wish to raise a complaint about the ers if they ever find themselves in West February article, “Cosmic firecrackers,” Texas. — Richard Taylor, Odessa, TX about fast radio bursts. The first para- Taking note graph talks about Duncan Lorimer of Thank you for the Jeff Hester article, “The West Virginia University and his student, mind’s siren call,” in the March issue. A trip through time David Narkovic. It implicitly states that It’s a good reminder of the need to think I loved Bob Berman’s March column Narkovic was the person who, comb- about thinking. — John Wallbank, Howell, MI about memorable sky sightings. My ing through pulsar survey data, found a own list includes the 1976 Mars twilight “signal unlike anything anyone had seen occultation of Epsilon Geminorum; or predicted before.” Yet Lorimer is the A scientist’s responsibility the spectacular aurora displays of 1989; person who, in the next paragraph, “had The April 2018 edition of Astronomy was the long-tailed and colorful Comet discovered the first fast radio burst.” unusually interesting and informative. Hyakutake in 1996 (easily the most If I’m reading this correctly, Narkovic Your publication constantly challenges exciting comet I’ve ever seen); the thrill- is the one who actually discovered it, what we think we know about the uni- ing 1999 Leonid shower; the once-in- yet the article goes on to hail Lorimer verse and presents new and fascinating my-lifetime 2012 transit of Venus; the as the discoverer and the one who is ideas for our consideration. Kudos and superb 1998 solar eclipse, seen from a remembered. The lack of recognition congratulations on a brilliant publication. sailboat in the Caribbean; and the 2017 for Narkovic isn’t right. — Debbie Butler, I firmly believe that our knowledge eclipse in Kentucky, memorable because Vashon, WA base in science should contribute to we took the whole family to see it. greater understanding of our world, the — John Bevan, Alford, MA Astronomy responds universe, and the realities we face day to You concern is valid — Narkovic did day. We should be careful in changing our make the initial find, but it appears that ideas and theories into facts until such Locating the zenith Lorimer generally receives the credit. time (if ever) they prove to be a basic I enjoyed Stephen James O’Meara’s col- Narkovic was an undergraduate at the principle. umn in the March 2018 issue. I’ve found time. While it’s true he spotted something I believe that there’s a special responsi- that an effective way to practice locat- unusual in the data, he had neither the bility upon those who claim to be scien- ing the zenith is to simply use eyedrops. knowledge nor the experience to deter- tists. The search for scientific facts and Holding the eyedropper several inches mine exactly what it was — or whether it principles should be unbridled, and the above the eye and hitting the pupil forces was real at all. This is likely why he use of those facts and principles should accurate identification of what’s overhead. brought it to Lorimer, his supervisor. bear the upmost scrutiny. Then, when under the night sky, imagine What happened next is pretty common Science is bounding ahead in areas that you’re holding a virtual eyedropper. in academia. When students discover such as information sharing, cosmology, — John Westfall, Antioch, CA something, it is often their advisers who basic theories of physics, medicine, and get the public credit, perhaps for recog- genetics. So fast, in fact, it challenges us to We welcome your comments at nizing it as worthwhile. For example, use this new information in productive Astronomy Letters, P. O. Box 1612, Jocelyn Bell Burnell discovered the radio ways and to limit the misuse and destruc- Waukesha, WI 53187; or email to letters@ blips of the first pulsar while working tion of information in front of us. Let us astronomy.com. Please include your under Antony Hewish. She, too, took her hope that we awaken from our current name, city, state, and country. Letters find to her adviser. Even though Bell administrative and government stupor, may be edited for space and clarity. Burnell today is credited with the discov- and let us not forget the responsibility that ery, Hewish won the Nobel Prize in all scientists bear. — Donald Craig, Indianapolis

8 ASTRONOMY • AUGUST 2018 QUANTUM GRAVITY QGEVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH . . .

FOR THE RECORD IN A SPIN PUFF UP HOT BYTES >> The International Astronomers have A new study shows that Astronomical Union discovered that “solar galaxies like the Milky TRENDING released a list of the tornadoes” aren’t Way grow larger with TO THE TOP first officially named actually rotating, but age, as newly formed features on Pluto’s appear to move due to stars on their outskirts largest moon, Charon. projection effects. expand their borders.

SNAPSHOT Stellar hide- and-seek

Astronomers use multiple CRAWFORD KEN ASA/SDO/GSFC; observatories to pinpoint a strange source of X-rays.

New data from the MUSE instrument on the European Southern Observatory’s Very Large Telescope (red), when combined with images from NASA’s Hubble Space Telescope (green) and Chandra X-ray Observatory (blue and purple), have helped researchers define a mysterious X-ray source. The distinct ring of dust (red) discovered with MUSE appears inside a larger supernova remnant 200,000 light-years away in the Small Magellanic Cloud, and perfectly circles an observed but previously undefined X-ray source dubbed p1. Combining these clues, astronomers determined that the gaseous ring sur- I/AURA)/F. VOGT ET AL.; TOP FROM LEFT: NASA/JOHNS HOPKINS UNIVERSITY APPLIED PHYSICS LABORATORY/SOUTHWEST RESEARCH INSTITUTE; N INSTITUTE; RESEARCH LABORATORY/SOUTHWEST UNIVERSITY PHYSICS HOPKINS APPLIED NASA/JOHNS LEFT: FROM TOP ET AL.; VOGT I/AURA)/F. rounds an isolated neutron star C with a low magnetic field, the first of its kind identified outside the Milky Way. Neutron stars are created when a massive star dies. Their dim X-ray luminosity makes these stars difficult to spot, but when literally surrounded by light, it’s tough for ESO/NASA, ESA AND THE HUBBLE HERITAGE TEAM (STS TEAM HERITAGE HUBBLE THE AND ESA ESO/NASA, even these elusive objects to XRAY FILM. New observations of the Small Magellanic Cloud with the MUSE instrument allowed astronomers to locate the hide. — Amber Jorgenson corpse of a star that went supernova 2,000 years ago.

WWW.ASTRONOMY.COM 9 INTERSTELLAR SYNTHESIS. Low-energy electrons, which are prevalent in space, can induce the formation of glycine, ASTRONEWS a relatively complex amino acid essential for life, according to scientists at Université de Sherbrooke in Quebec, Canada.

DETACHABLE DARK MATTER? times slower than they would be if the gal- NGC 1052-DF2 axy contained a typical amount of dark mat- is a large, diffuse ter. The researchers used the motions of galaxy 65 million these globular clusters to determine the gal- light-years away in the constellation axy’s overall mass, ultimately finding that “if Cetus. The Hubble there is any dark matter at all, it’s very little,” Space Telescope’s said van Dokkum. Advanced Camera Though the researchers are not entirely for Surveys took this image of the sure how DF2 formed without dark matter, galaxy, which is one possibility is that gas initially flowing thought to contain toward NGC 1052 broke away due to shocks a negligible amount of dark induced by jets from the elliptical’s central matter. NASA/ESA/P. VAN black hole, reforming into DF2. But no mat- DOKKUM (YALE UNIVERSITY) ter how the strange galaxy formed, the team’s findings demonstrate that dark matter is indeed a physical building block that can be separated from normal (baryonic) matter. A GALAXY WITHOUT DARK MATTER And, as a result, the findings cast doubt on ark matter is a gravitationally galaxy without it is unexpected. It chal- some alternative theories to dark matter, inferred, but optically elusive, type of lenges the standard ideas of how we think such as modified Newtonian dynamics. Dmatter believed to form the founda- galaxies work, and it shows that dark mat- Even though there have been several tion upon which all galaxies are built. ter is real: It has its own separate existence recent challenges to current theories of dark But in a study published March 28 in the apart from other components of galaxies.” matter, inconsistencies are not necessarily a journal Nature, a team of astronomers The oddball galaxy in question, NGC bad thing. Instead, they simply mean that uncovered the first galaxy ever found with a 1052-DF2 (DF2 for short), is an ultra-diffuse our current understanding of dark matter is dearth of dark matter. galaxy about 65 million light-years away in a not entirely correct. Confusing results often “We thought that every galaxy had dark galaxy cluster dominated by the large ellipti- force scientists to look at problems in differ- matter and that dark matter is how a galaxy cal galaxy NGC 1052. Using spectroscopic ent ways, which typically lead to revised begins,” said lead author Pieter van data to map out stellar motions, the theories that better describe reality. Dokkum, an astrophysicist at Yale researchers were surprised to find 10 globu- After all, to paraphrase Isaac Asimov, the University, in a press release. “This invis- lar clusters (large, spherical groups of typi- most exciting phrase to hear in science is ible, mysterious substance is the most dom- cally old stars) on the outskirts of DF2 that not “Eureka!” but rather, “Hmm . . . that’s inant aspect of any galaxy. So finding a were revolving around the galaxy three odd.” — Jake Parks

HOW MUCH DO PLANETS TIP? ALKER, ESA/XMM, ROSAT ESA/XMM, ALKER,

Cold front NASA/CXC/GSFC/S. W NASA/CXC/GSFC/S. Mercury Venus Earth Mars 0.3° 177.4° 23.4° 25.2° Jupiter This cold front won’t quit 3.1° LOW TEMPS. A group of astronomers recently used data from NASA’s Chandra X-ray Observatory, the European Space Agency’s XMM-Newton, and the German Roentgen satellite to study the persistent cold front that has plagued the Perseus galaxy cluster for 5 billion years. The cold front, which appears as a curved arm on the left side of this image, spans 2 million light-years Saturn Uranus Neptune and is hurtling away from the cluster’s center at 26.7° 97.8° 28.3° 300,000 mph (about 483,000 km/h). It measures about 30 million degrees Fahrenheit (17 million TILTAWHIRL . From the time we were in grade school, one fundamental lesson we learned about degrees Celsius), while the surrounding region is our planet is that its axis — the line through the planet from the north to south poles around which it about 80 million F (44 million C). The front was rotates — tilts 23.5° to the plane of its orbit around the Sun. But what about the other planets? Do they likely created when Perseus collided with another also tip by this amount? As you can see, three others tilt about galaxy cluster, sending gas from the cluster’s as much as Earth, two have either a tiny or no tilt, and another KELLY ROEN : As of the beginning center sloshing outward. Researchers think the pair tilt like crazy. — Michael E. Bakich FAST of 2018, Earth’s FACT cold front has strong magnetic fields surrounding exact axial tilt, ASTRONOMY it, protecting it from erosion. — A.J. called obliquity, is 23.436938°. 10 ASTRONOMY • AUGUST 2018 BIG PICTURE. Rochester Institute of Technology researchers are modifying a device found in many digital cinema ASTRONEWS projectors to simultaneously observe multiple astronomical targets within the same star field. TESS is on its way to hunt for planets QUICK TAKES NEW LEADS LIFTOFF. At 6:51 P.M. EDT on April 18, NASA’s The SpaceX rocket ferrying TESS Former U.S. representative Transiting Exoplanet Survey Satellite launches April 18 from Cape Canaveral and Navy pilot Jim Bridenstine (TESS) shot into the sky aboard a Air Force Station in Florida. The satellite’s is the new administrator SpaceX Falcon 9 rocket launched final orbit is elliptical, with a perigee of of NASA. Jim Green is the from Space Launch Complex 40 at 67,300 miles (108,300 kilometers) and an agency’s new chief scientist. Cape Canaveral Air Force Station in apogee of 233,500 miles (375,800 km). Florida. Just over an hour later, the NASA/KIM SHIFLETT WATER CANNON• spacecraft’s solar arrays deployed High-powered projectile successfully, and TESS began its experiments carried out at journey to the 13.7-day orbit that it Brown University show that up will maintain around Earth. Within to 30 percent of an asteroid’s two months of reaching orbit, the water can survive when it spacecraft’s systems are expected to impacts another body. complete checkout and testing, and TESS will begin observing. WHAT’S THAT• SMELL? TESS is NASA’s next-generation Astronomers have found that, planet-hunting satellite, designed to unlike Jupiter and Saturn, pick up where the Kepler space tele- Uranus’ atmosphere contains scope will soon leave off. The satellite hydrogen sulfide. will map the sky in 26 sections over • the course of the next two years, SWEPT CLEAN looking for transits as exoplanets X-ray flares from red dwarfs cross the face of their parent star and (the most common type of cause the starlight’s brightness to dip star) may sterilize exoplanets FOUR EYES. TESS carries four wide- in otherwise habitable zones. slightly in the process. field cameras, with which it will survey While its method for finding plan- 85 percent of the sky over the next MASSIVE• CRASH ets is the same as Kepler’s, TESS will two years. Its targets are hundreds of New research suggests Mars’ focus on stars 30 to 100 times TESS will provide a list of our top thousands of bright stars, and the mission moons formed after an impact brighter than Kepler’s targets, all neighboring worlds for any follow-up is expected to identify thousands of between a dwarf planet-sized within about 30 to 300 light-years of observations, as well as any far future exoplanets for follow-up with both ground object and proto-Mars. Earth. The satellite’s main goal is to travel plans.” In all, TESS will cover and space-based observatories. NASA/GSFC identify promising exoplanet candi- 85 percent of the sky, targeting ALREADY• WET date systems, which can then be fol- 200,000 bright stars and efficiently which will be Earth-sized. In addition An extensive study found lowed up in detail from the ground sending back full-frame images each to spotting transiting exoplanets, 70 to 95 percent of Earth’s and with the James Webb Space time its unique, slightly elliptical TESS can also be used to study stellar water was present before the Telescope when it launches no earlier orbit brings it close to Earth. The properties and behavior, including massive impact that likely than May 2020. spacecraft is equipped with four flares, starspots, and supernovae. formed the Moon. “The Kepler space telescope found wide-field cameras, developed by “TESS is small, but it is mighty an astounding number of exoplanets, MIT’s Lincoln Laboratory, that are because it will search the whole sky, CHILLED• WATER but most of them are many, many designed to work together to achieve all the bright stars we can see at Two newly discovered light-years away, too dim for us to a combined 24°-by-96° field of view. night, for worlds orbiting them,” salty subglacial lakes in the learn much about them,“ said Lisa A study led by Thomas Barclay Kaltenegger said. “When looking up Canadian Arctic may serve as Kaltenegger, director of Cornell at the NASA Goddard Space Flight at night, we will be able to point at ideal analogs for the lakes University’s Carl Sagan Institute, in a Center and the University of bright stars in the night sky and say, within Jupiter’s moon Europa. press release. “That’s why TESS is so Maryland, submitted for publication ‘Right there, there is a star that hosts important: It will find exoplanets in April, estimates TESS will find more another Venus, Mars, or maybe even FAMILY• TREE around stars in our cosmic backyard. than 4,000 new exoplanets, many of another Earth.’ ” — Alison Klesman The GALAH survey compiled spectra for 340,000 stars to find the Sun’s long-lost birth cluster, which has since A pair of CubeSats spread throughout the sky. heads for Mars UNDERWAY• NASA’s Mars InSight mission TINY MESSENGERS. While many of the most is on its way to the Red Planet. famous spacecraft rival a school bus in size, miniature It is set to land November 26. CubeSats ranging from 4 inches (10 centimeters) to 24 inches (60 cm) on a side and weighing 3 pounds GAMMA-RAY• SKY (1.33 kilograms) or less have made observations from The HESS collaboration space much more affordable. The twin CubeSats of published the largest-ever the Mars Cube One mission, or MarCO, are the first catalog of high-energy CubeSats to venture into deep space, trailing NASA’s gamma-ray sources located InSight lander to Mars after a successful launch May 5. within the Milky Way. The MarCOs will relay data about InSight’s entry and landing to help improve future Mars landings, which SMASH-UP• are difficult because of the planet’s thin atmosphere. Globular clusters may contain If successful, they will also serve as proof of concept “second-generation” black that CubeSats can be used for deep space missions. holes that have merged multi- Prior to launch, the CubeSats underwent extensive ple times, an MIT-led research testing (pictured) to ensure their components will work — J.P. — A.K. team has found.

NASA/JPL-CALTECH properly once in orbit around the Red Planet.

WWW.ASTRONOMY.COM 11 STRANGEUNIVERSE BY BOB BERMAN Mars closes in The Red Planet’s opposition this summer comes with a few caveats.

e’re all excited about this extraordinary Mars opposi- Above: Mars will be easy to pick out in the night sky as it shines bright orange, like a cosmic pumpkin. NASA, ESA, AND A. DYER tion. That’s WJuly 27, with Mars shining next Left: The Hubble Space Telescope imaged Mars when it was 50 million to the Full Moon. And Mars miles (80 million kilometers) from Earth, during the Red Planet’s 2016 comes closest to Earth on the opposition. At that distance, the telescope revealed features just 20 to 30 miles (30 to 50 km) across. This year, Mars comes even closer, sitting just 31st — the nearest martian visit 35,785,000 miles (57,590,000 km) away. NASA, ESA, THE HUBBLE HERITAGE TEAM (STScI/AURA), J. BELL since 2003. It won’t be surpassed (ASU), AND M. WOLFF (SPACE SCIENCE INSTITUTE) until 2035. Very cool stuff. It’s a “perihelic opposition,” erroneously This Mars opposition’s width because Mars appears this large with Mars close to its nearest assumed this was an annual of 24" is superb. It’ll stay bigger only a few times in one’s life. point to the Sun and Earth near event. The fake “Mars is bigger than 20" through August, the Just for fun, consider Saturn’s its farthest, placing our two than the Moon” business every same apparent size as a dime oppositions for a comparison. orbits at very nearly their nar- single summer stems from that 2½ city blocks away. That’s big When the ringed planet comes rowest gap. Mars at magnitude Old Farmer’s Almanac article, enough to let telescopes discern particularly close to us, it always –2.8 is now brighter than with the part about “through a surface features. But there’s a happens during our Northern Jupiter, which doesn’t happen telescope” omitted. problem. Although it’s mid- Hemisphere winter when the very often. After Venus sets So don’t be surprised if you spring in the martian southern planet is at its highest, in Taurus around nightfall, for the rest of see that same ridiculous head- hemisphere, so the carbon or Gemini. At the same time, its the night, even a total newbie line appear this year — except dioxide-based southern polar rings are always then tilted most could find Mars just by picking now, unlike all those previous cap is still large and striking, favorably, in a maximally out the sky’s brightest “star.” summers, Mars actually has the word “south” has a darker “open” orientation, at their best. For confirmation, it’s as orange come close once again. And side for most of us. So Saturn’s big perihelic opposi- as a pumpkin — astronomy yes, 100x power Fantastic tions automatically happen with made easy. will make it up-close mar- optimal ring angle and maxi- The opposition will be a appear bigger Mars appears this tian perihelic mum sky elevation. Everything media event, and one of those than the Moon. large only a few oppositions that comes together like gears mesh- cases where popular hyped It’s fun to times in one’s life. happen in July ing. We get three such winners, astronomy is fully in sync with compare mar- and August give each a year apart, and then wait what we actual astronomers tian opposi- Mars a very 27 years for the next trifecta. enjoy. Not that Mars is immune tions, which happen every 26 high southern declination. It’s By contrast, a series of two to hype. In August 2003, we months, to those of, say, Saturn, extremely low in the sky for very close martian approaches, had the closest martian visit in which occur once a year. In U.S., Canadian, Japanese, a bit more than two years apart, more than 50,000 years, and in both cases, speedier Earth Chinese, and European observ- happens every 15 years, but the Old Farmer’s Almanac, passes the slower superior ers. The present martian decli- with the planet mostly a low- where I’ve been the astronomy planet, making it appear big nation of –25° makes it even down smudge — unless you can editor since forever, I wrote that and bright for a month or two. lower than the winter solstice hang out with Aussie, Kiwi, or through any telescope at a mere But while Saturn always looks Sun. Its disk shines through South African astronomers who 100x, Mars would look bigger telescopically amazing, Mars three times more air than if it have hit the jackpot. than the naked-eye Moon. usually has such a tiny disk that were high overhead, which Still, around here, you play Well, guess what? Almost it’s hard to see detail on its almost always blurs the image. the hand you’re dealt. This every summer after that, there 4,000-mile-wide (6,000 kilome- This is no problem if you’re summer it’s the god of war at have been splashy “news” head- ters) body unless the opposition merely admiring Mars’ rare his closest and brightest. lines on the web urging people, occurs near the narrowest gap extreme brilliance. But if you’re “Look up — Mars looks bigger between our orbits. And that hunting telescopically for sur- Join me and Pulse of the Planet’s than the Moon!” Mars wasn’t occurs only during martian face detail, you need a steady Jim Metzner in my new podcast, Astounding Universe, at even visible most of those sum- oppositions between July and night when the stars are not http://astoundinguniverse.com. mers; it’s simply that websites October. Like right now. twinkling. Still, give it a try,

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12 ASTRONOMY • AUGUST 2018 THE IN CROWD. A dozen newly discovered stellar-mass black holes found around Sagittarius A* suggest that ASTRONEWS 10,000 such objects could orbit within 3 light-years of the Milky Way’s galactic center. Hubble spots farthest star ever seen An international team of researchers has 2011 announced the discovery of the most distant star ever observed, nicknamed Icarus. The team, whose study was published April 2 in Nature Astronomy, detected the blue supergiant star, which shone roughly 10 billion years ago, with the help of the Hubble Space Telescope and an observational phenomenon known as gravitational lensing. Gravitational lensing is an effect predicted by Einstein’s general theory of relativity. When 5” a galaxy cluster serendipitously wanders directly between Earth and a distant back- 2016 ground object, gravitational lensing can mag- nify the distant object by up to a factor of about 50. Furthermore, if there is a smaller, impeccably aligned object within the lensing galaxy cluster, the background object can be magnified — in a process called gravitational microlensing — by a factor of up to 5,000. This is exactly what happened to Icarus. “This is the first time we’re seeing a magnified, individual star,” said Patrick Kelly, an astrophysicist of the University of Minnesota and lead author of the study, in a press release. COSMIC COINCIDENCE. The blistering blue star Icarus, which existed almost 10 billion years ago, is visible “You can see individual galaxies out there, but thanks to a chance alignment that magnified it by a factor of at least 2,000. NASA/ESA/P. KELLY (UNIVERSITY OF CALIFORNIA, BERKELEY) this star is at least 100 times farther away than the next individual star we can study, except into the nature of dark matter, which affects the of more powerful instruments such as the for supernova explosions.” lensing ability of the intervening galaxy cluster. James Webb Space Telescope, astronomers are The discovery provides astronomers with No matter what astronomers are able to optimistic that more microlensing events like insight not only into the formation and evolu- glean from Icarus, this chance discovery of an this will allow them to study the evolution of tion of stars in the early universe, but also into extremely distant and magnified star is not the universe’s earliest stars in unprecedented the composition of galaxy clusters, and even likely to be the last. With the upcoming launch detail. — J.P. Kepler solves a stellar mystery The Kepler space telescope has already found over 1,000 exoplanets, with thousands more candidates awaiting confirmation. But it is also contributing to other areas of astronomy by serendipitously catching some stars at the moment of death, when they explode as a supernova. DOUBLE BLAST. Thanks to Kepler, astronomers have caught a large Astronomers estimate that number of supernovae on camera, including outliers called FELTs, which one supernova explodes in flare up and disappear in days. Astronomers believe these events occur the universe every second. when a star first sends out a shell of material (left) prior to exploding Dissecting a They typically brighten only in a supernova (center). The shell then flares up briefly (right) when the for weeks, meaning many are supernova’s shock wave hits it. NASA, ESA, AND A. FEILD (STSCI) missed. And one category cosmic butterfly of supernovae, called fast- studied the FELT’s light curve, may not have been particu- WHAT’S INSIDE? Observations with NASA’s evolving luminous transients, which tracks brightness over larly strong. Thus, the only vis- Hubble Space Telescope, the European Southern or FELTs, dims within just a time, to determine what hap- ible sign of the star’s death is Observatory’s Very Large Telescope, and Apache few days, so even fewer are pened. The group believes the brief flare-up of the shell. Point Observatory have shed light on NGC 6240, spotted. Kepler’s unique this explosion and other FELTs With the Kepler telescope two merging galaxies that form an unmistakable butterfly shape. Recent research shows that two method of staring at stars for like it occur when an aging low on fuel and its mission different forces are driving its distinct silhouette: star first “burps” out material, ending soon, the researchers long periods, to look for dim- The gas in its northwest corner is produced ming associated with transit- which expands outward in a are now looking to the

by stellar winds, while the gas in its northeast UNIVERSITY) BROOK CHARLOTTESVILLE/NRAO/STONY VIRGINIA, (UNIVERSITY OF EVANS A. AND COLLABORATION, I/AURA)-ESA/HUBBLE C ing planets, has uncovered shell. When the star explodes Transiting Exoplanet Survey corner has been ejected by the galaxies’ merging more than 20 supernovae, a few years later, the shock Satellite mission, which supermassive black holes. These differing wind including a FELT spotted in wave runs into the shell, launched April 18, to spot sim- sources may be good for appearance, but neither 2015 that flared up and then briefly lighting it up. The ilar events. They will also con- is good for star formation. The strong gusts are disappeared in just 10 days. whole thing then fades tinue combing through blowing away the material needed to form new In a March 26 paper in quickly, researchers believe, Kepler’s data long after the stars, slowing down the typical starburst that Nature Astronomy, researchers because the actual supernova spacecraft goes dark. — A.K. occurs during a galactic collision. — A.J. NASA, ESA, THE HUBBLE HERITAGE (STS HERITAGE HUBBLE THE ESA, NASA,

WWW.ASTRONOMY.COM 13 AI ASSISTANCE. Machine learning can help astronomers better predict the long-term habitability of exoplanets ASTRONEWS in multistar systems by more accurately analyzing regions of instability. HOW SMALL IS THAT STAR? TINY SUNS. While many of the brightest stars in the sky dwarf our Sun in terms of size, the Sun is much larger than many others. In particular, the red dwarfs around which many extrasolar planets have been found are, on average, about one-tenth to half the size of our Sun. Between stars and planets lie brown dwarfs, or failed stars, which don’t have enough mass to create the Alpha Centauri A temperatures and pressures 1.2 RSun needed for hydrogen fusion. They are too large to be Sun planets, but too small to be stars. Brown dwarfs can fuse 1 RSun deuterium, an isotope of hydrogen, for a short time before they eventually cool Alpha Centauri B off and go dark. — A.K. 0.9 RSun Dwarf stars FAST have estimated FACT ET AL. BAGANOFF NASA/CXC/MIT/F. TRAPPIST-1 CENTRAL BODY. The center of the Milky Way, imaged by the Chandra X-ray 0.12 R lifetimes Proxima Centauri Sun longer than the Observatory, contains a supermassive black hole 4 million times the mass 0.15 RSun current age of of the Sun. Astronomers believe most galaxies contain similar behemoths in their centers, some ranging up to billions of times the Sun’s mass. the universe. Gliese 229 Gliese 229B TRAPPIST-1 is (brown dwarf) 0.26 RSun already between : ROEN KELLY ROEN : Supermassive black holes disrupt 0.1 RSun 5.8 billion and 9.8 billion years old.

ASTRONOMY their smaller counterparts Encounters with the supermas- observable gravitational waves,

I) sive black holes in the center they must be in very compact, C of most galaxies could cause eccentric orbits. The research smaller black hole binary systems team showed that coming into to emit observable gravita- close contact with a supermas- tional waves, a new study led by sive black hole’s extremely researchers at Liverpool John strong gravitational field could Moores University says. The find- affect the orbits of some black ing was announced April 3 at the hole binaries, causing them to European Week of Astronomy orbit even closer together. This and Space Science in Liverpool, could result in black holes that England. otherwise would never have As the orbits of black holes in merged spiraling inward and ); ACKNOWLEDGEMENT: NASA, ESA, AND J. ANDERSON (STS ANDERSON J. AND ESA, NASA, ACKNOWLEDGEMENT: ); I

C binary systems decay, gravita- emitting gravitational waves. tional waves expand outward. The group’s simulations showed Albert Einstein predicted these that binaries that managed to waves, but he believed they were survive massive black hole too small to detect. Nevertheless, encounters had merger times the effect was first seen in 2015 reduced by more than a factor of when a black hole duo, each 100 in 10 percent of cases. about 30 times the Sun’s mass, Thanks to instruments such drew together and merged. as the Laser Interferometer A handful of binary black Gravitational-wave Observatory,

NASA, ESA, AND T. BROWN AND S. CASERTANO (STS CASERTANO S. AND BROWN T. AND ESA, NASA, hole systems have since been researchers hope to observe Hubble precisely measures observed, suggesting that black more gravitational waves and hole mergers are fairly common gain additional insight into their distance to ancient cluster in the universe. However, to emit origins. — A.J. LONGDISTANCE MEASUREMENT. In a scientific first, astronomers have accurately measured the distance to one of our universe’s oldest residents: the 13.4 billion-year-old globular cluster NGC 6397. Using NASA’s Hubble Space Telescope and a technique called trigonometric parallax, which measures the apparent motion 4,000 of a star as Earth orbits the Sun, they calculated a distance of 7,800 light-years between Earth and NGC 6397. To do this, the team measured the parallax of 40 The number of newly discovered early galaxies, many of pulsating stars, known as Cepheid variables, in NGC 6397 four times over the course of two years. With this data, the researchers were able to determine the cluster’s which will evolve into galaxies like the Milky Way, spotted distance with only a 3 percent margin of error. — A.J. in one of the largest 3-D maps of the early universe.

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WWW.ASTRONOMY.COM 15 FORYOURCONSIDERATION BY JEFF HESTER One step at a time Walking a not-so-mysterious path to life.

ew words in science Morowitz’s whole point was can provoke quite so that of course life didn’t just pop visceral a reaction as into existence one day! You don’t abiogenesis. I get it. get to the top of a mountain by The idea of life arising leaping from the valley floor. Funbidden from nonliving mat- You get there one step at a time. © N.L | DREAMSTIME.COM ter can be hard to wrap your Let’s glance at some of the England. Sometimes described with 3.7 billion-year-old fossil- head around. But like any idea, evidence. Over 65 years ago, as “the next Darwin,” England ized stromatolites and chemical it deserves to be examined hon- Stanley Miller and Harold Urey has shown that as chemical sys- signatures of life in 4.1 billion- estly on its merits. Unfortunately, mixed a few simple compounds tems far from thermodynamic year-old zircons, what we can a lot of people don’t. like carbon dioxide and ammo- equilibrium convert useful say is that life was fast out of Probably the biggest miscon- nia in a flask and added a spark. energy into entropy, they inevi- the gate! ception about abiogenesis is that The murky liquid that emerged tably make things with lifelike Think of it like this. Suppose it requires some absurd event was full of complex organic properties. “Life,” he says, you live in L.A., and a friend like atoms randomly sticking molecules including amino “should be as unsurprising as comes from Arizona to visit. together one day to make a living acids, the building blocks of rocks rolling downhill.” They might have flown. They thing. A willfully dishonest criti- proteins. Since then, similar Not that many years ago, might have ridden a bus. They cism, common in some circles, experiments have produced a there might have been room to might have driven on Interstate often reads something like this: plethora of biologically impor- reasonably question whether 10, or taken I-8 across and then “Biophysicist Harold Morowitz tant compounds, including the abiogenesis was possible. That is came up the coast from San says that the probability of a sim- nucleotides that encode infor- no longer the case. With our Diego. Regardless, it’s no sur- ple cell forming by chance is 1 in mation in DNA and RNA. deeper understanding of the prise that they made it. It’s not 10340,000,000. Even scientists know A recent breakthrough inexorable processes of thermo- that hard to get to L.A. from that abiogenesis is impossible!” came from Jan Sadownik and dynamics and evolution, and the Arizona. Anyone who has ever fallen Given a menagerie of for that claim has every right organic molecules, emergent to be annoyed, because they’ve Today the facts show that abiogenesis, self-replication, nonequilib- been sold a bill of goods. once a questionable and even radical idea, rium thermodynamics, (Perhaps I should be more blunt: Darwinian selection, plenty They’ve been lied to.) Were there was no fluke. of energy, a beaker the size an Oscar for Most Shameless of Earth and millions of years, Misrepresentation of Another’s the journey from geochemistry Work, that little straw man colleagues at the University of fundamental chemistry of life to life doesn’t look very hard, would be a shoo-in. Groningen, in the Netherlands. emerging in laboratories around either. When not taken out of con- In their experiment, self- the world, the major hurdles to Today the facts show that text, here is what Morowitz had replicating molecules emerged abiogenesis are gone. abiogenesis, once a questionable to say: “The ultimate emergence spontaneously from a mixture Was life’s first molecule a and even radical idea, was no of metabolism seems embedded of simpler compounds, then protein, or was it RNA? Did it all fluke. Quite the contrary, it was in the laws of chemistry. . . . In mutated into different species start with UV-driven chemistry a likely and perhaps this realm, novelty piles upon that competed for resources. beneath ice sheets, reactions even unavoidable product of novelty, and we change from the Quoting from their 2016 paper catalyzed by clay, chemistry dynamic, natural, unguided rule of systems of physical chem- in Nature Chemistry, “these around hydrothermal vents, or processes at work on a recently istry to those allowable rules of results mark an important step some combination thereof? With formed Earth. biology. With the emergence of towards achieving Darwinian such a smorgasbord of ideas to If that isn’t mind-blowingly distinguishable competitive evolution with a system of fully play with, scientists working on cool, I don’t know what would protocols, the world becomes synthetic molecules and the abiogenesis face an embarrass- be! Darwinian, and we move from synthesis of life.” ment of riches. the domain of relative simplicity Seeing complex organics and We may never know exactly Jeff Hester is a keynote speaker, to the kind of complexity that self-replicating molecules pop what path led to terrestrial life; coach, and astrophysicist. eventually leads to the emer- up unbidden is no surprise to you can’t always reconstruct the Follow his thoughts at jeff-hester.com. gence of mind.” MIT biophysicist Jeremy details of contingent history. But

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16 ASTRONOMY • AUGUST 2018 MAGNIFIED IMAGE. Astronomers hope the James Webb Space Telescope will spot the first stars and black holes ASTRONEWS with the help of rare gravitational lensing events that can amplify light by 10,000x. Sagittarius A* may have a dozen nomadic siblings

HD 69830 system

Solar system NASA/JPL-CALTECH Visualizing a dusty star system BEYOND THE VEIL. From a dark location SUPERMASSIVE SIBLINGS. Like most galaxies, the Andromeda Galaxy (pictured) and the Milky Way are on Earth, observers can see the glow of the thought to house supermassive black holes in their cores. According to new research, galaxies roughly the mass zodiacal light — sunlight scattered by dust shed of the Milky Way also likely contain about a dozen more “wandering” supermassive black holes as remnants of from comets and generated by collisions within past mergers. NASA/JPL-CALTECH the asteroid belt. While our own solar system’s asteroid belt is just a few percent of the mass Most supermassive black holes (SMBHs) are over long stretches of cosmic time. From the of the Moon, systems such as HD 69830 have thought to remain relatively sedentary through- sample, the researchers were able to determine dust generated by an asteroid belt about 20 out their lives, lingering in the centers of their that any galaxy roughly the mass of the Milky times more massive. Here, an artist illustrates the host galaxies as Sagittarius A* does within the Way, regardless of its recent merger history or difference between the zodiacal light in our solar Milky Way. However, if a small galaxy merges morphology, likely contains about a dozen system (bottom), and what it would look like from with a larger one, the smaller galaxy’s SMBH can SMBHs over 10 million solar masses, with about a hypothetical planet around HD 69830. From this be thrown into a wide orbit through the newly five of those located within 30,000 light-years of planet, the zodiacal light would shine 1,000 times brighter, drowning out portions of the Milky Way formed galaxy, becoming a “wandering” super- the galaxy’s center. in the night sky. Not only is visualizing such thick massive black hole. Although such a slew of vagabond SMBHs dust intriguing, but it also highlights the difficulty Although astronomers have previously found may seem intimidating — especially consider- of finding exoplanets around stars that have evidence of these nomadic SMBHs on the out- ing they roam free for at least a few billion years significant shrouds of interplanetary dust. — A.K. skirts of other galaxies, their overall prevalence — they pose little threat to our tiny corner of in the universe is still largely unknown. But such the cosmos. wandering SMBHs may in fact be quite common “It is extremely unlikely that any wandering (and even observable) within many types of gal- supermassive black hole will come close enough axies, including the Milky Way, according to a to our Sun to have any impact on our solar sys- new study published April 24 in The Astrophysical tem,” said lead author Michael Tremmel, a post- 100 th Journal Letters. doctoral fellow at the Yale Center for Astronomy To carry out the study, the researchers pulled and Astrophysics, in a press release. “We esti- The anniversary the a sample of Milky Way-mass galaxies from a mate that a close approach of one of these wan- International Astronomical state-of-the-art cosmological simulation called derers that is able to affect our solar system ROMULUS25, which models how billions of indi- should occur every 100 billion years or so, or Union will celebrate in 2019. vidually resolved particles interact and evolve nearly 10 times the age of the universe.” — J.P.

IN THE SHADOWS. Astronomers recently used Blocking starlight shows disks around young stars the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument on the European Southern Observatory’s Very Large Telescope to suppress the bright light of young T Tauri stars and reveal the faint disks that surround them. The new imagery shows a veritable zoo of disks, each with a unique size, shape, and brightness. The young stars, which are less than 10 million years old, show disks made up of dust, gas, and planetesimals — precursors to planetary systems. By studying the properties of these usually overshadowed objects, researchers can begin to RXJ 1615 AS 209 IM Lup correlate different types of disks with the planets that form within them. — A.J. ESO/H. AVENHAUS ET AL./E. SISSA ET AL./DARTT-S AND SHINE COLLABORATIONS SHINE AND ET AL./DARTT-S SISSA ET AL./E. AVENHAUS ESO/H.

WWW.ASTRONOMY.COM 17 OBSERVINGBASICS BY GLENN CHAPLE

The author sketched Mars using a 4.5-inch reflecting telescope The return of during the planet’s 2003 opposition. In typical viewing conditions, the finer surface features disappear for seconds or the Red Planet minutes due to the blurring effect of Earth’s atmosphere. To combat It’s been 13 years since Mars’ last favorable opposition, this, don’t try to sketch the planet during temporary moments of clear so don’t let this one slide by your scope. viewing. Instead, create a detailed mental image, and begin sketching only when turbulence returns. The Hubble image of Mars shows the same ear ye, hear ye. 13 years, during which opposi- hemisphere as the sketch. SKETCH: G. CHAPLE; Mars is back! Raise tions yield a sub-20" disk diam- IMAGE: NASA/ESA/THE HUBBLE HERITAGE TEAM (STSCI/AURA) the banners, strike eter. The last reasonably “ up the band.” These favorable opposition was the stability) will determine how settle down, and a crystal-clear opening words of aforementioned 2005 event. much detail you’ll spot. Mars will snap into view. For Hmy November 2005 column The next will happen in And, unfortunately, this a few fleeting seconds, you’ll heralded a favorable appear- October 2020 (angular diam- opposition of Mars occurred see grayish areas and a bright ance of Mars in the evening eter 22.6"). After that, we’ll when the planet was in the white spot (the southern polar sky that month. And I’ve writ- have to wait until 2033 (22.1") southern part of the zodiac. For cap). All too soon, the turbu- ten absolutely nothing about and 2035 (24.6") for the next those of us in mid-northerly lence will return and minutes Mars in the 13 years since. favorable pair. latitudes, Mars barely breached will pass before another It’s not that Mars doesn’t But let’s forget about both the treetops along the southern detailed view. deserve the attention of back- past and future Mars opposi- horizon, its disk subject to the If you’re an astrosketcher yard astronomers; it most tions and concentrate on the whims of atmospheric turbu- attempting a pencil and paper certainly does. It’s just that current one. Mars reached lence. Be sure to observe Mars rendering of Mars, wait for favorable apparitions of Mars opposition late last month, but when it’s as high above the hori- those momentary steady peri- — those occasions when the it remains well-seen through- zon as possible, around 1 A.M. ods. When one comes, soak in planet is close to Earth during out August. It’s an impressive local time on August 1, 10:30 P.M. as much detail as you can. an opposition and its angular naked-eye sight — a ruddy local time at month’s end. When the turbulence returns, diameter equals or exceeds 20" magnitude –2.8 “star” domi- If you’ve never viewed Mars put pencil in hand and sketch — are few and far between. nating the southwest part of through a telescope, you’ll be in as much as you can. Then The last time Mars appeared Capricornus at the beginning surprised to discover that the wait for the next clear view and this large was November 2005. repeat. Hang in there, and A martian year is slightly you’ll come up with a surpris- longer than two Earth years, so Yes, Mars is back. Be patient and persistent. ingly detailed drawing. A oppositions occur every two And most of all, enjoy the show! crude sketch of Mars I made years and two months on aver- during the 2003 opposition age. So far, so good. But not all using a 4.5-inch reflecting tele- martian oppositions are equal. of August, and still prominent Red Planet isn’t red at all. scope appears on this page. If one occurs when Mars is at magnitude –2.1 at the close It’s more of an ochre, or Compare it with a Hubble near aphelion (the portion of of the month. yellow-orange color. You’ll also image taken when the same Mars’ orbit when it’s farthest Telescopically, Mars is a be surprised (unpleasantly) side of Mars was facing Earth. from the Sun), the Earth-Mars challenging target even for to discover that the martian Yes, Mars is back. Be patient distance will be on the order of experienced astronomers. disk is constantly quivering and persistent. And most of all, 60 million miles, and the mar- When it’s at its closest this and contorting, teasing you enjoy the show! tian disk will be less than 14" year, you’ll still need a magni- with vague glimpses of surface Questions, comments, or across. When Mars is near the fication of 75x to enlarge the detail. If only Mars would suggestions? Email me at Sun in its orbit (perihelion), its disk to the apparent size of the settle down and give you a [email protected]. Next distance from us at opposition Full Moon as it appears to the clear view! month: Stellar duos you missed shrinks to some 35 million unaided eye. While you can Mars isn’t the culprit; the air during the Double Star miles, while its angular size pick out some planetary detail above us is to blame. Under Marathon. Clear skies! balloons to as much as 25". at that power, 100x to 150x for typical seeing conditions, Favorable martian opposi- a small-aperture scope and atmospheric turbulence will Glenn Chaple has been an tions occur in groups of two or 200x and up for medium to “boil” Mars’ image for minutes avid observer since a friend three. Between these pairs or large scopes is a must. Seeing on end. Then, just when you’re showed him Saturn through a small backyard scope in 1963. triplets are gaps that average (a measure of atmospheric about to give up, the air will

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18 ASTRONOMY • AUGUST 2018 WHO ARE YOU? A machine-learning technique previously used for facial recognition has been retooled by ASTRONEWS an international team of astronomers to classify galaxies in Hubble images. Gaia creates detailed star catalog The European Space Agency’s (ESA) Gaia space observatory spent 22 months conducting a comprehensive survey of over 1 billion stars in our sky, creating the most detailed star catalog to date. The dataset, which was released April 25 and will appear in an upcoming special issue of the journal Astronomy & Astrophysics, is expected to contribute to countless discoveries over the course of its analysis. Gaia’s catalog contains the positions, motions, and distances of nearly 1.7 billion stars. The spacecraft also charted the orbits of 75 globular clusters and 12 dwarf galaxies sur- GAIA’S SKY. Astronomers turned Gaia’s measurements of nearly 1.7 billion stars into this map of the sky. The rounding the Milky Way Galaxy. Gaia’s stellar brighter regions occur where stars are densely concentrated, while dimmer areas show regions of the sky where motion and population data could help fewer stars are found. The dark, filamentary features are clouds of gas and dust that absorb starlight, blocking researchers better understand the Milky Way’s Gaia’s view of objects behind them. ESA/GAIA/DPAC formation and evolution. In addition to its stellar measurements, Gaia observed the motions However, Gaia’s meticulous measurements will within 5,000 light-years of the Sun, will allow of more than 14,000 asteroids in our solar sys- allow researchers to differentiate between a astronomers to study the population and evo- tem, enabling researchers to refine their orbits. star’s parallax and its actual movement across lution of stars throughout the Milky Way. The mission also pinpointed the positions of our line of sight (called proper motion) within With so many celestial objects in and nearly half a million distant quasars. the galaxy. Both the parallax and proper around our galaxy, it’s important to under- Launched in 2013, Gaia compiled the motion of 1.3 billion stars will be featured in stand how everything coexists. Mapping the motions and distances of 2 million stars during the catalog, with direct distances estimated for locations and motions of those objects helps its first year in operation. Now, with star data about 10 percent of those stars. researchers understand our galaxy’s forma- climbing into the billions, a staff of 450 scien- Gaia also recorded the brightness of each tion, evolution, and future. tists and software engineers is responsible for star and measured the color of almost all stars “Gaia is astronomy at its finest,” said Gaia cataloging it all. surveyed. From this, the researchers created an mission manager Fred Jansen of ESA in a press To calculate the distances to stars, research- extremely detailed Hertzsprung-Russell dia- release. “Scientists will be busy with this data ers typically measure stellar parallax, the per- gram, a chart that compares the luminosity of for many years, and we are ready to be sur- ceived movement of a star based on Earth’s stars to their temperature. Gaia’s diagram, prised by the avalanche of discoveries that will changing position in its orbit around the Sun. which is made up of 4 million stars that lie unlock the secrets of our galaxy.” — A.J.

A star on Earth LABORATORIES NATIONAL SANDIA OF COURTESY MONTOYA, RANDY HENZE, NASA HENZE, STAR STUFF. The Z-machine at Sandia National Laboratories is the world’s most powerful X-ray source. Astrophysicists at The University of Texas at Austin When black holes collide will now use it to conduct experiments on matter subjected to the temperatures and pressures heretofore found only in stars. Researchers have never before been BACKGROUND NOISE. Gravitational waves are ripples in space-time that able to study stellar material directly here on Earth, so these experiments will propagate outward from violent cosmic events. In a paper published April 16 in help to answer key questions about why theory and observation don’t always the journal Physical Review X, researchers outline a new technique for detecting match. Inside the machine, a combination of electrical capacitors and lasers turns the faint “hum” of the more than 100,000 gravitational wave events believed to gas into plasma. For a few nanoseconds, this plasma exists in the same state as occur each year. This hum is generated by merging stellar-mass black holes; the the photosphere of a white dwarf, literally giving astronomers a piece of a star researchers estimate a pair coalesces somewhere in the universe every two to to study. Each of the visible flashes that spiderweb across the apparatus when 10 minutes. The technique will use a supercomputer to sift through seas of noisy this occurs has the energy of a lightning bolt. The flashes are not part of the data and tease out signals from previously hidden black holes. — J.P. experiment at all; they’re the leaked energy given off in the process. — A.K.

WWW.ASTRONOMY.COM 19 PLANETSIZED SELFIE. The asteroid-bound OSIRIS-REx mission trained its instruments on Earth during a recent flyby, ASTRONEWS finding clear signs of life in addition to CO levels 14 percent higher than the Galileo spacecraft measured in 1990. 2 Astronomers trace a cosmic spiderweb Our universe contains both voids and dense cats in photographs.” areas where matter — galaxies — has gath- To map the filaments, researchers exam- ered. Between these clumps of matter lies a ined the cosmic microwave background (CMB) spiderweb-like network of dark matter, called left over from the Big Bang. Using sophisti- the cosmic web, along which intergalactic cated software, they looked for areas where gas gathers and flows into the centers of gal- the CMB is affected by gravitational lensing, in axies and clusters. Combining observations which intervening matter has distorted the from all-sky surveys with image-recognition light, to trace out the imprint of these dark software and current models and theories, matter filaments throughout the universe. an international group of researchers has The study found that filaments stretch, identified the gravitational effects of the dark bend, and twist over time, and that their matter in these invisible filaments, allowing length is affected by the competing forces of SEEING THE them to trace out their shape. The research gravity and the expansion of the universe. was published April 9 in Nature Astronomy. They arc across hundreds or millions of light- INVISIBLE. A network of filaments of “Usually researchers don’t study these fila- years and deposit matter into the halos sur- dark matter and gas (blue) ments directly — they look at galaxies in rounding giant galaxy clusters. connects areas of high density, observations,” said study leader Shirley Ho of Studying these filaments and the gas they such as galaxy clusters (pink spots). Berkeley Lab and Carnegie Mellon University contain, the researchers believe, will not only Researchers have found the imprint of these in a press release. “We used the same meth- fill in our picture of the material in the voids filaments by studying how the light of the cosmic ods to find the filaments that Yahoo and between galaxies, but could also provide microwave background is affected as it travels Google use for image recognition, like recog- constraints on dark matter and dark energy, through this network to Earth. UC RIVERSIDE nizing the names of street signs or finding both of which remain a mystery. — A.K.

HOW DOES THE FALCON HEAVY STACK UP AGAINST THE COMPETITION?

Organization NASA NASA SpaceX ULA Arianespace Blue Origin NASA SpaceX

Space Launch Saturn V System Big Falcon Rocket New Glenn

Delta IV Falcon Heavy Heavy

Height (ft) Space Shuttle 363 Ariane V 365 326 348

229 236 184 173

Liftof thrust (lbf) 7.6 1.2 5.1 2.1 2.9 3.9 9.2 11.8 million million million million million million million million : ROEN KELLY ROEN :

Payload* (lbs) 310,000 60,500 141,000 62,50046,000 99,000 290,000 330,000 ASTRONOMY *Estimated payload capacities are based on launching into low-Earth orbit

LIFTOFF. With three boosters and 27 engines generating more than 5 million pounds of thrust, SpaceX’s Falcon Heavy is the most powerful rocket in operation today — by a factor of two. Standing 229 feet (70 meters) tall, the Falcon Heavy is Each of the four engines on a FAST able to launch 141,000 pounds (64,000 kilograms) of payload into low Earth orbit. The Falcon Heavy is also the first rocket Boeing 747-400 produces FACT since NASA’s Apollo-era Saturn V that is capable of sending humans to the Moon. a thrust equal to about But over the next few years, the Falcon Heavy will be introduced to some new competition. This includes Blue Origin’s 63,000 pounds of force (lbf). New Glenn rocket, which aims to siphon commercial satellite launches away from the Falcon Heavy, as well as NASA’s Space Launch System (SLS), which has the ultimate goal of launching humans to Mars. To compete with the Mars- oriented SLS, SpaceX is currently developing the Big Falcon Rocket (BFR). — J.P.

20 ASTRONOMY • AUGUST 2018 ELDERLY POPULATION. Researchers have found the Milky ASTRONEWS Way’s bulge contains stars no younger than 7 billion years old.

AHEAD OF SCHEDULE. An artist’s impression shows the merging galaxies that make up SPT 2349–56, only about 1.5 billion years after the birth of the universe. ESO/M. KORNMESSER Massive galaxies merged earlier than expected Astronomers previously believed that young universe was just 1.5 billion years old. They’re star-forming galaxies first began to collide and the most active regions of star formation ever form larger galaxies about 3 billion years after witnessed in the infant universe — and the the Big Bang. But new observations point to a near-colliding galaxies will go on to create the pileup occurring 1.5 billion years sooner. cores of massive galaxy clusters. Two teams of international researchers used The discovery is puzzling, though. Because the Atacama Large Millimeter/submillimeter they form stars so quickly, starburst galaxies Array (ALMA) and the Atacama Pathfinder rapidly burn through their gas and have short Experiment (APEX) telescopes to study two newly life spans, making it tough to spot just one, let discovered groups of merging starburst galaxies, alone a group. “At any time, in any corner of the which pop out thousands of stars each year. universe, these galaxies are usually in the minor- They’re separated from Earth by 90 percent of the ity. So, finding numerous dusty starbursts shin- observable universe, so the light just now reaching at the same time like this is very puzzling, ing us shows these galaxies as they appeared bil- and something that we still need to under- lions of years ago. The research was published stand,” said Iván Oteo of the University of March 26 in The Astrophysical Journal and will also Edinburgh, whose team discovered one of the appear in an upcoming issue of Nature. clusters, in a press release. Additionally, current The groups were initially identified as indis- models predict that protoclusters like these tinct smudges of light by researchers using the should have taken longer than 1.5 billion years South Pole Telescope and the Herschel Space to evolve into such massive objects. Observatory. Further observations by ALMA and The researchers are now planning to further APEX revealed two extremely dense packs of study these and other starburst galaxies to crack galaxies, one made up of 14 galaxies and the the case of such dense, fast-forming galactic other made up of 10, coalescing when the groups. — A.J.

SPT APEX ALMA

FINE DETAIL. SPT 2349–56 is a distant group of merging galaxies. Imaged with the South Pole Telescope (SPT), it appeared as a single bright source, but astronomers subsequently zoomed in with the Atacama Pathfinder Experiment (APEX) and finally the Atacama Large Millimeter/submillimeter Array (ALMA) to resolve it into 14 separate galaxies. ESO/ALMA (ESO/NAOJ/NRAO)/MILLER ET AL.

WWW.ASTRONOMY.COM 21 22 ASTRONOMY • AUGUST 2018 n May 27, 1973, Beyond our solar system, astrono- Stephen Walther mers have discovered and cataloged — a Wisconsin several thousand planets orbiting native, graduate of other stars, from the very first detec- the University of tion in 1992 to the present day. It’s OWisconsin, and astronomy enthusi- now clearer than ever that the galaxy ast — founded this magazine. Before is populated with countless planets, a decade was out, Astronomy would and who knows how many are suit- grow to become the highest circula- able for the chemistry of life? tion title on the subject in the world. It Since the early 1990s, the Hubble maintains that distinction to this day. Space Telescope has revolutionized To celebrate our founder’s legacy our understanding of the universe and the 45th year of our publication, at large — and shown us amazing

we’re offering up a special package of MEDIA KALMBACH pictures. Other spacecraft, chiefly stories. I hope you will enjoy this cel- Wisconsin native Steve Walther COBE, WMAP, and Planck, ebration of the biggest things that founded Astronomy magazine in 1973 have provided us with an ever- as an outgrowth of a college project. have occurred in the world of astron- By 1980, it had become the most sharpening picture of the universe’s omy during the past 45 years. They widely read astronomy magazine cosmological parameters — origin, cover the entire scope of our shared in the world — and it still is today. age, size, evolution, and so on. interests, from astronomy and astro- And it’s widely believed by cosmolo- physics to planetary science to cosmology. gists that an early era of inflation impacted the uni- This package also represents an “alumni issue,” verse that we inhabit. mostly written by folks who have been editors for the Deep mysteries abound, and we’ve cracked some of magazine in the past: Korey Haynes, Liz Kruesi, their codes. We now know that black holes are ubiqui- Bob Naeye, Frank Reddy, and John Wenz. Current edi- tous in the cosmos, and we’ve even detected them after tor Michael Bakich also contributed material, as did they crash into each other and ripple space-time with science journalist Mara Johnson-Groh. We hope you’ll gravitational waves. And that greatest of mysteries, like the diversity of big astro stories. dark energy, still beckons: What is the nature of the The world of astronomy has changed radically over repulsive force that is speeding up the last half-century. We’ve explored Mars, from the the expansion of the universe? Viking landers through the most recent Curiosity mis- I hope you’ll enjoy this special sion, with many a lander, orbiter, and rover in between. package of content that cele- The quest to understand why Mars’ atmosphere has brates big discoveries of the evolved over time still rolls on. We experienced the past 45 years. May the next 45 amazing Voyager Grand Tour, which explored the years be just as interesting, and outer planets for the first time in history and revealed filled with a sharpening view numerous moons as dynamic worlds in the process. of the cosmos. And of course, just three years ago, New Horizons provided the first up-close look at Pluto, Charon, and the David J. Eicher is editor system’s other small moons. of Astronomy.

First issue, August 1973

WWW.ASTRONOMY.COM 23 1 BIGGEST THINGS THAT CHANGED ASTRONOMY

Above: The Curiosity rover set down in the vast bowl of Gale Crater in 2012. Layers of rock appear in the foreground while Mount Sharp rises behind. Scientists adjusted the color balance in this scene so that the landscape appears as it would if it were on Earth. THE RED PLANET REVEALED Left: The Viking 2 lander touched down on the rock-strewn plains of Utopia Planitia in 1976. The trench in the center of the image marks soil that was disturbed when the spacecraft collected a sample for analysis. The shroud that protected the sampler’s More than a dozen spacecraft have shown Mars collector head rests just to its right. NASA/JPL to be a wonderfully diverse world with hidden stores of water. by John Wenz

y 1974, the United Each Viking lander included States and the an astrobiology lab. Three Soviet Union had experiments were meant to test thrown some two for microbial life on Mars. Two dozen missions at of the three showed nothing, BMars, with only a few sticking. but the third produced strange The Mariner 4 flyby in 1964 and inconclusive results, revealed Mars — Earth’s revealing trace evidence for smaller sibling and a seem- some kind of metabolic activ- ingly promising abode for life ity despite the nil results from “Blueberries” collect at the bottom of this bowl in Eagle Crater. The Opportunity rover — as nothing more than a the other experiments. discovered these bluish spheres and analyzed their composition, confirming they are barren, cold, reddish desert. After the Viking successes made of hematite. On Earth, this mineral forms only in water. NASA/JPL/CORNELL When Mariner 9 entered of the mid-1970s, Mars explo- orbit in 1971, it painted a ration crept to a standstill for slightly brighter picture. Sure, two decades. It’s a stark con- heel: “Water not found on riverbeds, canyons, and valleys there was no surface water trast to the situation today, Mars after all.” A volley back that cut into the planet, though right now, but signs of dry river- when multiple missions are and forth ensues. Often, the they are a far cry from the sup- beds hinted at water in the dissecting the planet from debate ends in a stalemate. But posed “canals” astronomer past. Scientists began to under- several angles. the gradual accumulation of Percival Lowell reported seeing stand that, however sterile it evidence paints two different on the Red Planet starting in currently appears, Mars is still Water, water everywhere pictures for water on Mars: its the 1890s. an active world of dust storms, And it seems that every few past abundance, and its pres- Mars Global Surveyor — fog, and carbon dioxide hazes. years, these missions produce ent possibility. which arrived in 1997, becom- There was more than meets headlines declaring, “Water Nearly every Mars mission ing the first successful mission the eye, and Vikings 1 and 2 found on Mars.” Sometimes, a since Mariner 9 has found to the planet since Viking — intended to find out what few months later, the scientific evidence for past water. The discovered gullies. On Earth, that was. community and media turn Viking orbiters found dry water typically carves such

24 ASTRONOMY • AUGUST 2018 Above: The European Space Agency’s Mars Express captured this patch of water ice in an unnamed crater near the planet’s north pole. Traces of water ice also appear on the walls and rim of this 22-mile-wide (35 kilometers) crater. ESA/DLR/FU BERLIN (G. NEUKUM)

Right: The Mars Reconnaissance Orbiter discovered this fresh impact crater in May 2012, in a region that showed no such scar in July 2010. This close-up shows the 100-foot- wide (30 meters) crater and its debris field, which spreads 9 miles (15 km) from the

NASA/JPL-CALTECH/MSSS impact site. NASA/JPL-CALTECH/UNIVERSITY OF ARIZONA

correlation without further evidence. The two most impressive rovers, the ongoing Opportunity and Curiosity, The Curiosity rover discovered these mineral veins on the lower slopes of Mount Sharp. Scientists think the veins, which appear have spent several years as a network of ridges, formed when water flowed through fractured rock and deposited minerals in the cracks. NASA/JPL-CALTECH/MSSS exploring craters and ancient lake beds, all but confirming that Mars was a lush water features. But Mars’ current circling the planet, have dis- Mars 96, Mars Polar Lander, world billions of years ago. In atmosphere is much too thin covered their share of intrigu- and Beagle 2 — the bulk of the fact, Opportunity has been for water to survive on the ing geologic features. MRO landing missions in the past exploring Mars since 2004 surface for long, so a conten- famously found “recurring 20 years have been unqualified after being given an initial tious debate has developed as slope lineae,” brine-filled flu- successes. It started with Mars 90-day mission. (Its twin, to what causes the gullies. ids that some researchers iden- Pathfinder, whose Sojourner Spirit, lasted “only” until 2011.) Some scientists think dry ice tify as seasonal flows of water, probe proved the viability of But where did the water go? or natural sedimentary move- albeit in a salty slush that rovers on Mars in 1997. The ments could be at the root; eventually dries into toxic salts Phoenix lander, which touched Mars’ atmosphere others think seasonal flows of — far from friendly to life. down in 2008, found abundant In order to have water on the water could stick around just Odyssey has mapped several water ice near the north pole surface, Mars needs a much long enough to create the fea- regions where water ice seems of Mars. thicker atmosphere. So what tures. The orbiter also found to exist just below the martian In 2007, the Spirit rover happened? evidence for subsurface gla- surface, out of reach of the found bizarre structures made NASA’s MAVEN mission ciers and old lake beds. Sun’s sublimating gaze. of opaline silica on the floor of has gradually pieced the story Mars Odyssey and Mars But the biggest gains argu- Gusev Crater. On Earth, such together. The orbiter, which Reconnaissance Orbiter ably have come from landers features appear as microbial reached Mars in 2014, grazes (MRO), both still operating and rovers. Despite some nota- colonies near geysers, though the planet’s atmosphere to after more than a decade ble failures — Deep Space 2, scientists are loath to make the study its history. It has found

WWW.ASTRONOMY.COM 25 ACTIVE MARS MISSIONS

Launch Name Agency Description 2001 Mars Odyssey NASA An orbiter that studies the surface with spectrometers and a thermal imager. 2003 Mars Express ESA This craft is in a polar orbit performing high-resolution imaging and mapping. 2003 Opportunity NASA A rover that has spent 14 years on the surface, traversing nearly 30 miles (50 km). 2005 Mars Reconnaissance NASA The sharpest eye in the sky above Mars, Orbiter it studies the surface and atmosphere, and it probes beneath the polar caps. 2011 Curiosity NASA This rover is exploring Gale Crater, an ancient lake bed, and Mount Sharp. So far, it has covered about 11 miles (18 km). 2013 Mangalyaan ISRO Also known as the Mars Orbiter Mission, the craft is India’s first to the planet and a test bed for future missions. 2013 MAVEN NASA The Mars Atmosphere and Volatile Evolution mission studies the planet’s atmosphere and its loss over time.

Olympus Mons, the solar system’s largest volcano, rises 16 miles 2016 ExoMars ESA/Roscosmos This orbiter-lander combo was intended (25 km) above the surrounding plains. In 2004, Mars Express to study biological conditions. Although captured this view of its summit caldera, which spans some the lander crashed, the Trace Gas Orbiter remains operational. 63 miles (102 km). ESA/DLR/FU BERLIN (G. NEUKUM)

will probe the planet’s interior, hoping to learn whether there’s any seismic activity indicating that Mars is still an active world. What remains of the European and Russian ExoMars program will carry out some of the most intensive tests for present habitability on Mars. And NASA’s Mars 2020 rover may include a sample return, bringing martian soil to Earth. What’s desperately needed is a new orbiter. The orbiters that serve as primary points of contact between Mars’ surface and Earth are 13 and 17 years NASA/JPL-CALTECH/UNIVERSITY OF ARIZONA OF NASA/JPL-CALTECH/UNIVERSITY old, and they are not guaran- Mars’ moon Phobos is tiny — 16 miles (26 km) across in its longest The Spirit rover discovered these nodules of opaline silica dimension — but it’s full of craters. The largest, Stickney (lower in Gusev Crater, near a plateau called Home Plate. On Earth, teed to survive far into the right), spans 5.6 miles (9 km). Scientists think tidal forces one day biological processes form similar structures near active geysers next decade. will tear Phobos apart when it ventures too close to Mars. and hot springs. NASA/JPL/CORNELL All of these explorations are helping scientists build a comprehensive — and sometimes that Mars likely lost its mag- Besides the discovery of debris launched into orbit after puzzling — picture of Mars. In netic field long ago. Because of water and atmospheric loss, an asteroid impact with the some ways, we were already this, the solar wind had unlim- planetary probes have turned planet. In addition, it appears assembling our present view of ited access to strip away much up other interesting bits about Phobos one day will come Mars at Astronomy’s founding of the atmosphere. When that Mars in the past 45 years. For close enough to Mars to get in 1973. But in so many other happened, some of the surface instance, while Mars’ two ripped apart and briefly form ways, we were just scratching water froze underground, but small moons, Phobos and a ring around the planet. the surface. most of it turned into vapor Deimos, were once thought to The next few years will and escaped the planet entirely, be captured asteroids, some deepen our understanding of John Wenz is a science writer in turning it from a water world scientists now suspect they Mars. When the InSight lander Madison, Wisconsin, and a former into a cold desert. may have coalesced from arrives at Mars late this year, it Astronomy associate editor.

26 ASTRONOMY • AUGUST 2018 2 BIGGEST THINGS THAT CHANGED ASTRONOMY

Cygnus X-1 is a very bright X-ray source in our galaxy and one of the first widely accepted black hole candidates. In this artist’s concept, SHINING LIGHT ON BLACK HOLES the black hole (right) is shown stealing gas from its companion star. It took 200 years to find the first black hole. Now we know NASA/ESA/HUBBLE/M. KORNMESSER they’re everywhere. by Francis Reddy

n 1937, an astronomi- The other source, Cygnus A, active galaxies like Cygnus A astronomers. Although unde- cally inclined electrical is a galaxy more than 750 mil- could emit such vast amounts tectable by its own light, he engineer named Grote lion light-years away. Reber of energy, and finally the dis- noted, the object could be Reber built a homemade had made the first observation covery of small stellar-mass found by observing irregulari- radio dish out of lumber of a black hole, one more than black holes in our home galaxy ties in any “luminous bodies” Iand sheet metal in his back- 2 billion times the mass of the and beyond. that happened to revolve yard in Wheaton, Illinois. The Sun. Astronomers now think Although the term black around it. Next, France’s following year, he confirmed such enormous “supermassive” hole wasn’t coined until 1967, Pierre-Simon Laplace indepen- Karl Jansky’s 1931 discovery of black holes reside in the cen- when American physicist John dently postulated the existence radio waves emanating from ters of most big galaxies, A. Wheeler first mentioned it of black holes in a book pub- the center of the Milky Way. including our own. Recently, in a talk, the quest for these lished in 1796. Over the next few years, observations of Cygnus A enigmatic objects began much Fast forward to late 1915, Reber extended his research of using the Very Large Array in earlier. when Albert Einstein pub- what he called “cosmic static,” New Mexico revealed another In May 1783, the English lished the defining paper on eventually publishing sky bright radio source — possibly scientist-turned-clergyman his general theory of relativity. maps showing prominent another giant black hole — John Michell envisioned a star A few weeks later, fellow radio sources in the constella- near the galaxy’s center. so large that its escape velocity German physicist Karl tions Cassiopeia and Cygnus. Our growing understanding equaled the speed of light. Schwarzschild published an The first, called Cassiopeia A, of black holes has unfolded He wrote that “all light emitted exact solution to Einstein’s new is the brightest extrasolar along three distinct but related from such a body would be relativity equations, which radio source in the sky, now tracks: first through efforts to made to return towards it, revealed the radius that a given known to be a young super- understand gravity itself, then by its own proper gravity,” mass would need to be com- nova remnant. explanations of how so-called rendering the star invisible to pressed to before collapsing

WWW.ASTRONOMY.COM 27 1989

3,000 light-years 2015

ZOOMING IN ON CYGNUS A

Radio data from the Very Large Array (VLA) was used to construct this image of Cygnus A, the brightest extragalactic radio source in the sky. Long, thin particle jets produced by a 2.5 billion-solar-mass black hole in the galaxy connect to the two extended lobes, where speeding electrons are trapped in magnetic fields and emit radio waves. From tip to tip, the structure spans nearly half a million light-years. Left inset: VLA observations in 2015 show a brightening source not present in 1989, possibly a flare-up by a second supermassive black hole. Right inset: A color image of the central galaxy constructed from Hubble and Keck data. NRAO/AUI; PERLEY, ET AL., NRAO/AUI/NSF; R.A.E. FOSBURY, ET AL. 8,500 light-years

into a black hole. This neutron star at the moment is astronomers to determine the Cygnus X-1. Located about “Schwarzschild radius” marks the pulsar PSR J0348+0432, masses of both members of the 6,100 light-years away, the the event horizon — the point which tips the scales at 2.01 system. If the mass of the com- binary system is made up of a of no return, from which noth- solar masses. We know now pact object exceeds three Suns, 15-solar-mass black hole orbiting, not even light, can escape that when a massive star runs then it must be a black hole. ing the common center of — for a non-rotating black out of fuel and collapses under Most of these systems mass it shares with a hot, blue hole. Half a century later, the its own weight, the resulting remain dormant for decades O-type supergiant, which is New Zealand mathematician supernova crushes the stellar while gas streaming from the estimated at 25 or more solar Roy Kerr described an exact core to extreme densities. If the companion star slowly accu- masses. The star’s surface is so solution for rotating versions mass of the core falls some- mulates into a storage disk hot it continuously sheds of these gravitational beasts. where between around the black plasma into space, creating a In 1934, astronomers Walter 2.01 and 3 solar hole. Eventually, fast-moving outflow called a Baade and Fritz Zwicky sug- masses (exactly A mountain the disk stellar wind. As the black hole gested that supernova explo- where depends on of astronomical becomes unsta- orbits the star, it sweeps up a sions represented the unknown details ble, and gas small part of this outflowing transformation of a normal of neutron star evidence shows begins spiraling gas, which produces X-rays star into a neutron star, an structure), then a that black holes inward. Friction when the gas falls toward the object with the Sun’s mass, but stellar-mass black are here to stay. heats the gas up black hole. crushed into a sphere the size hole is born. to millions of The closest confirmed black of Manhattan. A few years Confirmed degrees so it hole is A0620–00, about 2,800 later, physicists J. Robert stellar-mass black holes exist in glows in X-rays before plung- light-years away, but it’s likely Oppenheimer and George 20 or so X-ray-emitting binary ing past the event horizon. there are many others much Volkoff, using work by physi- systems found in our galaxy Outbursts like this typically closer that we haven’t yet cist Richard Tolman, showed and its neighbors. In these sys- last about a year before the found. Astronomers estimate that if neutron stars become tems, a normal companion star system returns to dormancy. that our Milky Way may hold too massive, they must con- orbits close to the black hole. A few systems persistently as many as 100 million stellar- tinue to collapse. Observing the orbital motions produce X-rays, and the most mass black holes. “That means, The heaviest measured of the companion allows famous and best studied is with high probability, there is a

28 ASTRONOMY • AUGUST 2018 HOW DO BLACK HOLES STACK UP TO THEIR COMPANION STARS?

These illustrations show accurate Companion distances, sizes, and inclinations for Mercury Sun star the accretion disks and companion stars in 11 black hole binaries in our Accretion disk own and neighboring galaxies. In and black hole these systems, the black hole mass has been dynamically confirmed through observations of the companion’s motion. The system sizes are largely determined by the orbital period, which ranges from GRS 1915+105 LMC X–3 33.9 days (GRS 1915+105) to 0.2 day (XTE J1118+480). M33 X-7, LMC X-1, and Cygnus X-1 are all persistent X-ray sources, where the black hole is fed by stellar winds from a massive companion star. The others illustrated are transient sources seen only during outbursts, which typically last about a year and Cygnus X–1 LMC X–1 M33 X–7 recur over decades. About 40 more are classified as containing black hole candidates, but because they lack dynamical mass measurements, some could contain neutron stars. ASTRONOMY: ROEN XTE J1550–564 GX 339–4 GRO J1655–40 XTE J1118+480 A0620–00 KELLY AFTER J. A. OROSZ GS 2023+338

Cygnus X-1 is the black hole within about 20 distance. STIS measured a brighter (bottom) of light-years of Earth — we just gas velocity of 880,000 mph the two points of light don’t know where,” says (1.4 million km/h) within near the center of Jeremy Schnittman, an astro- 26 light-years of the galaxy’s this image. Part of its companion star’s physicist at NASA’s Goddard center. From this motion, stellar wind is swept Space Flight Center in astronomers calculated that up by the black hole. Greenbelt, Maryland. Unless the black hole at the heart of Before it falls in, the black holes are “fed” with mat- M84 contains at least a whop- gas becomes so hot it ter that emits light as it heats ping 300 million solar masses. emits X-rays. A small fraction of the gas is up on the way in, they can be Since 2015, striking new evi- redirected into particle hard to find. dence for the existence of black jets that shoot away Hard, but not impossible. holes has come from the Laser from the black hole. Over the past two decades, Interferometer Gravitational- Jet-powered The bluish umbrella- bow shock like shape at top teams using the Keck telescope wave Observatory (LIGO), center is a bow shock in Hawaii and the European which so far has detected caused by material in Southern Observatory’s Very nearly half a dozen merging the jet colliding with Large Telescope in Chile have black hole pairs. Large orbiting interstellar gas. T.A. RECTOR (UNIVERSITY OF ALASKA mapped the orbits of massive masses lose energy by creating ANCHORAGE) AND H. SCHWEIKER stars in the center of our gal- ripples in space-time called (WIYN AND NOAO/AURA/NSF) axy. These stars loop around a gravitational waves, yet another common but invisible center of prediction of Einstein’s relativ- mass 4 million times that of ity. And as they lose energy, the Sun. Other evidence indi- binary black holes draw closer cates that a radio source together over millions of years known as Sagittarius A* and until they coalesce. the unseen object inferred by From stellar-mass black stellar orbits are one and the holes that pepper our galaxy same — and almost certainly a to their monster brethren that supermassive black hole. drive jets spanning hundreds Peering into the centers of thousands of light-years, of other galaxies provides there’s still much to learn about even more evidence. For exam- these mind-bending objects. ple, in 1997, astronomers But a mountain of astronomi- aligned the slit of Hubble’s cal evidence shows that black Hubble’s Space Telescope Imaging Spectrograph (STIS) captures the motions of gas Space Telescope Imaging holes are here to stay. in the grip of M84’s black hole. Left: The blue outline shows the region at the center Spectrograph (STIS) with the of M84 where STIS acquired data. Right: STIS reveals the rotational motion of stars and center of galaxy M84 in the Francis Reddy is the senior gas. The change in wavelength records whether an object is moving toward (blue) or away from (red) the observer, and the larger the deviation from the center, the greater Virgo cluster, revealing a dra- science writer for the Astrophysics the speed. STIS found a velocity of 880,000 mph (1.4 million km/h) within 26 light-years matic swing in gas motions Science Division at NASA’s Goddard of the galaxy’s center, indicating M84’s black hole weighs at least 300 million solar over a comparatively small Space Flight Center in Maryland. masses. NASA, GARY BOWER, RICHARD GREEN (NOAO), THE STIS INSTRUMENT DEFINITION TEAM

WWW.ASTRONOMY.COM 29 3 BIGGEST THINGS THAT CHANGED ASTRONOMY

After visiting Jupiter and Saturn, Voyager 2 was sent to Uranus and Neptune, completing the Grand Tour. ASTRONOMY: ROEN KELLY

VOYAGER’S SATURN TERMINATION UPITER 8 J SHOCK GRAND TOUR 2 1 HELIOSHEATH 3 4 7 The twin probes explored 6 more planets, discovered 5

more moons, and offered URANUS 9 11 more breaking news than any other spacecraft. by Michael E. Bakich NEPTUNE

VOYAGER 1 March 5, 1979 Closest approach to Jupiter VOYAGER 1 VOYAGER 1 VOYAGER 1 VOYAGER 1 Sept. 5, 1977 Nov. 12, 1980 Dec. 16, 2004 Aug. 25, 2012 Launch Closest approach Crosses the Passes the reach from Earth to Saturn termination shock of the solar wind 23 5 9 11

19771978 1979 1980 1981 1982 19831984 1985 1986 19871988 1989 2004 2005 2006 2007 2008 2009 2010 2011 2012

1 4 678 10 VOYAGER 2 VOYAGER 2 VOYAGER 2 VOYAGER 2 VOYAGER 2 VOYAGER 2 Aug. 20, 1977 July 9, 1979 Aug. 25, 1981 Jan. 24, 1986 Aug. 24, 1989 Aug. 30, 2007 Launch Closest approach Closest approach Closest approach Closest approach Crosses the from Earth to Jupiter to Saturn to Uranus to Neptune termination shock

hen NASA accomplish all this, engineers become early history. As of image, navigation engineer launched built into them a generous (for 2018, both Voyagers have fin- Linda Morabito discovered a Voyager 1 the 1970s) five-year lifetime. ished their fourth decade of feature along Io’s edge. What and Voyager At Jupiter, and then Saturn, operation — and they show she initially thought was a 2 in the the mission achieved far more no signs of stopping. moon turned out to be a plume Wsummer of 1977, its engineers than its original objectives. from an active volcano. were sending the spacecraft on Then came the big news: By One surprise after another Planetary geologists subse- specific missions. Originally, carefully tweaking Voyager 2’s The Voyager spacecraft made quently learned Io’s interior is the space agency tasked the flight path, flybys of Uranus enough discoveries to fill this in turmoil: Jupiter’s gravity Voyagers with conducting and Neptune were possible. magazine — and we did just stretches it differently depend- close-up studies of Jupiter and The two-planet addition that for our October 2017 issue ing on how far the moon is Saturn. They would compile became the Grand Tour. The — but most scientists would from the planet. Such an inter- data on magnetic fields, the projected lifetimes stretched view the following as the top 10. action creates intense heating Sun’s influence, Saturn’s rings, to 12 years for the Neptune 1) Volcanoes on Jupiter’s due to friction. The result is a few large moons, and send encounter August 24, 1989. moon Io. This was the biggie. that Io has 100 times as much back lots of great images. To And that date would, in turn, While processing a Voyager 1 volcanic activity as Earth.

30 ASTRONOMY • AUGUST 2018 The Great Red Spot is a spinning anti-cyclone in Jupiter’s southern hemisphere. At the time Voyager 1 snapped this close-up of swirling clouds, the Great Red Spot was three and a half times the size of Earth. NASA/GODDARD SPACE FLIGHT CENTER

Above: As Voyager 1 flew by the jovian moon Io, it captured this image of an active plume (left edge, bluish white) coming from Loki, a volcano then on Io’s limb, from 340,000 miles (490,000 km) away. The dark heart-shaped feature near the bottom shows fallout deposits from the active plume Pele. NASA/JPL/USGS

This Voyager 2 image is just one of hundreds of high-resolution views of Saturn’s rings. Below: Voyager 2 revealed Europa’s surface to be devoid of mountains or craters as the The probe took this shot from a range of 2 million miles (3.3 million kilometers). At spacecraft flew by the jovian satellite July 9, 1979. The main feature it did show was a lower right, you can see the planet through the rings. The Cassini Division is the dark network of crisscrossing streaks. The lines are where warmer ice broke through the gap that extends from lower center to upper left. NASA/JPL colder surface when tidal forces from Jupiter and its other large moons cracked the outer layer of the moon. NASA/JPL/TED STRYK

2) Jupiter’s turbulent moon and Jupiter. But Europa’s atmosphere. After watching orbit is closer to circular than the giant planet’s cloud bands Io’s, so the internal heating and Great Red Spot from afar isn’t enough to create volca- for three centuries, scientists noes — just enough to melt got their first up-close look vast quantities of ice. with Voyager 1. They saw doz- 4) The Io torus. Voyager 1 ens of interacting hurricanes, found a thick ring of ionized some as large as planets. And sulfur and oxygen shed by Io the Red Spot itself displays that inflates Jupiter’s giant layers of complex activity. magnetic field. The material It lies 5 miles (8 kilometers) originates within the moon’s above the surrounding clouds, volcanoes, some of which are and time-lapse movies so powerful that they erupt it confirmed its counterclock- directly into space. wise rotation. 5) Saturn’s ring structure. 3) An ocean within Before 1980, astronomers rec- Europa? As the two spacecraft ognized fewer than six rings flew by the fourth-largest around Saturn. But Voyagers’ jovian moon, its icy crust cameras showed that each ring showed a dizzying series of had numerous subdivisions. In intersecting cracks. addition, Voyager 1 discovered Calculations indicated the pos- that the enigmatic F ring has sibility of a liquid ocean deep two small “shepherding” satel- beneath the ice. Such a feature lites, Pandora and Prometheus, likely exists because of the whose gravity keeps the ring tidal interaction between the in place.

WWW.ASTRONOMY.COM 31 This Voyager 2 photograph of Titan, taken August 23, 1981, from a range of 1.4 million miles (2.3 million km), shows some detail in the cloud systems on this saturnian moon. Titan’s southern hemisphere appears lighter than its northern half in this image. It also shows a dark collar near the north pole. This large moon’s atmospheric circulation is responsible for forming these bands. NASA/JPL

Astronomers created this image of Neptune from the last whole-planet exposures taken with the Voyager 2 narrow angle camera. The picture shows the Great Dark Spot and its companion bright smudge in the center, the fast-moving bright feature called “Scooter,” and the little dark spot at lower left. NASA

6) Titan’s atmosphere. much energy as it receives Voyager 1 showed that Titan from the Sun, researchers has a nitrogen atmosphere with think the decay of radioactive a surface pressure 45 percent elements deep within Neptune greater than on Earth. Voyager powers the currents. data hinted at the possibility 9) Geysers on Triton. In (later confirmed) that this sat- addition to observing clouds ellite experiences clouds of and hazes in the thin atmo- methane and other hydrocar- sphere of Neptune’s largest sat- bons, and that rain falling from ellite, Voyager 2 found evidence Voyager 2 took this global color mosaic of Triton, Neptune’s largest moon. It is one those clouds creates lakes of of cryovolcanoes — otherwise of only three objects in the solar system with an atmosphere of mainly nitrogen. (The others are Earth and Titan.) But this moon is so cold (–391 degrees Fahrenheit) liquid methane on the surface. known as ice volcanoes. These that most of the nitrogen has condensed on the surface as frost. NASA/JPL/USGS 7) The Great Dark Spot. active geysers within the As Voyager 2 approached moon’s southern polar cap Neptune, scientists identified spew dust-laden nitrogen up to followed in 2007. As each among the stars and the a gigantic dark feature. It was 5 miles (8 km) above the sur- spacecraft crossed the helio- boundary between the Sun’s dubbed the Great face, which lies pause, their Voyager Interstellar influence and interstellar Dark Spot, and in perpetual Mission commenced. space. Communications will be researchers were More than cold at a tem- maintained until the Voyagers’ at a loss to 40 years after their perature of A new horizon power sources no longer can explain how such 37 kelvins As of February 8, 2018, run critical subsystems. a storm could launches, Voyager 1 (–393 degrees Voyagers 1 and 2 are 13.16 bil- form given the and Voyager 2 Fahrenheit). lion miles (21.18 billion km) A legacy of discovery small amount of continue to go 10) The edge and 10.91 billion miles The grand tour of the solar energy Neptune where no one of the solar (17.56 billion km) from Earth, system (and beyond) continues. receives from the system. The respectively. And the craft are The primary explorers are two Sun. Further has gone before. Voyager space- still making news. In 2011, workmanlike spacecraft that study showed the craft didn’t stop Voyager 1 crossed into a zone achieved the goals scientists set Great Dark Spot, working after astronomers call the stagna- before them, far surpassed their and similar features observed their planetary encounters. tion region. There, at the planned life spans, and adapted since Voyager 2 passed by, are In 2014, Voyager 1 passed an boundary of interstellar space, to new expectations by evolving cyclones that exist as holes in important boundary within the solar wind is less intense, technologically. Indeed, more the planet’s upper atmosphere. our solar system called the but the magnetic field mea- than 40 years after their 8) Neptune’s supersonic heliopause. This is where the sures twice as strong. launches, Voyager 1 and winds. The discovery of the strength of the solar wind isn’t Voyager 1 is leaving the solar Voyager 2 continue to go where fastest winds in the solar sys- powerful enough to overcome system at about 320 million no one has gone before. tem in the atmosphere of the the stellar winds of nearby miles (520 million km) per most distant planet was a stun- stars. Voyager 1 crossed year. Meanwhile, Voyager 2 is Michael E. Bakich, a senior ner. Voyager 2 measured wind another border, the termination exiting at about 290 million editor of Astronomy, remembers speeds of 1,100 mph (1,600 shock, where the solar wind miles (470 million km) per year. the Voyager missions and the km/h) above Neptune. Because abruptly slows to subsonic Both spacecraft continue impact they had on visitors to the the planet radiates 2.6 times as speed, back in 2004. Voyager 2 to study ultraviolet sources planetariums where he worked.

32 ASTRONOMY • AUGUST 2018 4 BIGGEST THINGS THAT CHANGED ASTRONOMY

Gravitational waves

Time Cosmic microwave background

Big Bang

Protons Nuclear fusion Nuclear fusion Neutral Modern Inlation form begins ends hydrogen universe forms Diameter of the observable universe Diameter

0 10-32 s 1 +s 0.01 s 3 min 380,000 13.8 Age of the universe years billion years

Inflation took place a fraction of a second INFLATION LEAVES ITS MARK after the Big Bang, well before the signal from the cosmic microwave background (CMB). This process dramatically Alan Guth’s remarkable theory provides a master key to the universe increased the size of the universe in an incredibly short amount of time, both we see today. by Mara Johnson-Groh magnifying local density fluctuations and diluting the universe’s temperature profile to create the smooth CMB we observe. Other evidence of inflation is anticipated to come from identification of the gravitational waves produced during inflation, which should have a ust a trillionth of a tril- evidence remains scant, and characteristic polarization (or pattern). lionth of a trillionth of a some scientists are raising ASTRONOMY: ROEN KELLY, AFTER BICEP2 COLLABORATION second after the Big doubts about its legitimacy. Bang, the infant universe underwent a titanic The birth of a theory all the magnetic monopoles? Jgrowth spurt. In the In 1980, Alan Guth, a theoreti- In an elegant solution, Guth briefest of moments, the cal physicist and cosmologist at proposed that the early uni- universe swelled 1026 times — Cornell University, was verse, where monopoles were that’s 100,000,000,000,000,000, attempting to solve fundamen- common, had expanded rap- 000,000,000 — the equivalent tal problems left unaddressed idly, scattering the monopoles of expanding an apple to over by the Big Bang theory and across such a wide space that 100,000 times the size of the modern quantum mechanics. they became incredibly rare Milky Way. On Earth, we know all mag- — so rare that we could search This brief but dramatic nets, and indeed the planet endlessly without ever encoun- epoch is what astronomers call itself, have two poles: north and tering one. Coining the theory Alan Guth developed inflation theory in 1980 to address issues with both the inflation. As a theory, it has south. Yet some particle physics “cosmic inflation,” Guth current Big Bang theory and quantum had marked success in theories suggest that magnets resolved the monopole prob- mechanics. BETSY DEVINE recounting the universe’s with only one pole — mono- lem and simultaneously solved infancy and describing the poles — should exist. But if the two other major issues with cosmos as we see it today. Yet theories are correct, where are the Big Bang theory.

WWW.ASTRONOMY.COM 33 Rapid expansion WHAT SHAPE IS SPACETIME? Inlation era 1040 10-35 to 10-32 Although it’s impossi- >1 ble to actually picture, 0 1030 the universe can be 20 10 Standard model simplified as a two- 1010 dimensional surface in three dimensions: 10-10 Imagine space-time as 10-20 a plane that can be -30 curved into a sphere <1 10 Inlationary model 0 Now or a saddle, or laid flat. 10-40 The shape of the -50

Radius of the universe (meters) Radius of the universe 10 universe is deter- -60 10 mined by the total 10-45 10-35 10 -25 10 -15 10-5 105 1015 density of matter and Time (seconds) energy it contains. At a critical density, The universe’s inflationary period began roughly 10-35 second after the Big Bang and space-time becomes lasted until about 10-32 second after the Big Bang. During this short period, the size of 26 not curved, but flat. the universe jumped by a factor of about 10 . ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY =1 (If you’re curious, this 0 value is 10-26 kg/m3, or 10 hydrogen atoms The seeds of massive clusters per cubic meter.) The density param-

Wide-ield Infrared Survey Explorer WIYN/Subaru eter, Ω0, is the ratio of the average density of matter and energy in (WISE) image Telescope image the universe to the critical density. If Ω0 is greater than 1, the universe has sufficient matter to exert enough gravity and eventually slow its expansion, ultimately contracting again. This is the case for a positively curved universe, which can be pictured as a sphere. A triangle drawn on this surface would have internal angles that add up to more than 180°.

If Ω0 is less than 1, the universe does not have enough matter to stop its expansion, and will continue to expand forever. In this case, space-time can be pictured as saddle-shaped, and a triangle drawn on the surface would have internal angles that total less than 180°.

When Ω0 equals 1, the universe has exactly enough matter to NASA/JPL-CALTECH/UCLA/WIYN/SUBARU eventually cause it to stop expanding, but only after infinite time. In this case, space-time is flat, and the sum of a triangle’s angles is always 180°. This is the case for our universe. Based on astronomers’ Inflation scaled up tiny density fluctuations in the very early universe, creating larger best measurements to date, the density of matter and energy in our areas that have continued to gather mass due to their greater density as the universe ages. Identifying distant, massive galaxy clusters allows astronomers to test the universe makes it indistinguishable from the flat scenario — to the predictions set forth by inflation; one such cluster is MOO J2342.0+1301, which sits best of our ability to measure, the density equals exactly the critical 7.7 billion light-years away, when the universe was only half its current age. density, and the universe is flat. — Alison Klesman

The density of energy and How did it happen? the universe is so uniform. If showed that a virtually uniform matter in the universe dictates While the cause of inflation is you gaze skyward with a sensi- universe, post-inflation, would whether space-time, the fabric uncertain, the leading theory tive radio telescope, you’ll create the signature of the CMB of our universe, is flat or postulates it occurred because of encounter the soft glow of the almost exactly. curved. Remarkably, in our an extremely high-energy state first light emitted — the cosmic Now, the CMB isn’t per- universe this density is just in the early universe. This state microwave background (CMB) fectly smooth. Across the precisely the specific value, was left over after the separation radiation — spread almost universe, there are small called the critical density, that of the fundamental forces (grav- evenly across the sky in every deviations, measured as slight makes it flat instead of curved ity, electromagnetism, and the direction. The fact that this differences in temperature, — something that cosmolo- weak and strong nuclear forces), radiation could be nearly uni- typically just a millionth of a gists have a hard time believ- which were unified when the form in such disparate regions degree. These changes origi- ing happened at random. universe began. The extreme long puzzled cosmologists nated with tiny quantum fluc- However, if the universe amount of energy left the uni- because the temperature of the tuations in the early universe expanded rapidly, as inflation verse unstable, and as it settled universe — already 90 million and would eventually become proposes, any curvature that to a lower, more stable state, the light-years across — couldn’t the backbone of galactic struc- had existed prior to inflation surge of released energy caused have equalized to a uniform ture in the cosmos. would now be diluted, creating the remarkable expansion. value by the time the light was Back in the brief moments the flat universe we see today. Inflation also explains why emitted. Guth’s theory neatly before inflation, the universe

34 ASTRONOMY • AUGUST 2018 THE SPIDER COLLABORATION; THE SPIDER COLLABORATION AND THE BRITISH ANTARCTIC SURVEY ANTARCTIC BRITISH THE AND COLLABORATION SPIDER THE COLLABORATION; SPIDER THE The SPIDER balloon-borne instrument (left) was launched from Antarctica and carried six telescopes for over two weeks to look for a characteristic signal left by gravitational waves associated with inflation, which should be present in the cosmic microwave background. It was recovered in February 2015 (right); data analysis continues. was hot and dense. While at Princeton University. “We elapsed since inflation; simi- have to go back to the drawing energy was evenly distributed, can trace from the early times larly, laboratory experiments board and very seriously on a microscopic scale, quan- to the later times, and we’re can’t produce the high levels of rethink inflation.” tum mechanics dictated that seeing a very consistent story.” energy needed to simulate con- In the past four decades, random spots have slightly Today, this consistency ditions during inflation. inflation has become a central higher or lower densities. As between the variation in the However, gravitational waves pillar around which cosmology inflation shook the universe, CMB and the large-scale struc- associated with the universe’s is organized. But while it tidies those tiny over- and under- tures of galaxies across the uni- rapid expansion, similar to up outstanding problems with densities supersized into larger verse is one of the few pieces of those triggered by colliding the Big Bang theory, it remains regions. And as the universe observational evidence for black holes and neutron stars, unverified. And the lack of evolved, the denser regions inflation found so far. could offer evidence, if found. evidence leaves some scientists became galaxy clusters as they “With inflation, the number While detectors like the with reservations. slowly accumulated more and of observational quantities we Laser Interferometer “It’s had incredible success more mass. have is limited,” says Marc Gravitational- at describing “If we take a galaxy survey, Kamionkowski, a professor wave Observatory what we see . . . where we can map out the and theoretical physicist at are insensitive to Inflation remains and people talk positions of where galaxies are Johns Hopkins University. inflationary about it as the and how they’re clustered “Therefore, it limits the level of gravitational cosmologists’ only option, but together across the sky, there’s detail at which the model or waves, other best theory many theorists a remarkable agreement in the theory can be specified.” searches are to explain the think that we distribution of galaxies after underway. One current structure should be con- billions of years and the pre- Searching for signs such search uses sidering other diction that you get right at the Finding evidence in the cos- the Background and infancy options for what beginning of the universe,” mos is challenging because of Imaging of of our universe. happened in the says Jo Dunkley, a cosmologist the length of time that has Cosmic early universe,” Extragalactic Dunkley says. Polarization (BICEP) instru- “I think it’s unlikely that it ments, which have been oper- happened exactly the way ating from the South Pole since we’ve been thinking about it. I 2006. Another, dubbed don’t think we’ve got the whole SPIDER, sends balloons high story yet.” into the atmosphere to look for For now, inflation remains polarized gravitational waves. cosmologists’ best theory to “Theory predicts [an infla- explain the current structure tionary] gravitational wave and infancy of our universe. signature should be detectable Perhaps it will be another half- in the next five to 10 years. If century before scientists have a we detect a signal, it provides clearer understanding of what an additional data point that happened in the universe’s The BICEP2 telescope at right (with the South Pole Telescope at left) operated from the suggests we’re on the right first breaths. Dark Sector Laboratory at the Amundsen-Scott South Pole Station from 2010 until 2012. track,” Kamionkowski says. “If BICEP3, now in its place, is collecting data on the cosmic microwave background to Mara Johnson-Groh search for hallmarks of inflation imprinted on this signal by the gravitational waves it 10 years from now we haven’t is a spurred. Although evidence of inflation based on BICEP2 observations was announced found these inflationary gravi- freelance science writer and in 2014, the signal has since been attributed to cosmic dust. AMBLE (WIKIPEDIA) tational waves, we’re going to frequent contributor to Astronomy.

WWW.ASTRONOMY.COM 35 SKYTHIS Visible to the naked eye MARTIN RATCLIFFE and ALISTER LING describe the Visible with binoculars MONTH solar system’s changing landscape as it appears in Earth’s sky. Visible with a telescope

August 2018: Prime time for the Perseids

Jupiter lies in Libra near magnitude 2.8 Zubenelgenubi (Alpha [α] Librae). The giant world begins the month 1.4° northwest of this star, but the planet’s eastward motion carries it to a point 0.6° due north of it on the 14th. By month’s end, Jupiter lies 1.9° east of Alpha. Shining at magnitude –2.0 in mid-August, the planet appears nearly 100 times brighter than the star. Plan to observe Jupiter through a telescope while it lies highest near the end of twilight. The planet’s large A Perseid fireball blazes across a partly cloudy sky at the peak of the 2013 shower. Moon-free circumstances apparent size (36" at mid- promise nearly perfect viewing conditions for this year’s Perseids. JAMIE COOPER month) affords great views of its dynamic atmosphere. he longer nights of late hour after sundown August 1. diameter grows from 21" to 29" Two dark equatorial belts, one summer mean more It appears 2° lower when the while its phase wanes from on either side of a brighter time for observing, and waxing crescent Moon passes 57 to 40 percent lit. zone that coincides with Tthat’s great news this by on the 13th and 14th. Luna As Venus dips low in the Jupiter’s equator, stand out. year. Venus, Mars, appears 10° to Venus’ right on west, Jupiter, Saturn, and Mars Finer details come into view Jupiter, and Saturn will dazzle the 13th and 7° above the (in that order) appear to its through larger scopes or dur- evening viewers whether they planet the following night. east. The span of the four plan- ing moments of steady seeing. watch with their naked eyes or Venus reaches greatest elon- ets shrinks from 127° in early As you view Jupiter, you’ll through a telescope. And the gation August 17, when it lies August to 95° at month’s end. also see up to four moons early morning hours provide 46° east of the Sun. You might some of the year’s best views of think the planet would appear

Mercury. But the month’s pre- highest then, but you’d be The Moon passes Venus mier event arrives the night of wrong — Venus stands only 7° BOÖTES August 12/13, when Moon-free high an hour after sunset. The SCORPIUS skies promise the best display problem is that the ecliptic, the Arcturus of Perseid meteors in years. apparent path of the Sun and If you look west as twilight planets across the sky, makes a LIBRA deepens, you can’t miss Venus. shallow angle to the western Earth’s neighbor brightens horizon after sundown in Jupiter VIRGO from magnitude –4.3 to –4.6 late summer. How much differ- this month and appears prom- ence does this make? Venus Spica Moon Denebola inent a half-hour after sunset. appeared more than twice as The planet moves eastward high in early June when it was LEO against the backdrop of Virgo, only 35° from the Sun. Venus CENTAURUS CORVUS pulling closer to 1st-magnitude Despite its low altitude, Spica. It approaches within 1.3° Venus sports two distinct 10° of this star at month’s end. advantages over its late spring Northern Hemisphere appearance. Not only does it August 14, 45 minutes after sunset observers won’t see Venus shine brighter than it did in Looking west-southwest climb high. From 40° north early June, but it also looks latitude, the planet stands 10° more appealing through a tele- The brilliant planet lies farthest from the Sun on August 17, but viewers above the western horizon an scope. In August, the planet’s shouldn’t miss its date with Luna on the 14th. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY

36 ASTRONOMY • AUGUST 2018 RISINGMOON A southern treat at First Quarter Maurolycus

The Moon slinks low across the The large crater Maurolycus Gemma southern sky on summer eve- resides in the southern third of Frisius nings. It often shows a color that the lunar disk. Named after the can range from pale yellow to 16th-century Italian mathemati- orange. The warm color arises cian Francesco Maurolico, who because its light takes a longer disagreed with Copernicus’ the- path through Earth’s atmosphere, ory of a Sun-centered solar sys- Maurolycus which scatters out shorter wave- tem, the crater spans 71 miles. lengths, and occasionally passes The impact that carved out this through smoke from late sum- beauty fractured the lunar crust. N mer forest fires. Lava later seeped up and paved Plan to target Luna on the the floor around its central E evening of August 17. The First peaks. The event partially wiped Quarter Moon then displays a out a crater to its immediate This 71-mile-wide crater appears prominent in the southern highlands breathtaking series of seas in its south and one to the northwest. the evening of August 17. CONSOLIDATED LUNAR ATLAS/UA/LPL; INSET: NASA/GSFC/ASU north. After enjoying these lava- More recent impacts left filled basins, scan southward sharper craters and softened northwestern edge? Your obser- As the Moon approaches its along the craters that line the earlier ones. The freshest are the vations on the 17th and subse- Full phase on the 26th, notice terminator — the dividing line small craterlets on Maurolycus’ quent evenings will help set the that a ray from Tycho Crater between lunar day and night floor. Can you figure out the timeline straight. A similar story crosses Maurolycus. Use high that nearly bisects the Moon sequence of impacts that cre- plays out on the crater Gemma magnification to pick out this this evening. ated the group on the crater’s Frisius a short hop to the north. subtle stripe. arrayed around the planet. METEOR The jovian satellites change WATCH positions noticeably within an hour or two, but the most A New Moon means Perseid meteor shower exciting times to observe are when one of them passes in great Perseid viewing front of the planet and casts Polaris its shadow on the cloud tops. Most observers rank the Perseid ARIES Dozens of such events meteor shower as the best of the Radiant year. This annual treat offers a con- occur in August, but perhaps PERSEUS the most appealing for North sistently high rate of meteors, pro- American observers happens duces a high percentage of bright on the 23rd. That evening ones, and takes place in August when nice weather is common. Pleiades finds both Io and Europa in Capella And this year promises the best front of Jupiter after darkness TAURUS display in three years. The shower falls along the East Coast and peaks the night of August 12/13, AURIGA during twilight across the Aldebaran less than two days after New Moon, Midwest. Io’s shadow first so our satellite won’t shed any touches the jovian cloud tops unwanted light into the night sky. at 10:02 p.m. EDT, 19 minutes The best views will come before 10° before Europa itself moves off dawn on the 13th, when the radi- the planet’s western limb. ant — the point in Perseus from August 13, 2 A.M. Perseid meteors Looking northeast Europa’s shadow appears where the meteors appear to ema- Active dates: July 17–Aug. 24 on the planet’s eastern limb nate — climbs highest. Observers Peak: August 12 starting at 10:34 p.m. EDT. For This year’s finest meteor display Moon at peak: New Moon under a dark sky can expect to see should light up the Moon-free the next 97 minutes, both an average of between one and Maximum rate at peak: sky at its peak the night of 110 meteors/hour shadows transit Jupiter’s disk. two meteors per minute. August 12/13. (From eastern North America, the planet sets during the OBSERVING Venus puts on a brilliant show at dusk when it reaches greatest early part of this double HIGHLIGHT — Continued on page 42 eastern elongation August 17. It then shines at magnitude –4.5.

WWW.ASTRONOMY.COM 37

STAR N

DOME CAMELOPARDALIS

_ a

How to use this map: This map portrays the

PERSEUS sky as seen near 35° north latitude. Located 884 NGC

inside the border are the cardinal directions M81

and their intermediate points. To find M82

NGC 869 NGC stars, hold the map overhead and CASSIOPEIA

orient it so one of the labels matches

NE ¡

the direction you’re facing. The ANDROMEDA

Polaris

stars above the map’s horizon b

NCP

now match what’s in the sky. d

M33

_ a

MINOR

CEPHEUS `

The all-sky map shows URSA

how the sky looks at: `

M31

11 P.M. August 1 a 10 P.M. August 15 b

P.M. b

9 August 31 b

_ DRACO

Planets are shown + c

d _

at midmonth k LACERTA

Deneb

b _

CYGNUS

HERCULES

¡ Vega

PEGASUS M13

d c _

c r ` E a M57 P LYRA a PISCES t ` h VULPECULA + o c M27 f b t _ Enif SAGITTA h M15 ` e DELPHINUS S a _ u a n ( ` e e ¡ c _ l Altair ip EQUULEUS a _ ti c) c _ _ ` _ g SERPENS CAUDA S STAR AQUARIUS b ` MAGNITUDES ` d e AQUILA h OPHIUCHU Sirius d _ 0.0 b M11 3.0 _ 1.0 a SCUTUM i 4.0 b 2.0 5.0 CAPRICORNUS ` M16 M17 d / j + PISCIS Saturn AUSTRINUS M22 STAR COLORS M20 A star’s color depends m h e o M8 on its surface temperature. Mars A The hottest stars shine blue SE b M6 • SAGITTARIUSc a • Slightly cooler stars appear white MICROSCOPIUM • Intermediate stars (like the Sun) glow yellow ¡ M7 h _ Lower-temperature stars appear orange CORONA d g p • ` The coolest stars glow red AUSTRALIS f • _ Fainter stars can’t excite our eyes’ color NGC 6231 • c receptors, so they appear white unless you ` TELESCOPIUM e d use optical aid to gather more light _

_ ARA

38 ASTRONOMY • AUGUST 2018 S Note: Moon phases in the calendar vary

in size due to the distance from Earth e AUGUST 2018 and are shown at 0h Universal Time.

SUN. MON. TUES. WED. THURS. FRI. SAT. MAP SYMBOLS

Open cluster 1234

Globular cluster

URSA MAJOR URSA ` Diffuse nebula

567891011 _

s NW Planetary nebula Galaxy

12 13 14 15 16 17 18 a

b KELLY ROEN :

¡ 19 20 21 22 23 24 25 ASTRONOMY

Mizar

M51 26 27 28 29 30 31

_ BY ILLUSTRATIONS

VENATICI d CANES

NGP Calendar of events

M64 a 1 Asteroid Vesta is stationary, 17 The Moon passes 5° north of

` 7 P.M. EDT Jupiter, 7 A.M. EDT

BERENICES COMA

_ 3 BOÖTES The Moon passes 5° south of Venus is at greatest eastern

Uranus, 5 P.M. EDT elongation (46°), 1 P.M. EDT b

¡ _

¡ 4 18 d Last Quarter Moon First Quarter Moon _ W occurs at 2:18 P.M. EDT occurs at 3:49 A.M. EDT

The Moon passes 1.2° north of Mercury is stationary, 8 A.M. EDT

BOREALIS CORONA

Arcturus asteroid Juno, 7 P.M. EDT 21 The Moon passes 2° north of

` 6 The Moon passes 1.1° north of Saturn, 6 A.M. EDT

c Aldebaran, 3 P.M. EDT 23 The Moon is at apogee (252,119 7 Asteroid Pallas is in conjunction miles from Earth), 7:23 A.M. EDT

_ A.M. SERPENS with the Sun, 9 EDT CAPUT The Moon passes 7° north of VIRGO P.M. P.M. S M5 Uranus is stationary, 5 EDT Mars, 1 EDT 8 26 b Mercury is in inferior Full Moon occurs at conjunction, 10 P.M. EDT 7:56 A.M. EDT ¡ ` 10 The Moon is at perigee Mercury is at greatest western Jupiter (222,500 miles from Earth), elongation (18°), 5 P.M. EDT c a _ 2:07 P.M. EDT LIBRA 27 The Moon passes 2° south of 11 New Moon occurs at Neptune, 6 A.M. EDT ` 5:58 A.M. EDT; partial 28 A.M. b solar eclipse Mars is stationary, 6 EDT

/ m 30 m SPECIAL OBSERVING DATE The Moon passes 5° south of ntares Uranus, 11 P.M. EDT A _ M4 12 The Perseid meteor o SW shower peaks under ideal, Moon-free skies. SCORPIUS ¡ 14 The Moon passes 6° north of + LUPUS Venus, 10 A.M. EDT d a

BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT www.Astronomy.com/starchart.

WWW.ASTRONOMY.COM 39 PATH OF THE PLANETS The planets in August 2018

Objects visible before dawn LYN PER AND LAC AUR UMa CYG Mercury appears brightGEM LMI during morning twilight TRI in late August ARI LEO PEG VUL P ath o f the CNC Sun SGE Uranus (ec PSC EQU DEL Hebe Juno lipt TAU ic) Mercury Sun CMi AQL Celestial equator SEX ORI Neptune AQR MON HYA CET ERI CAP

CMa Pluto LEP PsA PYX FOR Mars ANT SCL MIC COL PUP CAE PHE VEL HOR

Moon phases Dawn Midnight

13 12 11 10 9 8 7 6 5 4 3 2 1

31 30 29 28 27 26 25 24 23

The planets These illustrations show the size, phase, and orientation of each planet and the two brightest dwarf planets at 0h UT for the dates in the data table at bottom. South is at the top to match the view through a telescope. in the sky Mars Jupiter

Saturn Venus Mercury Uranus

WE Pluto N Ceres Neptune 10"

Planets MERCURY VENUS MARS CERES JUPITER SATURN URANUS NEPTUNE PLUTO Date Aug. 31 Aug. 15 Aug. 15 Aug. 15 Aug. 15 Aug. 15 Aug. 15 Aug. 15 Aug. 15 Magnitude –0.7 –4.4 –2.5 8.8 –2.0 0.3 5.8 7.8 14.2 Angular size 6.5" 23.6" 23.4" 0.4" 36.4" 17.7" 3.6" 2.4" 0.1" Illumination 61% 50% 98% 99% 99% 100% 100% 100% 100% Distance (AU) from Earth 1.029 0.707 0.400 3.417 5.416 9.373 19.515 29.010 32.766 Distance (AU) from Sun 0.309 0.727 1.389 2.578 5.387 10.064 19.878 29.941 33.622 Right ascension (2000.0) 9h29.9m 12h26.6m 20h12.2m 11h35.1m 14h50.8m 18h11.6m 2h01.2m 23h08.0m 19h22.2m Declination (2000.0) 15°20' –4°14' –26°33' 11°14' –15°26' –22°39' 11°45' –6°39' –21°59'

40 ASTRONOMY • AUGUST 2018 This map unfolds the entire night sky from sunset (at right) until sunrise (at left). Arrows and colored dots show motions and locations of solar system objects during the month.

Jupiter’s moons Dots display positions Io of Galilean satellites at 11 P.M. EDT on the date Europa shown. South is at the

top to match S the view Ganymede through a WE telescope. N Callisto

8 Europa

9 Jupiter

10 Io

13 Ganymede

16 Callisto

28 : ROEN KELLY ROEN :

30 ASTRONOMY

31 ILLUSTRATIONS BY BY ILLUSTRATIONS

WWW.ASTRONOMY.COM 41 — Continued from page 37 Jupiter sports a pair of black eyes WHEN TO VIEW THE PLANETS S EVENING SKY MIDNIGHT MORNING SKY Venus (west) Mars (south) Mercury (east) Mars (southeast) Saturn (southwest) Uranus (south) Jupiter Jupiter (southwest) Uranus (east) Neptune (southwest) Saturn (south) Neptune (southeast) Neptune (east) W shadow transit and prime embedded in a star-studded viewing locations shift farther setting of the rich Milky Way. west.) Io moves off Jupiter’s Of course, Saturn looks limb at 9:59 p.m. CDT, while even more stunning through a Io the shadows lift back into space telescope. The planet’s disk between 9 and 10 p.m. PDT. spans 18" while the rings Europa Europa’s Io’s shadow shadow Move east about 50° and extend 40" and tip 27° to our you’ll come to Saturn. The line of sight. The large tilt 10" ringed planet shines at magni- affords dramatic views of ring August 23, 10:15 P.M. CDT tude 0.3 in northwestern structure. Even the smallest The giant planet looks like it was in a bar fight when the shadows of two Sagittarius. It lies 2.7° east of scope reveals the dark gap large moons traverse the bright cloud tops the evening of August 23. the Trifid Nebula (M20) in that separates the outer A ring early August and moves 1.0° from the brighter B ring. Rhea — show up through harder to spot so far from its closer by month’s end. Several Small instruments also 4-inch and larger instruments. parent body. other bright deep-sky objects show several moons. Titan is All three circle Saturn inside As nice as Venus, Jupiter, also reside in this vicinity. The the biggest and brightest; its Titan’s orbit. Distant Iapetus and Saturn are, Mars is the delicate wisps of the Lagoon 8th-magnitude glow shows up proves more challenging. It star of the planetary world Nebula (M8) appear 1.4° south easily through any scope. The appears as an 11th-magnitude in August. The Red Planet of M20, while open cluster satellite’s distance from Saturn dot 1.7' south of Saturn on reached its peak during July’s M21 stands 0.7° northeast of ranges from 1.3' when it lies August 8. Although it doubles final week, and the view in M20. Be sure to scan this area due north or south to 2.9' in brightness by the time it early August hardly suffers. through binoculars from when it is farthest east or west. reaches greatest western elon- On the 1st, it shines at magni- under a dark sky. Saturn Three 10th-magnitude gation on the 28th, it then lies tude –2.8 and its disk spans gleams like a yellow sapphire moons — Tethys, Dione, and 8.8' from the planet and will be 24". By the 31st, however, Mars COMETSEARCH Old faithful performs for the Queen Comet 21P/Giacobini-Zinner N Comet 21P/Giacobini-Zinner midnight. Your best views will t loops from beyond Jupiter’s come when the Moon is gone s orbit to just outside Earth’s. This from the morning sky during Aug 10 circuit brings it into the inner mid-August. Astroimagers won’t CAMELOPARDALIS f 12 solar system every 6.6 years, want to miss the comet when it though some returns produce passes through the photogenic 14 better displays than others. region of the Heart and Soul neb- E 16 Path of Fortunately, this should be one ulae (IC 1805 and IC 1848, respec- Comet 21P of the comet’s best. tively), from the 15th to the 18th. 18 ¡ If predictions hold, Comet 21P Comet 21P should sport a 20 should reach 9th magnitude in well-defined eastern flank. This is August. To see it well, you’ll need where the solar wind pushes on CASSIOPEIA IC 1848 IC 1805 a 4-inch or larger telescope and a dust ejected from its icy nucleus. dark-sky site. The comet moves The comet typically develops a from Cassiopeia the Queen into short gas tail, too. High power 2° Camelopardalis the Giraffe this should reveal a bright spot at the month. Although this region is comet’s center — the so-called circumpolar north of 30° north false nucleus — where dust This periodic visitor to the inner solar system should reach 9th magnitude latitude, it climbs highest after hides the true nucleus from view. in August as it cuts across the star fields of Cassiopeia.

42 ASTRONOMY • AUGUST 2018 The Moon takes a bite of the Sun LOCATINGASTEROIDS Easy observing in the summertime

After viewing August’s four Then, shift your view northeast bright evening planets, how to where the chart shows you about a low-stress diversion? Vesta’s location. Alternatively, This is a great time to watch from a dark-sky site, you can asteroid 4 Vesta shift from jump to the spot on the south- night to night as it crosses one ern flank of the star cloud of the emptiest zones near the dubbed the “Teapot’s steam.” Milky Way’s heart. Like all solar system objects, Whether you use binoculars Vesta changes position relative or a telescope, this main belt to the background stars as it Observers across northern Canada, northern Europe, and a good portion object should be simple to find. orbits the Sun. But the aster- of Asia can witness a partial solar eclipse August 11. BURLEY PACKWOOD With only a handful of nearby oid’s motion this month is quite stars brighter than our target, slow. Although its night-to- appears half as bright (magni- need binoculars or a telescope the chart below will take the night movement is obvious, guesswork out of identifying you’ll be hard-pressed to see it tude –2.1) and 13 percent to track down the planet, 7th-magnitude Vesta from the shift position during a two-hour smaller (21" across). Still, those which glows at magnitude 7.8 distant suns. observing session. Your best numbers are impressive — the in eastern Aquarius. Use 4th- The asteroid lies in southern chance comes the evenings of ϕ Red Planet hasn’t appeared magnitude Phi ( ) Aquarii Ophiuchus. To find it, zero in on August 20 and 21. Vesta then brighter or bigger since 2003. as a guide. Neptune lies 1.4° this region’s brightest star, mag- skims less than 10' south of the Mars spends August on the west-southwest of this star on nitude 3.3 Theta (θ) Ophiuchi. 5th-magnitude star 51 Oph. border between Capricornus the 1st and drifts 0.7° farther and Sagittarius. The planet’s away by month’s end. A tele- Vesta inhales the Teapot’s steam westward motion against this scope reveals the planet’s blue- background carries it from the gray disk, which measures NN former into the latter constella- 2.4" across. OPHIUCHUS tion on the 24th, but it comes Uranus trails three hours to a halt four days later and behind Neptune. The closer then slowly heads east. planet won’t tax your observ- Aug 1 This region remains visible ing skills as much because it 6 all night in early August, shines at magnitude 5.8 — EE 11 climbing highest in the south bright enough to see with the 5151 shortly before 1 a.m. local day- naked eye under a dark sky 16 4444 light time. It reaches the same and an easy binocular object 21 k position by 10:30 p.m. in late after it climbs high in the 26 31 Path of Vesta August. Mars peaks at an alti- southeast in the hours before tude of about 25° from mid- dawn. Uranus lies in Aries, e northern latitudes; the finest 12° south of Hamal (Alpha 0.5° views through a telescope come Arietis). Three 6th-magnitude within two hours of this peak. stars lie within 2° of the This bright asteroid glows at 7th magnitude as it passes in front of dark Features on Mars appear planet. Use a telescope to dis- dust clouds near the Milky Way’s center. best when they lie near the tinguish Uranus, which shows center of its disk. For observers a 3.6"-diameter disk and a in North America, Aurorae distinctive blue-green hue, before sunrise. When viewed in these regions can watch the Sinus stands front and center from the pointlike stars. through a telescope, the mag- Moon pass in front of the Sun. on August’s first few evenings; The last week of August nitude –0.2 planet spans 7" From Beijing, the Moon cov- Sinus Meridiani takes center finds Mercury in the pre- and appears 43 percent lit. ers one-third of our star’s stage on the 9th and 10th; dawn sky. The planet reaches Last August brought per- diameter just before sunset. Syrtis Major and Hellas follow greatest elongation 18° west haps the most watched total suit from the 16th to the 18th; of the Sun on the 26th. But solar eclipse in history. On Martin Ratcliffe provides plane- and Mare Sirenum takes over unlike August evenings, the August 11, 2018, observers in tarium development for Sky-Skan, on the 30th and 31st. ecliptic angles nearly straight northern Canada, northern Inc., from his home in Wichita, Neptune rises by 10 p.m. up from the eastern horizon Europe, and parts of Asia can Kansas. Meteorologist Alister local daylight time August 1 before dawn, and Mercury witness a partial solar eclipse. Ling works for Environment and during twilight by month’s stands 8° high 45 minutes With a safe solar filter, viewers Canada in Edmonton, Alberta. close. But the best views will come after midnight. You’ll GET DAILY UPDATES ON YOUR NIGHT SKY AT www.Astronomy.com/skythisweek.

WWW.ASTRONOMY.COM 43 5 BIGGEST THINGS THAT CHANGED ASTRONOMY

The Planck satellite (left) produced DECODING THE COSMIC the most detailed image of the cosmic microwave background (CMB) to date. Planck This all-sky map (above), released in satellite March 2013 and based on 15.5 months MICROWAVE BACKGROUND of observation, shows tiny fluctuations NASA in the temperature of the CMB. These variations correspond to minute The Big Bang left behind a unique signature under- and over-densities of matter that ultimately led to the large-scale on the sky. Probes such as COBE, WMAP, and structure we see in the universe today. The redder areas represent above- Planck taught us how to read it. by Liz Kruesi average temperatures, and bluer areas show temperatures colder than average. EUROPEAN SPACE AGENCY, PLANCK COLLABORATION

glow undetectable University cosmologist Joanna satellites launched above The surface of to the human eye Dunkley. “And it gives us so Earth’s obscuring atmosphere last scattering permeates the much information because all to map that microwave glow The universe began with the universe. This the things that we now see out to precisions on the order Big Bang 13.8 billion years ago light is the rem- in space — the galaxies, the of millionths of a degree. as a fiery sea that expanded Anant signature of the cosmic clusters of galaxies — the very Specifically, three satellites rapidly. A few minutes later, beginning — a dense, hot fire- earliest seeds of those, we see — COBE, WMAP, and Planck the universe’s constituent pri- ball that burst forth and cre- in this CMB light.” — revealed that our current mordial subatomic particles ated all mass, energy, and time. Extracting these clues from cosmos, which is complex and glommed together into an ele- The primordial cosmic micro- the CMB has taken multiple filled with clusters of galaxies, mental soup of atomic nuclei wave background (CMB) radi- generations of telescopes on stars, planets, and black containing hydrogen, helium, ation has since traveled some the ground, lofted into the holes, evolved from a and trace amounts of lithium. 13.8 billion years through the atmosphere, and launched into surprisingly simple Electrons and light collided expanding cosmos to our tele- space. In the mid-1960s, early universe. and scattered off of those scopes on Earth and above it. when Arno Penzias and atomic nuclei. Over the next But the CMB isn’t just light. Robert Wilson discovered thousands of years, the cosmos It holds within it an incredible the CMB’s pervasive continued to expand, giving wealth of knowledge that microwave static the particles more room to Wilkinson astronomers have been teasing across the sky, it Microwave move and allowing the uni- out for the past few decades. appeared identical Anisotropy verse’s temperature to cool “It’s the earliest view we have everywhere. It Probe (WMAP) bit by bit. Around 380,000 of the universe,” says Princeton would take NASA years after the Big Bang, the

44 ASTRONOMY • AUGUST 2018 temperature dropped to about 3,000 kelvins, cool enough for CLOSEUP VIEWS OF THE CMB electrons to latch onto hydrogen nuclei. The universe COBE: 1990 map became mostly neutral hydro- Resolution: 7° gen, with some heavier elements swirled in. With fewer individual particles zooming around, light could finally move about freely. And so it has traveled, mostly unhindered, in the approximately 13.8 billion TEAM SCIENCE NASA/WMAP WMAP: 2013 map years since that time of “last Resolution: 0.5° scattering.” These photons (5 times carry a snapshot of the more sensitive 380,000-year-old universe. than COBE) Since the 1960s, telescopes on Earth have captured that glow in every direction of the sky. While the light 380,000 years into the universe’s his- TEAM SCIENCE NASA/WMAP tory would have been visible to Planck: 2013 map Resolution: 0.16° human eyes if we were around, (15 times cosmic expansion has since more sensitive stretched the light into the than COBE) longer wavelengths of micro- waves — at least, that’s the wavelength astronomers had predicted. But would observations match theory? COLLABORATION PLANCK Each successive cosmological probe has improved astronomers’ view of the CMB with better resolution, revealing ever-finer details The three probes (anisotropies in temperature and density) that hold the key to assembling an accurate picture of our young universe. ASTRONOMY: ROEN KELLY The Cosmic Background Explorer (COBE) launched in 1989. One of its instruments The radiation’s intensity plot- light, and thus how much measured the intensity of the ted by wavelength makes it energy that light carries, is microwave glow at wavelengths obvious that the CMB has a directly related to its tem- ranging from 0.1 to 10 milli- very specific intensity curve, perature; redder light has less meters across the entire sky. where the strongest signal is at energy and a lower tempera- The COBE science team’s first 2 mm. That wavelength corre- ture than bluer light.) announcement, in 1990, was sponds to a temperature of COBE’s other instrument the result of that measurement. 2.725 K. (The wavelength of broke apart the seemingly uniform 2.725 K glow into more Cosmic detail, looking for spots where Background the temperature is warmer or Explorer THE UNIVERSE TURNS TRANSPARENT colder than average. It turned (COBE) NASA out there is a difference of only Electron a tiny fraction of a degree, Hydrogen about 0.00001 K, between hot- atom ter and colder spots. But there is so much more Photon This nearly identical cosmic that scientists can do with the glow with exactly the right CMB than confirm the Big temperature was concrete evi- Bang. “From the anisotropies, dence that the entire sky — the the hot and cold spots, we get entire observable universe — the initial conditions — how Proton began in a Big Bang. With bumpy was the early universe such tiny temperature differ- and also what is its composi- The CMB represents the moment at which the universe became “transparent.” ences across vast regions of tion,” says Mather. Immediately after its birth, the universe was hot and dense. As it expanded, it cooled, sky, those spots must have The next CMB satellite was and its density dropped. Within the young universe, photons couldn’t travel very far been in contact at early times. designed to improve upon these — a few inches — before colliding with a nearby particle. As the matter in the universe COBE leaders John Mather anisotropy measurements, transitioned from plasma (left) to atomic hydrogen (right) 380,000 years after the Big Bang, photons could travel much farther — the width of the universe — without and George Smoot won the mapping them at finer angular necessarily experiencing a collision. This moment, also called the surface of last 2006 Nobel Prize in Physics resolutions. COBE could map scattering, is encoded in the CMB we see today. ASTRONOMY: ROEN KELLY for their work. hot and cold spots of about 7°

WWW.ASTRONOMY.COM 45 This full-sky map from the Planck satellite shows the distribution of normal matter in Planck mapped out the distortions in the CMB due to gravitational lensing caused by our universe. It charts the location of matter between Earth and the observable edge of dark matter’s presence. This all-sky map shows where the dark matter is in the universe; the universe, with darker regions representing greater mass and lighter regions repre- darker blue areas show greater dark matter density, and lighter colors show areas of less senting less mass. The grayed-out regions are where light from the Milky Way blocked dark matter density. Grayed-out regions are where light from the Milky Way and other Planck’s ability to detect distant matter. ESA/NASA/JPL-CALTECH nearby galaxies blocked Planck’s ability to take measurements. ESA AND THE PLANCK COLLABORATION on the sky, while the Wilkinson in this fashion, and software Dunkley. “If you drop a whole computer models to the signal Microwave Anisotropy Probe generated a map of all those bunch of pebbles in, the ripples strengths (which astronomers (WMAP), launched in 2001 tiny differences. That map will sort of combine together, obtained from analyzing and operated until 2010, could holds a treasure-trove of and you see a whole pattern of WMAP and Planck data), zoom in to a resolution of bet- cosmic secrets. ripples across the water. We researchers can piece together ter than 0.5°. Planck, the CMB think of this pattern of slightly what the early universe looked satellite that operated from Unlocking the different temperatures of this like and how it has evolved. 2009 to 2013, zoomed in even early universe light on the sky a little bit like Thanks to these three cosmic further, to 0.16°. To reveal those secrets, cos- the pond after it’s covered in probes, we know the universe All of these missions mologists study the pattern of ripples.” began in a Big Bang, and around mapped temperatures to the hot and cold spots frozen into The size breakdown of the 380,000 years later, electrons and order of 0.00001 to 0.000001 K. the CMB and decompose those CMB’s temperature spots, or protons combined, letting light To minimize measurement spots into their constituent fluctuations, is like a cosmic roam free. We know our cosmos errors related to such small sizes. While most of the hot Rosetta Stone. The strength of is 13.8 billion years old and how signals, the spacecrafts’ detec- and cold spots are about 1° on the fluctuations’ signals at dif- fast it is expanding. We know tors pointed toward two spots the sky, they are overlaid on ferent scales is associated with that 31 percent of the universe is on the sky at the same time fluctuations with larger sizes. the universe’s age, its ingredi- matter, but only 5 percent is and measured the temperature “Imagine looking at a ents, its expansion rate, and made of ordinary matter like difference between them. The smooth pond of water that we when the first stars lit up you and me, while 26 percent is satellites swept the entire sky might drop pebbles into,” says the cosmos. By comparing invisible dark matter. Much more of the cosmos is composed of a mysterious, repulsive dark energy — 69 percent. WHAT CAN THE CMB TELL US? And perhaps most impor- tantly, astronomers now have a

) 1.2

-1 way to find out pieces of infor- Curvature of 80 the universe (lat) mation not literally encoded in 1.0 the CMB itself. That’s because Density of the CMB maps and their statis- sr sec cm 2 0.8 60 dark matter tics have led to the so-called standard model of cosmology. 0.6 “We now have a really simple

ergs/cm 40

-4 model that describes basically all 0.4 Amount of baryonic of our observations,” says 20 dark matter Dunkley. “We can track from 0.2 the very first moments of time Temperature luctuations (μK) Temperature all the way through today and Intensity (10 Intensity 0 0 0 5 101520 10 100 1,000 make predictions about how Waves/cm Multipole moment (l) large-scale structure evolved. And it has remarkable success. In 1990, the Far Infrared Absolute Spectrophotometer (FIRAS) The power spectrum of the CMB measures the scales over which instrument on COBE measured the spectrum of the CMB in temperature variations in the CMB occur. This figure shows the That’s the big thing these satel- frequency versus intensity to measure the temperature of the CMB size of temperature fluctuations in the CMB (in microkelvins) lite missions have given the and compare it to the temperature predicted by the hot Big Bang plotted against the multipole moment (l), which is a measure of community.” theory. The result, based on 43 measurements at equal spacing the scale over which the fluctuations are seen. The peaks hold information about the curvature of space-time and the amounts along the curve, matches the theoretical expectation so exactly Liz Kruesi is a contributing editor that the data points fall precisely on the curve, with uncertainties of normal and dark matter in the universe, as well as constrain that are smaller than the width of the blue line used to draw it. the type of dark matter present. ASTRONOMY: ROEN KELLY, AFTER WAYNE HU of Astronomy who writes about ASTRONOMY: ROEN KELLY, AFTER FIXSEN ET AL. 1996 the universe from Austin, Texas.

46 ASTRONOMY • AUGUST 2018 6 BIGGEST THINGS THAT CHANGED ASTRONOMY

HUBBLE’S ASTOUNDING LEGACY NASA’s finest instrument has captured more images and generated more research than any telescope in history. by Michael E. Bakich

ong before humanity telescope and its instruments. could venture into Funding issues cut the pro- space, astronomers posed mirror diameter and dreamed of a tele- also prompted collaboration scope above Earth’s with the European Space Lobscuring atmosphere. In Agency (ESA), which joined 1962, this dream took a step the project in 1975. But at last, toward reality when a National the project began. Academy of Sciences study group recommended the devel- What’s in a name? opment of a space telescope. When NASA secured funding Hubble’s errant mirror returned an unfocused image of M100 (left) with the Wide Field NASA launched two for the space telescope, the and Planetary Camera. Astronauts replaced the camera with one that incorporated a Orbiting Astronomical agency decided to name it after lens array that corrects Hubble’s mirror. The result (right) is much improved. NASA/STSCI Observatories in 1968 and Edwin Powell Hubble, whose 1972. Both produced a wealth studies expanded the universe of information, and support beyond our galaxy. us). Now known as Hubble’s most crucial parts. Telescopes for a space telescope grew. But Hubble conclusively proved law, it marked the first obser- typically have mirrors polished it was the space shuttle, with spiral “nebulae” were galaxies vational support for the Big to an accuracy of about 1⁄10 the its capacity to deliver and ser- like the Milky Way. And he, Bang theory. wavelength of visible light. vice large payloads, that finally along with Milton L. Humason, Because Hubble would observe made the concept feasible. discovered that galaxies’ dis- Construction, engineering, from near-ultraviolet to near- NASA selected a team of tances are proportional to their and operation infrared wavelengths, its scientists in 1973 to establish redshifts (the speeds at which For the telescope, the mirror mirror needed to be polished the basic design of such a they are moving away from and optical systems were the to an accuracy twice as good.

WWW.ASTRONOMY.COM 47 spectacular images we enjoy. The Goddard High Resolution Spectrograph, the Faint Object Camera, and the Faint Object Spectrograph all operate in the ultraviolet range of the spectrum. The High Speed Photometer conducts observations of objects that vary in brightness. It can take up to 100,000 measurements per second with a measuring accuracy better than 2 percent. Hubble’s three Fine Guidance Sensors keep the telescope accurately pointed during observations. Power comes from two solar panels, which also charge This view of the center of the Eta Carinae Nebula (NGC 3372) is one of the largest panoramas Hubble ever acquired. Scientists took 48 Hubble exposures and then added color information taken at the Cerro Tololo Inter-American Observatory in Chile. six batteries that keep things HUBBLE IMAGE: NASA/ESA/N. SMITH (UNIV. OF CALIFORNIA, BERKELEY)/THE HUBBLE HERITAGE TEAM (STSCI/AURA); CTIO IMAGE: N. SMITH (UNIV. OF CALIFORNIA, BERKELEY)/NOAO/AURA/NSF going during the roughly 25 minutes per orbit that Hubble spends in Earth’s shadow. Hubble’s problem and fix On June 25, 1990, astronomers learned that the space telescope’s images were out of focus. Why? The main mirror had been ground to the wrong shape! Fortunately, because engineers knew what went wrong, they were able to design new optical components with the same error but reversed. And the upgrades continued. In 1997, astronauts replaced Barred spiral galaxy NGC 1300 is unlike regular spirals, whose long, Hubble snapped Saturn’s aurora, which appears blue sinuous arms start at their centers. NGC 1300 possesses a thick bar, and because of the glow of ultraviolet light. At Saturn, it would instruments, repaired thermal its spiral arms begin at either end. This composite is one of the largest appear red because of ionized hydrogen. NASA/ESA/J. CLARKE insulation, and boosted Hubble images of a complete galaxy. NASA/ESA/THE HUBBLE HERITAGE TEAM (STSCI/AURA) (BOSTON UNIV.)/Z. LEVAY (STSCI)/ ERICH KARKOSCHKA (UNIV. OF ARIZONA) Hubble’s orbit. In 1999, a different group replaced all six gyro- scopes, a Fine Guidance Sensor, and thermal-insulation blan- kets. March 2002 saw the installation of a new main camera. During this mission, astronauts also replaced the solar arrays with new ones two- thirds as large that provided Twenty-one fragments of Comet Shoemaker-Levy 9 struck Jupiter in July 1994. The planet’s gravity had torn the comet into pieces as it 30 percent more power. passed by in summer 1992. Hubble imaged the chunks before they plunged into the jovian atmosphere. The 11 largest comet fragments measured 1.5 to 2.5 miles (2 to 4 km) in diameter. This mosaic image shows 20 fragments. H.A. WEAVER (JHU/APL)/T.E. SMITH (STSCI)/NASA Best. Telescope. Ever. What really sets Hubble apart, Hubble has a 94.5-inch (2.4 instruments arrived at NASA, Hubble’s instruments however, is the astounding meter) mirror. The wavelengths ready to go, in 1983, it took When launched, Hubble car- research scientists have con- Hubble can “see” range from engineers until 1985 to com- ried five scientific instruments. ducted with it. The pictures are 115 to 1,000 nanometers. Visible plete assembly of the entire All have since been replaced. nice (and some even important), light’s range is 400 to 700 nm. spacecraft. The Wide Field and Planetary but let’s face it: The images Building the space telescope NASA originally scheduled Camera (WFPC) is a high- Hubble has produced are the was a painstaking process that the launch for 1986 but was resolution imager. This instru- icing on the cake. Astronomers spanned almost a decade. forced to delay it after the ment, and its subsequent know it’s Hubble’s data that Contractors actually finished Challenger accident. Space shut- replacements, are the ones best rocks our universe. Need proof? the mirror in 1981 and then tle Discovery finally carried the known to Astronomy readers In December 1995, the completed the optical assembly Hubble Space Telescope into — it’s these, after all, that pro- Hubble mission team pointed in 1984. Although the science orbit on April 24, 1990. duce the majority of the the telescope toward a small

48 ASTRONOMY • AUGUST 2018 Galaxy cluster Abell S1063, located 4 billion light-years away, lies at center surrounded by galaxies much farther out. The cluster’s huge mass distorts and magnifies more distant galaxies due to gravitational lensing. This phenomenon allows astronomers to see galaxies that would otherwise be too small and faint to observe. NASA/ESA/J. LOTZ (STSCI) spot in the constellation Ursa gamma-ray burst identified the Major. Earthbound telescopes host galaxy and the cause: the showed nothing there. But collapse of a stellar core due to An iconic Hubble image, the “Pillars of Creation” are about 5 light-years tall and lie some after 342 exposures over 10 a supernova explosion. 6,500 light-years away. Ultraviolet light from a group of young, massive stars located off days, the resulting image — Also in 1999, astronomers the top of the image evaporates streamers of gas into space. Denser regions shadow the Hubble Deep Field — using Hubble completed an material from the powerful radiation. NASA/ESA/THE HUBBLE HERITAGE TEAM (STSCI/AURA) shocked the world. It eight-year survey of Cepheid contained some 3,000 objects, variable stars, those whose nearly all distant, previously periods correspond to their current estimate of 13.82 billion Saturn, documented the invisible galaxies. This and real brightnesses. The results years obtained by the Planck impact of Comet Shoemaker- subsequent “deep field” images revealed that the Hubble con- space observatory in 2013. Levy 9 as it hit Jupiter, and have revealed far more galaxies stant — the expansion Hubble observations also more. And, to be clear, it’s not than astronomers thought rate of the universe — is helped determine that quasars done yet. existed, including some that 70 kilometers per second per are powered by supermassive existed when the universe was megaparsec, a number within black holes. As matter falls The great eye in the sky less than a billion years old. 5 percent of the current value. into one, the surrounding Think of each of the images In January 1999, Hubble In addition, Hubble’s obser- region heats up and releases accompanying this story as a imaged what was, up to that vations of Cepheids in the gal- tremendous amounts of energy sight that early astronomers time, the most powerful explo- axy NGC 4603 estimated the and light. In addition, Hubble like Galileo could not have sion ever recorded. This opti- age of the universe at 13.7 bil- found that almost all galaxies even dreamed up. Hubble’s cal counterpart to a massive lion years, quite close to the with bright, active centers have 28-year success story has made supermassive black holes feed- it a household word — indeed, ing off the galaxy’s matter. a pop culture icon. Besides In the mid-1990s, astrono- being the greatest telescope mers discovered that a myste- ever built, Hubble may be the rious force they dubbed “dark single greatest technological energy” was causing the uni- achievement of all time. It not verse’s speed of expansion to only influenced scores of increase. Hubble added to the astronomers, but a generation precision of this measurement of everyday folk as well. in 2016 when it collected data Because of Hubble, millions that showed the universe was of people consistently turn their expanding 5 to 9 percent faster thoughts heavenward and con- than predicted, with an error template our universe’s beauty of only 2.4 percent. and unimaginable vastness. Closer to home, Hubble has tracked changes in the atmo- Michael E. Bakich, a senior spheres of the outer planets, editor of Astronomy, continues Globular cluster M79 lies 41,000 light-years away and contains 150,000 stars. Sun-like to be amazed at the high quality stars appear yellow while bright giants near the end of their lives are reddish. The discovered four of Pluto’s five fainter blue stars are “blue stragglers,” which form either by the merger of stars in a known moons, recorded huge and sheer quantity of images the binary system or by the collision of two unrelated stars in M79’s crowded core. NASA/ESA auroral displays on Jupiter and Hubble Space Telescope creates.

WWW.ASTRONOMY.COM 49 7 BIGGEST THINGS THAT CHANGED ASTRONOMY

This artist’s concept depicts a hypothetical form of dark energy called quintessence. Dark energy accounts for 69 percent of the total mass-energy in the cosmos and is thought to drive the accelerating expansion of THE WEIRD MYSTERY OF DARK ENERGY the universe. SCIENCE PHOTO LIBRARY

Though it dominates the universe, dark energy is the biggest discovery Heavy elements we don’t understand. by Liz Kruesi 0.03%

Neutrinos n the late 1990s, our Discovering dark energy WHAT MAKES UP 0.3% understanding of the To reveal this mysterious Baryonic universe’s ingredient list energy, scientists studied type THE UNIVERSE? matter underwent a major recali- Ia supernovae, all of which Dark matter 5% bration. It began when have light-intensity curves with 26% Itwo competing teams of astro- very similar shapes. How these Stars physicists studying the light blasts’ brightness changes over 0.5% from distant stellar explosions time — becoming brighter then found that the blasts were fading — follows a specific fainter than expected. shape that’s always nearly the This means that the explo- same. Astronomers can there- Hydrogen/ sions were farther from us than fore compare and standardize helium theory predicted; while theory these curves, providing a tool 4% already took into account our to study the distant cosmos. Dark knowledge that the universe is The curves have the same energy expanding, this discovery indi- shape because they start with 69% cated it is expanding at an the same type of source: a increasing rate. But the matter densely packed star remnant Estimates for the universe’s mass-energy distribution have varied over the years. But in we knew about in the universe made of carbon and oxygen. 2015, the Planck mission — a space observatory that mapped anisotropies in the cosmic — galaxies of stars and gas, That remnant — a white dwarf microwave background — derived these results. ASTRONOMY: ROEN KELLY AFTER MCDONALD OBSERVATORY called normal matter, plus — lives in a binary system invisible dark matter — works with a companion star. Over know how much energy a type Cosmology Project to use these by pulling, and therefore some- time, the white dwarf siphons Ia supernova gives off during a blasts to track how our uni- thing had to be pushing every- gas from its companion, and blast, so the observed bright- verse is expanding. In 1994, thing apart. That something, that material builds up on the ness gives them that super- astronomer Brian Schmidt, which astronomers now call surface. The white dwarf’s nova’s distance. who’s been at Australian dark energy, is a repulsive force, mass increases until it reaches In the late 1980s, Lawrence National University since 1995, and it also happens to make up a point where it undergoes a Berkeley National Laboratory co-launched a competing more of the universe than nor- runaway nuclear reaction, and astronomer Saul Perlmutter group, the High-Z Supernova mal and dark matter combined. the star explodes. Astronomers created the Supernova Search Team. Then in

50 ASTRONOMY • AUGUST 2018 DARK ENERGY: A CAUSE FOR ACCELERATION In the standard Lambda-Cold Dark Matter model of the universe, dark energy permeates throughout all of space. The repulsive effect of dark energy is steadily accelerating the expansion of the universe, and will ultimately lead to a “Big Freeze” scenario, where all of the matter in the cosmos indefinitely drifts apart. When this happens, clouds of gas and dust will be too diffuse to collapse in on themselves and form new stars. Over many trillions of years, the remaining stars will use up their fuel and blink out, leaving the universe a cold, dark, and desolate place. ASTRONOMY: ROEN KELLY

ACCELERATING EXPANSION A little more than 5 billion years ago, the repulsive force of dark energy began You are here to overpower the attractive force of gravity on large scales. This caused the universe to expand at an increasing rate, and this expansion will continue accelerating forever. INFLATION In less than 10–32 of a second after the Big Bang, the universe ballooned outward, growing faster than the speed of light and pulling all the matter and energy in the cosmos apart in all directions. BIG BANG The universe burst forth violently from an extremely hot and dense point of concentrated energy some 13.8 billion years ago.

September 1998, Schmidt’s expansion is slowing down — universe could be matter, studying the large-scale struc- team published a paper analyz- which is what astronomers’ which gravitationally pulls ture and evolution of the uni- ing 16 type Ia supernovae; in previous model of the universe on other material. verse for more than 30 years. June 1999, Perlmutter’s team had predicted. Instead, it “We have all of human Yet if our cosmos is accelerat- published their analysis of appeared the universe’s expan- experience telling us that grav- ing, something else is at play. 42 type Ia supernovae. Both sion is speeding up. It seemed ity pulls things together,” says “That either means that there’s groups found that more distant that for the cosmos to do what David Weinberg, a cosmologist some new component of the stellar blasts are fainter than the observations showed, only from The Ohio State universe with exotic physical they should be if cosmic about 25 to 30 percent of our University who has been properties, or that Einstein’s

WWW.ASTRONOMY.COM 51 cosmological constant, which is basically the idea that empty space is not actually empty. “Even if you take all of the matter out of space, the light, any neutrino, any particles whatsoever, in complete nothingness, you’re never left with complete nothingness,” says Davis. What’s left, according to quantum physics, are virtual particles, which pop in and out of existence. Because these last for short amounts of time, they don’t actually vio- late conservation of energy, adds Davis. And these virtual particles, which are effectively manifestations of the time- energy uncertainty principle in a vacuum, would create a type of negative pressure — they push instead of pull. But there’s a problem with the cosmological constant as the dark energy candidate: It’s not strong enough. Quantum mechanics predicts a vacuum energy density (the “weight” Virtual particles are another potential explanation for dark energy. These theorized particles are believed to form when a classical of empty space) that is 10 fol- particle (or even a region of empty space) temporarily transforms into a set of virtual particles, which may or may not have the same lowed by 120 zeroes more than mass as the original. Embodiments of the uncertainty principle within a vacuum, virtual particles would create a type of negative cosmologists observe. “I’m pressure that pulls things apart. AGSANDREW/SHUTTERSTOCK pretty confident in saying that’s the worst ever match theory of gravity is incorrect.” uncovering clues to an acceler- an alarming mystery. However, between a theoretical predic- Most astronomers are more ating universe, and they pro- when the researchers incorpo- tion and observation,” says convinced it’s the former: vided a way to solve several rated the fact that the universal Davis, laughing. Some weird newly found com- cosmic mysteries. expansion is accelerating and While scientists are much ponent, which they’ve nick- Perhaps the most concern- not decelerating, the age dis- more confident 20 years after named dark energy, is acting ing of those mysteries involved crepancy somewhat disap- dark energy’s discovery that it against gravity. the universe’s age. If the uni- peared. Perlmutter’s group does in fact exist, they clearly Perlmutter and Schmidt verse were made only of mat- calculated a 14.9 billion-year- haven’t identified it yet. But won the 2011 Nobel Prize in ter, it would despite this, Physics for their groundbreak- have decelerated the field is ing work, along with Adam since the Big still progress- Riess, who led the High-Z Bang due to the “Even if you take all of the matter out of ing, even Supernova Search Team’s anal- gravity of all space, the light, any neutrino, any particles though scien- ysis. The discovery of acceler- that matter pull- whatsoever, in complete nothingness, you’re tists don’t yet ating expansion was hailed as ing on itself. never left with complete nothingness.” understand the discovery of the decade. “If it had been the majority slowing down — cosmologist Tamara Davis of what makes The bigger picture over its entire up the This discovery wasn’t the only history, then its universe. reason the paradigm shifted so age, [astronomers] calculated, old cosmos, and Schmidt and Weinberg isn’t fazed. “I have easily. “Those were certainly would be about 9 or 10 billion Riess calculated a 14.2 billion- some history of living through important experiments,” says years old,” says University of year-old one. Today, scientists big questions about the nature Weinberg, but there was Queensland cosmologist know from a variety of studies of dark matter, and actually another reason they were Tamara Davis. However, stellar that the universe is 13.8 billion finding resolution of them,” he adopted as fact relatively astronomers were finding stars years old. says. “So that leaves me opti- quickly: “They were landing that appeared to be 13 billion mistic for the future.” on well-prepared ground.” The years old. “So we had a situa- What is it? work of Perlmutter, Schmidt, tion where the oldest stars were So what could dark energy be? Liz Kruesi is a contributing editor and Riess was the culmination older than the entire universe,” The leading theory is vacuum of Astronomy who writes about of a decade of research slowly says Davis, which was a bit of energy, also known as the the universe from Austin, Texas.

52 ASTRONOMY • AUGUST 2018 8 BIGGEST THINGS THAT CHANGED ASTRONOMY

In the past few decades, astronomers have found a wide variety of exoplanets, including hot Jupiters, pulsar planets, super-Earths, and hot EXOPLANETS BURST ONTO THE SCENE Neptunes. MARTIN VARGIC Since their discovery in 1992, planets outside our solar system have been found around thousands of stars in the galaxy. by Korey Haynes

t’s difficult to overstate world-class instruments and The early days examining a newly found pul- the explosive pace of massive space observatories, Humans, scientists included, sar, PSR 1257+12, when they exoplanet discovery. exoplanet science is now being have speculated about planets discovered timing variations in Within the span of just a conducted at all levels. While around other stars since the the pulses of light coming from few decades, the concept some of the field’s earliest puz- latter were first understood as the dead star. Such variations Iof planets outside our solar zles remain, new discoveries distant suns. But, setting the are common, since pulsars are system evolved from science and methods are growing at an tone for the field’s future, the often found in binary relation- fiction into a full-fledged field astounding pace. And exoplan- first exoplanets found weren’t ships with other stars; the vari- of scientific study. ets are still delivering surprises what astronomers had expected. ations can inform astronomers From backyard observers at every turn, just as they have In 1992, Aleksander about the system’s mass. But in with modest telescopes to since the birth of the field. Wolszczan and Dale Frail were this case, the masses of the

WWW.ASTRONOMY.COM 53 The first exoplanet orbiting a Sun-like star — circling 51 Pegasi, 51 light-years away — was discovered in 1995 by Swiss astronomers Michel Mayor and Didier Queloz. The breakthrough discovery suggested that the galaxy could be filled with solar systems much like ours. NASA/JPL-CALTECH

Kepler-35, a binary star system in Cygnus, lies some 5,000 light-years away. Amazingly, The first extrasolar planet discovered was a body orbiting the pulsar PSR B1257+12, the Kepler spacecraft found that this system of orbiting suns holds a planet that found by Polish astronomer Aleksander Wolszczan in 1992. This was the first confirmed circles both stars. It has just over one-eighth of Jupiter’s mass and a diameter of about discovery of planets outside our solar system, and it was huge news. Astronomers 70 percent that of Jupiter. The planet completes an eccentric orbit once every subsequently found that the pulsar has three planets, two with about four times Earth’s 131 days at a distance about 60 percent as great as the Earth-Sun distance. LYNETTE COOK mass and one that is tiny, with an Earth mass of just 0.02. LYNETTE COOK companions — there were two stage again. 51 Peg b, as it’s Over the next decade, exo- HARPS was one of the early — were tiny, only a few times known, is half the mass of planet discoveries trickled in. workhorses of exoplanet detec- the mass of Earth. Jupiter and orbits its star in only Astronomers found most of tion, and it still holds the The planets ignited a frenzy. four days. Mayor and Queloz these planets via the radial record for the most discoveries Only a year prior, a team of had found the first hot Jupiter. velocity method, which mea- by any ground-based search. astronomers had claimed a Scientists still aren’t sure how sures the slight motion of a star planet orbiting a pulsar using planets form or survive in a as an orbiting planet pulls it Model citizen the same method, only to pulsar system — not to mention toward and away from us. This In 1999, astronomers settled in retract it not long before the planets they found are of a motion is too tiny to measure to carefully observe the star Wolszczan’s announcement due size not observed in our solar except with a highly precise HD 209458. They already had to mistakes in their analysis. system. Nor are astronomers spectrometer, which can detect proven the star hosted a planet, But the new planets withstood sure how hot Jupiters, for which the Doppler shifting of the star- but they were after a new the firestorm of scrutiny, and our solar system has no equiva- light as the distant sun wobbles breakthrough. They were hop- science fiction became reality. lent, come to exist so close to back and forth. Teams around ing to catch the planet as it Just a few years later in 1995, their stars — if they migrate the globe put their instruments crossed its star in our line of Michel Mayor and Didier across their star systems, how and analyses to the test. Some sight, detectable as a tiny dip Queloz reported a planet and when do they do so, and failed, many succeeded, and the in the star’s light: a transit. around the star 51 Pegasi — a why do they stop? Despite exoplanet list grew. Two different groups suc- normal, main sequence star not plenty more examples and more In 2003, the High Accuracy ceeded, and suddenly astrono- so different from our Sun. But than 20 years to study the Radial velocity Planet Searcher mers had a new detection despite the more conventional question, these planets still (HARPS) saw first light at technique at their fingertips. setting, this planet reset the puzzle astronomers. La Silla Observatory in Chile. Years later, HD 209458 b

54 ASTRONOMY • AUGUST 2018 became the first exoplanet to have its spectrum — the finger- print of light that tells observers the components of the planet’s atmosphere — directly observed. Researchers detected sodium in the atmosphere’s outer layers. Astronomers would go on to take spectra of scores of exoplanets, but HD 209458 b’s brightness, size, and proximity to Earth made it a prime target, especially in the first heady days when scientists were proving they could not merely find planets, but study and characterize them as well. The modern era In 2009, the Kepler Space Telescope launched, and the trickle of planets became a del- The TRAPPIST-1 system contains seven terrestrial planets orbiting the cool red dwarf star Proxima Centauri. This illustration uge. Kepler, staring at tens of approximates the system as seen from a vantage point near planet TRAPPIST-1f (at right). NASA/JPL-CALTECH thousands of stars and watching for transits, delivered hundreds of planet candidates, then decades of previous studies. black holes — to study dim will provide details about thousands, within only a few Kepler also ran into mechani- exoplanets around bright stars exoplanet composition that years. Astronomers didn’t have cal difficulties, but astrono- in ever-increasing detail. Hubble can only dream of. to examine planets one by one mers learned to work with and Astronomers collected planets’ And in preparation, TESS (the any longer, but could perform around the telescope’s trouble- temperatures and chemical Transiting Exoplanet Survey population studies, comparing some reaction wheels to steer compositions by the dozens, Satellite) has begun its search these worlds in large numbers. it to new findings and new and pictures of formation and of the skies for the best targets Of course, Kepler had its missions. They innovated, evolution began to evolve. for Webb’s sights. Astronomers problems, too. Stars are more adapted, and overcame. With time, more precise expect TESS to revolutionize variable than scientists Scientists also grew adept at instruments, and more clever today’s exoplanet field as fully expected, and exoplanets using powerful ground- and analyses and techniques, as Kepler did. harder to find. But Kepler space-based instruments — astronomers found smaller Since Kepler, astronomers taught astronomers more designed for distant galaxies, planets and planets farther have found an Earth-sized about stellar activity than far-off nebulae, and accreting from their stars. Smaller tele- planet around Proxima scopes joined the fray, from Centauri, the Sun’s closest the Kilodegree Extremely neighbor; the TRAPPIST-1 1,400 Little Telescope (KELT) survey system, which contains seven EXOPLANET DISCOVERIES to amateur networks around temperate, rocky planets, three 1,200 the world. of them in the star’s habitable The richness of discovery zone; and even the outrageous 1,000 1,284 newly validated planets has opened up even more KELT-9 b, a hot Jupiter warmer Previous Kepler/K2 discoveries questions. Surveys uncovered than some stars, circling its 800 Non-Kepler discoveries scores of super-Earths and host on a polar orbit once every mini-Neptunes; these planets day and a half. Astronomers have no analogs in our solar have even taken pictures of 600 system, yet they appear to be more than a dozen exoplanets, the most common type of directly imaging these alien

Number of new planets Number of new 400 planet. Astronomers proved worlds. The last few decades that tiny planets around small have proven that the galaxy’s 200 stars are commonplace. Given stock of planets is far more that small stars are so frequent, diverse, confounding, and this led to numerous targets amazing than our solar system for detailed observing. ever prepared us for.

1997 1998 1999 2000

2001 2002 2003 2004 2005 2006

2007 2008 2009 2010 2011 2012

2013 2014 2015 2016 Discovery year The future Korey Haynes is a freelance The number of known exoplanets has exploded since the first detection in 1992. For years, astronomers have science journalist and a former Exoplanets now comprise more than 3,700 planets orbiting more than 2,800 been anticipating the James associate editor of Astronomy. stellar systems, many of which were found by the Kepler spacecraft. ASTRONOMY: ROEN KELLY Webb Space Telescope, which She lives in Minnesota.

WWW.ASTRONOMY.COM 55 9 BIGGEST THINGS THAT CHANGED ASTRONOMY

New Horizons revealed PLUTO FINDS Pluto as a wonderfully varied world where ancient cratered landscapes coexist with ITS PLACE fresh terrain no more than 10 million years old. Few scientists expected such diversity at the solar No longer a lone world at the edge of the solar system, Pluto is the brightest system’s outer edge. and best-studied member of a vast throng of Kuiper Belt objects. by John Wenz NASA/JHUAPL/SWRI

he astronomers of we were welcoming a new when seen through the best When Astronomy was Lowell Observatory planet. The New York Times telescopes of the day, Pluto founded in 1973, astronomers were on a mission reported that it was possibly appeared as no more than an still didn’t know a lot about in 1930, driven by bigger than Jupiter and some unresolved speck of light. this oddball outer planet. founder Percival 4 billion miles away. In 1950, prominent Dutch- Fortunately, technological TLowell to find a hypothesized Astronomers soon revised American astronomer Gerard advances would soon make ninth planet (dubbed “Planet the size to around that of Kuiper estimated Pluto to be the picture a lot clearer. X”) that would explain strange Mars. It wasn’t the massive as large as 6,500 miles (10,500 orbital movements in the outer Planet X everyone was looking kilometers) in diameter, which The 1970s and 1980s solar system. for, but it seemed, by all would have made it bigger The first breakthrough came in That year, a farmboy accounts, to be our solar sys- than Mars. Kuiper was wrong 1976, when infrared observa- from Kansas named Clyde tem’s ninth planet. Size esti- about this, but half-right about tions revealed the presence of Tombaugh used the observa- mates varied considerably over something more important. He methane ice on Pluto’s surface. tory’s 13-inch photographic the next few decades, however, suggested that a large belt of Because most solid bodies in telescope to discover a wander- with some researchers con- icy objects left over from the the outer solar system are much ing “star” that ended up being cluding the world might be formation of the solar system richer in water ice, Pluto sud- a distant solar system object. as big as Earth while others might have populated its outer denly became even more exotic. For the first time since thought it could be as small as fringes, though he didn’t think In 1977, Charles Kowal Neptune’s discovery in 1846, the Moon. Unfortunately, even it was still there today. discovered Chiron (note the

56 ASTRONOMY • AUGUST 2018 Details emerge on Pluto (left) and Charon less than two days before New Horizons made its historic flyby in July 2015. This composite image shows the two in approximate true color at their correct relative size, separation, and reflectivity. NASA/JHUAPL/SWRI

The slight bump seen in each of these 1978 images led astronomers James Christy and This unassuming dot turned out to be 1992 QB1, the first Kuiper Belt object scientists Robert Harrington to conclude that a large moon orbits Pluto. Christy named the moon recognized as such. The dim point of light (circled) moved slightly during the 3.4-hour Charon in part to honor his wife, Charlene. U.S. NAVAL OBSERVATORY interval between the top and bottom photos. DAVID JEWITT (UNIVERSITY OF HAWAII) letter i). Although some people quickly grasped that they must discovered that Pluto has a system. In 1992, David Jewitt initially hailed this strange be seeing a close-in satellite. thin atmosphere. and Jane Luu discovered object as the “tenth planet,” it Astronomers soon recognized 1992 QB1. Although the seemed too small — much that this was a moon so large The 1990s and 2000s tiny body is barely 60 miles closer to an asteroid than a that the Pluto system could be The 1990s brought the power (100 km) in diameter, its orbit planet. Chiron orbits the Sun considered a double planet. of the Hubble Space Telescope places it beyond Pluto. Soon, largely between Saturn and The 1980s brought several to bear on Pluto. Images from astronomers discovered more Uranus on a path that could revelations. First, the orbits of the Earth-orbiting observatory of these so-called trans- not be stable over millions of Pluto and Charon turned edge- revealed the first surface Neptunian objects, many years, and astronomers soon on to Earth starting in 1985, details of Pluto, though given significantly larger. Eventually, realized it must have origi- leading to a series of mutual the planet’s small size, the scientists realized these objects nated farther out. occultations where one passed observations didn’t reveal formed an entire belt and Then in 1978, James Christy in front of the other every much more than brightness named it after Kuiper. Pluto and Robert Harrington discov- 3.2 days. Scientists used these variations hinting at regions was simply the biggest and ered Charon (this time with occultations to accurately mea- with different compositions. brightest of these objects. an a). Photos taken at the U.S. sure the objects’ diameters and The 1990s also produced Quaoar, which is about half Naval Observatory in Arizona surface reflectivities. During some much-needed context for the size of Pluto, turned up in showed that Pluto appeared one such occultation in 1998, Pluto, and led to a huge shift in 2002. Sedna came along the elongated, and the scientists James Elliot and his colleagues our understanding of the solar following year; it is roughly

WWW.ASTRONOMY.COM 57 Pluto shows many signs of an active past. The informally named Alcyonia Lacus appears to be a frozen, former lake of liquid nitrogen with a distinct shoreline. The feature spans 19 miles (30 km) in the al-Idrisi Mountains north of Sputnik Planitia. NASA/JHUAPL/SWRI

probe also revealed that a little of Pluto’s atmosphere con- denses at Charon’s north pole, where solar radiation turns it a dark red. Despite its remoteness from the Sun’s warmth, Pluto shows signs of an underground ocean. Charon likely had one, too, before it froze and buckled the moon’s icy shell. Mountains, cliffs, craters, and other features abound, and diverse areas of methane, nitrogen, and water The huge tectonic belt that runs along Charon’s equator hints that Pluto’s large moon once held a vast water ocean. As the water froze, ice create a strange composite it expanded and buckled the surface, creating a network of fractures that stretches at least 650 miles (1,050 kilometers). NASA/JHUAPL/SWRI world. Images even revealed a frozen lake bed. We were seeing our classically designated Quaoar’s size. And in 2004, clears the neighborhood By 2012, Hubble added four ninth planet up close for the Haumea, egg-shaped and around its orbit. If it meets new members to Pluto’s family, first time, 26 years after we’d even closer to Pluto’s size, just the first two conditions, discovering the moons Nix, seen the eighth. came to light. it’s a dwarf planet. Hydra, Kerberos, and Styx. All Of all the large objects in our Then Eris showed up. Pluto did not make the cut. are much smaller than Charon solar system, we’ve perhaps Discovered in 2005, it has It’s worth noting — while but similar in composition, learned the most about Pluto slightly more mass than Pluto remaining agnostic on the hinting that they formed at the since 1973. It went from a point and a diameter just a few planet vs. not-a-planet debate same time in one of light at the dozen miles smaller. A crisis — that astronomers considered collision event. limits of earth- in confidence erupted among the first few asteroids found in New Horizons bound telescopes astronomers: If something the early 19th century to be flew past Pluto in Going back to a fully about as big as Pluto is out planets until they understood 2015 and finally could be the key fleshed-out there, who’s to say there might the sheer number of objects in nailed down the world. If New not be dozens more? the asteroid belt. world’s size. It to understanding Horizons showed Astronomers mulled rede- is 1,477 miles how our solar us anything fining the parameters of a The view from (2,377 km) in system came about Pluto, it’s planet. The International New Horizons diameter — together. that going back Astronomical Union (IAU) In 2006, Alan Stern saw his small, but not as could be the key proposed a draft that would decades-long dream come small as Eris — to understanding have made Eris, Ceres, and true: An Atlas V rocket lifted making Pluto the how our solar even Charon planets, but it got off from Cape Canaveral and current king of the Kuiper system came together. And for scrapped. Then, at its 2006 sent the first probe toward Belt. The spacecraft also that, it doesn’t matter whether meeting, the IAU voted to Pluto. The craft, chock full showed that Pluto seems to Pluto’s a planet or not. define a planet as something of science instruments and be geologically active, with a that orbits the Sun, has enough carrying the ashes of Clyde giant, heart-shaped nitrogen John Wenz is a science writer in mass for self-gravity to give it Tombaugh, took nine years glacier the size of Texas that Madison, Wisconsin, and a former a nearly spherical shape, and to reach its destination. constantly renews itself. The associate editor of Astronomy.

58 ASTRONOMY • AUGUST 2018 10 BIGGEST THINGS THAT CHANGED ASTRONOMY

When two merging black holes collide, they produce gravitational waves that travel through the fabric of space-time at the speed of light. GLIMPSING GRAVITATIONAL WAVES Now, astronomers can finally study these invisible cosmic ripples. LIGO COLLABORATION/SXS (SIMULATING The screams of colliding black holes allow astronomers to study the universe EXTREME SPACETIMES) PROJECT in a whole new way. by Robert Naeye

lbert Einstein earned questioned whether they were billion-dollar laser interferom- of science that can obtain his lofty reputation physically real. Moreover, he eters, each with arms 2.5 miles information on nature’s most for his many deep doubted that future scientists long. extreme events — events that insights into phys- could ever pick up these subtle It wasn’t until nearly a cen- would otherwise be hidden ical reality. space-time undulations. His tury after his 1916 paper that from view. Since colliding AAmong his greatest predic- calculations showed that these an international team of more black holes generate no form tions was that massive, rapidly waves would be so feeble by than 1,000 scientists proved of light, we can discern these moving objects throw off rip- the time they reached Earth that Einstein was indeed right cosmic cataclysms only from ples of energy at the speed of that detecting them would in that gravitational waves the way they contort the fabric light, causing space-time itself require instruments of extraor- exist, but wrong in assuming of space-time. to expand and contract. In dinary precision far beyond we could never detect them. In a mere fraction of a sec- papers published in 1916 and the technology of his era. At 5:50:45 a.m. EDT on ond, the black hole collision 1918, Einstein referred to this We can excuse Einstein’s September 14, 2015, the Laser detected by LIGO converted strange phenomenon as gravi- lack of faith; he was a product Interferometer Gravitational- about three times the mass of tationswellen. Today, we call of his time. Early 20th-century wave Observatory (LIGO) the Sun into gravitational-wave them gravitational waves. astronomers had yet to dis- picked up the faint yet unmis- energy — with a peak power But for all his creative cover black holes and neutron takable rumbles of gravita- output of nearly 50 times that insights into the natural world, stars, which are incredibly tional waves emanating from of the entire visible universe. Einstein underestimated the dense cosmic objects that pro- colliding black holes contain- But these waves diminished ingenuity of humankind. duce the strongest gravita- ing 36 and 29 solar masses. rapidly in intensity as they tra- He viewed his prediction of tional waves. And nobody LIGO’s discovery opened a versed space. By the time they gravitational waves as a mere back then was contemplating window on the universe, creat- reached Earth, after a journey mathematical curiosity, and the possibility of multiple ing a revolutionary new branch that took about 1.3 billion

WWW.ASTRONOMY.COM 59 holes with masses of about 50 THE INAUGURAL DETECTION Suns. These initial results have shone a light on a population Hanford, Washington (H1) Livingston, Lousiana (L1) of stellar-mass black holes previously undetected by astronomers. In the coming years, 1.0 astronomers will gain deep 0.5 insights into this group of rela- 0.0 tively massive stellar-mass black holes, how they pair up –0.5 in binary systems, and how

) –1.0 L1 observed their merger rate has evolved –21 H1 observed H1 observed over cosmic history. Even better, the European Virgo detector near Pisa, Italy, 1.0

Strain (10 Strain joined the fray in August 2017. 0.5 Just weeks after it commenced 0.0 scientific operations, Virgo and both LIGOs picked up –0.5 gravitational waves from Numerical relativity Numerical relativity –1.0 merging neutron stars. And Reconstructed (wavelet) Reconstructed (wavelet) with three detectors in action, Reconstructed (template) Reconstructed (template) scientists could localize the source with sufficient precision 512 to enable conventional tele- 8 256 scopes to catch the aftermath. 6 Over the following months, 128 astronomers around the world 4 studied the kilonova — a burst 64 2 of electromagnetic radiation Frequency (Hz) Frequency 1,000 times brighter than an 32 0 amplitude Normalized 0.30 0.35 0.40 0.45 0.30 0.35 0.40 0.45 erupting white dwarf (or nova), Time (seconds) Time (seconds) but not as bright as a supernova — as it brightened and The gravitational-wave event GW150914 was observed by the LIGO instruments in Hanford, Washington (H1, left panels) and Livingston, Louisiana (L1, right panels) on September 14, 2015, at 09:50:45 UTC. The upper panels show the observed and theoretically faded across the entire electro- predicted signals; the lower panels show the signal’s characteristic chirp. B.P. ABBOTT ET AL. (LIGO SCIENTIFIC COLLABORATION AND VIRGO COLLABORATION) magnetic spectrum. This ush- ered researchers into a new era of multi-messenger astronomy. years, they expanded and con- out the noise of passing trucks rumblings of four additional Despite all the hoopla, acco- tracted space-time by 1⁄10,000 and howling wolves. Plus, the black hole mergers (and a lades, and awards, the best is the width of a proton. It’s no LIGO team built two of these likely fifth), involving objects yet to come. Scientists and wonder Rainer Weiss, Kip facilities: one in Livingston, ranging from 8 to 36 solar engineers are upgrading LIGO Thorne, and Barry Barish, Louisiana, and a near-twin in masses. The most massive col- and Virgo, which will greatly three of LIGO’s key figures, Hanford, Washington. lision, the first one, produced a extend their reach deeper into shared the 2017 Nobel Prize in Since this initial detection, black hole of 62 solar masses. space and time. In the coming Physics: Detecting these LIGO has heard the gravitational Two others produced black years, new laser interferometers minuscule fluctuations required a precision akin to measuring the 25 trillion-mile distance to the nearest stars to the width of a human hair. Over many years, the LIGO collaboration met this formi- dable challenge by creating an engineering marvel. Scientists and engineers developed powerful and stable near-infrared lasers, tunnels evacuated to one-trillionth the density of air at sea level, mirrors polished so smooth that their roughness is measured at the scale of atoms, and sophisticated seismic iso- From left: Kip Thorne, Rainer Weiss, and Barry Barish shared the 2017 Nobel Prize in Physics for their “decisive contributions to the LIGO lation systems that can filter detector and the observation of gravitational waves,” according to the Nobel Prize committee. CALTECH ALUMNI ASSOCIATION; BRYCE VICKMARK; R. HAHN

60 ASTRONOMY • AUGUST 2018 LIGO: A GIGANTIC INTERFEROMETER Two black holes collide THAT’S ARMED AND READY The two LIGO interferometers are the largest and most sensitive such detectors ever built. Each one can perceive tiny fluctuations in space-time that are much smaller than the width of a proton. ASTRONOMY: ROEN KELLY Gravitational waves No gravitational waves detected Gravitational waves detected

Each individual light beam Gravitational waves distort the 3bounces back and forth in 3length of each arm dierently, its arm nearly 300 times. causing one arm to become longer Mirror Mirror than the other. By bouncing the The beam splitter lets half light back and forth many times, the 2the light into one arm, while distance discrepancy becomes even the other half delects 90° down more pronounced. another arm of equal length. 2 The light is split 2.5 miles into two paths. (4 km)

Light path Light path

Mirror Mirror Light is emitted Beam splitter Light is emitted 1from the laser. 1from the laser.

The beams Laser The beams Detector 4 4 recombine at recombine at the beam splitter. the beam splitter and hit the detector. 5 Since each light Light waves wave travels a dierent distance, their Light waves peaks and troughs no The light waves are aligned and cancel longer cancel, resulting 5 each other out. No signal is detected. in a detection signal.

in Japan and India will join the pulses of many dozens of the hunt, enabling even millisecond pulsars sprinkled THE TALLY SO FAR more precise localization of across the sky, radio astrono- GW150914 gravitational-wave sources for mers will almost certainly be Sept. 14, 2015 multi-messenger follow-up. able to eventually pick out the 70 GW1701104 GW170814 But to expand their search bass notes coming from the 60 Jan. 4, 2017 Aug. 14, 2017 for gravitational waves, astron- inspiral of supermassive black LVT151012 omers must move beyond holes, which contain millions 50 Oct. 12, 2015 (tentative) ground-based interferometers. or even billions of solar 40 These Earth-bound instruments masses. Furthermore, astrono- are tuned to hear gravitational mers combing through data 30 Solar masses waves coming from the inspiral from the European Space 20 and merger of stellar-mass black Agency’s (ESA) Gaia satellite 10 holes and neutron stars. In are conducting a similar LIGO-Virgo GW151226 GW170608 comparison, the gravitational experiment by looking for 0 neutron stars Dec. 25, 2015 June 8, 2017 waves produced by small black slight changes in the positions Aug. 17, 2017 hole collisions are relatively of nearly a billion stars. To date, researchers have confirmed the detections of five pairs of merging black high frequency, roughly akin And in the early 2030s, ESA holes, and one pair of merging neutron stars. Their relative sizes, both before to the glass-shattering, high- aims to launch its long- and after merging, are shown above. LSC/LIGO/CALTECH/SONOMA STATE (AURORE SIMONNET) pitched sound waves produced awaited Laser Interferometer by an opera singer. Space Antenna mission — And Mother Nature isn’t just three spacecraft that will orbit solar system. These waves hitting the high notes; she is the Sun as an equilateral tri- will reveal countless orbiting For years, freelance science performing a veritable sym- angle, firing lasers at one stellar binaries in our Milky journalist Robert Naeye phony with gravitational waves another to measure the distor- Way Galaxy, along with the wondered what would happen — though we cannot yet “hear” tions of space-time caused by mergers of monstrous black first: his Eagles winning the many of the instruments. low-frequency gravitational holes going nearly all the way Super Bowl, or LIGO detecting However, by precisely timing waves passing through our back to the dawn of time. gravitational waves. LIGO won.

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WWW.ASTRONOMY.COM 63 SECRETSKY BY STEPHEN JAMES O’MEARA

Trilithon 1 Slaughter A lunar Trilithon 5 Stone Blue Trilithon 2 Stonehenge Trilithon 4 Stones Our columnist observes a pattern on the Moon Trilithon 3 that resembles a well-known landmark on Earth.

Avenue et’s have some fun. suggesting here was a lunar But first, a disclaimer: Stonehenge! (Hillman was This Lunar Orbiter image shows Mare Insularum west (left) of Copernicus Crater. The I claim no archaeo- right. I couldn’t help it.) author overlaid the inner structures of Stonehenge III (traced from Bulfinch’s Mythology) astronomical, archaeo- On this night, the shadow and added the Avenue, which belongs to Stonehenge I. NASA mythological, or conditions were not optimum, It’s easy to see Larchaeomystical “whatevers” in and as time passed, the north- the location of what you are about to read. It’s eastern shadows gradually the M15 dome just plain old apophenia (see- slipped away, greatly diminish- and elongated M15 vent in this ing patterns in random data), ing the relief. Yet, my first Lunar Orbiter like transforming the pattern of visual impression was immedi- image. At this stars in open cluster NGC 457 ate, probably because I had just high Sun angle, into Astronomy Editor David recently begun reading Gerald the dome does not stand out Eicher’s friendly owl, creating Hawkins’ book Stonehenge in relief. NASA the Owl Cluster. In this case, Decoded. I’ve transformed a random pat- I referred to sketches of tern of hills on the Moon into Stonehenge in Hawkins’ book, something that bears resem- as well as some 1916 drawings blance to an ancient monument in Bulfinch’s Mythology. I on Earth. I dubbed it my “lunar found a striking, though not Stonehenge.” perfect, similarity between the The secret yielding a 0.6° slope; its rift vent ring pattern of hummocky As I’ve mentioned, the uprights measures 5.0 by 0.5 miles (8 by Pattern recognition terrain in Mare Insularum and of the lunar horseshoe lie in a 0.8 km). So you need a very low at work parts of Stonehenge III, espe- region of rough and hilly ter- Sun angle (on the order of 2° “Pattern recognition is one of cially its “horseshoe” of stand- rain made up of superimposed above the lunar horizon) to see the most fundamental cogni- ing stones and the long ejecta from the Copernicus, the feature well. M15 lies on a tive skills we possess,” says ditch-bank entrance known as Kepler, and Eratosthenes 130-mile-long (209 km) volca- British psychologist Keith the Avenue, which leads to the impacts, over which basaltic nic dike that includes several Hillman, adding that while famous Heel Stone and origi- lavas have flowed. Indeed, other domes (M3, M4, and we can actively look for pat- nally dates to Stonehenge I. inside the lunar Stonehenge, M10) whose vents are aligned in terns — as we do with the Owl the same direction. Cluster — pattern recognition You can look for other lunar also occurs unconsciously and It’s just plain old apophenia. Stonehenge formations that automatically. The latter hap- mimic the original’s, such as a pened to me February 25 while Slaughter Stone near the inner testing my new 8-inch reflector In Stonehenge III, the on its floor, is a low dome with base of the horseshoe, as well on the waxing gibbous Moon, horseshoe is composed of five an elongated vent at its sum- as some Blue Stones, which on 10 days after New Moon. trilithons, each consisting of mit. Known as M15, this dome Earth also trace out a horse- That night, the terminator two upright stones capped by a resembles a classic shield vol- shoe pattern. As always, send sliced through Mare Insularum third crosspiece. In my lunar cano (one with a pancakelike your thoughts, comments, and (the Sea of Islands) several horseshoe, only the five cross section), like those found imaginings to sjomeara31@ degrees west of Copernicus upright pairs are visible. Also, on the Big Island of Hawaii. gmail.com. Crater, where I noticed a ring the lunar Avenue is much As reported in the August of rugged terrain casting slight closer to the horseshoe pattern 2007 issue of Icarus, M15 mea- Stephen James O’Meara shadows. My mind’s neural than the one on Earth. Still, it’s sures 13 miles (21 kilometers) in is a globe-trotting observer pattern-recognition software fun to see the similarities. I did diameter but rises to an altitude who is always looking for the next great celestial event. automatically kicked in, not notice a Heel Stone. of only 360 feet (110 meters),

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....7 Astronomy ...... 67 ...... 63

...... 62 ...... 7 of ...... 65 ...... 3 ...... 65 ...... 15 ...... 65 ...... 7 ...... 65 ...... 5 ...... 2 ...... 15 ...... 65 ...... 15 ...... 5 ...... 67 INDEX INDEX ...... 15 ...... 5 ...... 65 ...... 65 ...... 75 ...... 65 ...... 65 ...... 76 ADVERTISERS ...... 21 Present Total Solar Eclipse Total Present Westfjords & Aurora Borealis Borealis & Aurora Westfjords Woodland Hills Cameras & Telescope Hills & Cameras Woodland Strange Science Special Issue Innovations Technical Inc Optics, Vue Tele Company Teaching The Jasper Tourism Stellarvue Milwaukee Public Museum Milwaukee Public Milky Way Inside & Out Special Issue . . .. . Inside & Out Special Issue Way Milky 63 Imager Revolution Rainbow Symphony ScopeStuff Messier Flashcards NexDome the to & Chile - Ancient Paths Peru Engineering Peterson Parts Precise Scope Buggy Jewelry Designs for MenJewelry for Designs Iceland 2018 - Great North, 2018 - Great Iceland iOptron Meade Instruments Mercury Globe Celestron Bob Berman Tours Bob Berman Astronomy magazine Astronomy Astro-Physics Armstrong MetalcraftsArmstrong Haven...... 7 Astro AP Theory magazine readers. The magazine is not responsible for magazine readers. omissions or for typographical errors in names or page numbers. The Advertiser Index is provided as a service to The BINOCULARUNIVERSE BY PHIL HARRINGTON Remembering An experienced observer’s legacy. John Davis HALLAS TONY BY IMAGES Davis’ Dinosaur is an asterism of two dozen 6th- to 8th-magnitude stars in Hercules.

ended last month’s col- stories about conquests through triceratops), while three other Theta (θ) Delphini, is smaller umn with two asterisms our binoculars. It quickly points due west, including than the previous two, making that were first spotted and became clear to me that not 49 Her, define its head and pro- it ideal for higher-power binocu- named by Massachusetts only did he have exceptional trusive third horn. A line under lars. Imagine Theta as the amateur astronomer observing skills, but his inven- the backbone from 60 Her to horse’s saddle. East of Theta, IJohn Davis. His keen eye and tive mind also let him see pat- 8th-magnitude SAO 102511 four faint stars form a backward unstoppable passion for observ- terns, or asterisms, through his completes the underside of the 7, defining the horse’s raised ing spanned half a century. binoculars where I just saw ran- tail, while an extension south- hind leg and tail. Southwest of Born in Manchester, New dom patterns of stars. westward to 6th-magnitude Theta, five faint points outline Hampshire, he spent most of Davis passed away earlier SAO 102474 forms its right leg its arched back, neck, and head. his adult life in Amherst, this year, and so in his memory, and foot. Finally, a faint mean- The cowboy himself is drawn Massachusetts. There, the I would like to devote this col- dering of stars hooks around to from an arc of 10th-magnitude University of Massachusetts’ umn to some of the asterisms 49 Her to complete the profile. stars northwest of Theta. thriving astronomy program, that he created. I mentioned It sounds complicated, but with Our final asterism lies within with its many public outreach two in last month’s column the aid of low-power binoculars, Cassiopeia, making it circum- events, stoked Davis’ interest. — the Garden Trowel and the the Dinosaur stands out nicely. polar for those of us viewing For many decades, he was a Hockey Stick. Here are four M27, the famous Dumbbell from mid-northern latitudes. well-known personality at ama- more. Can you see what he saw? Nebula in Vulpecula, is a show- It’s dubbed the Queen’s Kite, teur astronomy organizations Let’s begin at the red star piece of the summer sky. While and its distinctive diamond- and events across New England. Rasalgethi (Alpha [α] Herculis) viewing it once through his bin- shaped frame can be found He was an active longtime in far southern Hercules. Look oculars, Davis noticed that it about 2° southeast of Delta (δ) member of the Arunah Hill about 3° to its west, and you seemed to lie just south of a pat- Cassiopeiae. The top point Natural Science Center in should spot a fainter, orangish tern that looked like a Cowboy of the kite is marked by 5th- Worthington, Massachusetts, star. That star, SAO 102553, is Boot. The sole itself is drawn by magnitude Chi (χ) Cassiopeiae. as well as of the Amherst Area midway along an arc of stars connecting a line from 12 Half a dozen 6th- and 7th- Amateur Astronomers beginning at 60 Herculis and Vulpeculae through 14, 17, and magnitude stars fill out the rest Association (the 5As), from ending 4° northwest at 8th- finally 22 Vul, with 12 at the tip of the kite’s diamond-shaped whom he received a Lifetime magnitude SAO 102489. That’s of the toe and 22 the heel. body, seen cocked toward the Achievement Award. the tail and spine of Davis’ Continue northward from 22 to southeast. SAO 22566 forms the I first met Davis some Dinosaur, a collection of about 24, 19, and 23 Vul to create the southernmost tip. Five more three decades ago at the two dozen 6th- to 8th-magni- back of the boot. Westward stars in an arc wind toward the Astronomers’ Conjunction, a tude stars spanning 7°. Two from 23 to 15 Vul marks the west-southwest for the tail. small annual convention in faint stars north-northwest of Boot’s top opening, followed by We will visit more asterisms Northfield, Massachusetts. We SAO 102584 mark the tips of a southward plunge to 16 Vul for first drawn with John Davis’ became fast friends, sharing the dinosaur’s two horns (think the front. Finally, curve from 16 imaginative eyes in future col- to 13, and back to 12 Vul for the umns, including an airplane, a Nestled in Cassiopeia, pointy toe. Like the Dinosaur, muscleman, and a dog. the Queen’s the Cowboy Boot fills the 7° And when you look sky- Kite sports field of my 10x50 binoculars. By ward, pause to remember John. a diamond- His binoculars were always shaped frame scanning north-northeast of the of gemmy Boot, you will also see two long close at hand when stargazing. stars. strings of stars that he called the That’s because, like me, John Flying Bootlaces. believed that two eyes are What would a cowboy boot better than one. be without a cowboy? Davis gave us one on the back of a Bucking Phil Harrington is a longtime Bronco within the constellation contributor to Astronomy and Delphinus. This asterism, the author of many books. centered on 5th-magnitude

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WWW.ASTRONOMY.COM 67 ASKASTR0 Astronomy’s experts from around the globe answer your cosmic questions. THE EXPANDING UNIVERSE Q: HOW FAST IS THE UNIVERSE EXPANDING? HOW DO ASTRONOMERS CALCULATE THE

EXPANSION RATE? Antonio Fabre, Miami, Florida

A: The expansion rate of the an object and some other universe is called the Hubble observable parameter. For parameter. Because the fabric example, Cepheid variable stars of the universe is being pulsate periodically, and that stretched out as it expands, period is related to their lumi- galaxies farther away from us nosity. Using independent appear to be moving away methods of distance measure- faster. This is why the Hubble ment, we can determine the parameter is measured in units distance to Cepheids locally. of kilometers per second per And because we know how megaparsec (km/s/Mpc). bright they should be, we can We don’t know the rate use them as “standard candles” exactly, but in the last 50 years, to calibrate our distance mea- we’ve narrowed it down to surements when we see them Dramatic sweeps of clouds and intricate features make Jupiter look like a either 67 or 73 km/s/Mpc. in nearby galaxies. Moving whole new planet in this JunoCam image of the planet’s southern hemisphere. JunoCam can image Jupiter at resolutions nearly 10 times that of That’s not to say we believe outward, other studies have previous spacecraft, reaching resolutions of 2 miles (3 km) per pixel at the true expansion rate lies linked measurable galaxy closest approach to the planet near Jupiter’s equator, and 31 miles (50 km) between those two values, but properties to their luminosity, per pixel over the poles. NASA/JPL-CALTECH/SWRI/MSSS/GERALD EICHSTÄDT rather we think it’s reasonably allowing astronomers to then close to either one or the other. calculate their distances. 67 km/s/Mpc, while those based A: There are several reasons So a galaxy 1 Mpc away — 3.26 Approaching from another on Cepheids and galaxies favor why the JunoCam images of million light-years — is moving side, scientists also study the 73 km/s/Mpc. Jupiter look so different. One away from us at 73 km/s (or cosmic microwave background It’s possible that the Laser of them is the perspective from 67 km/s, depending on which (CMB), which hints at the Interferometer Gravitational- Juno: The spacecraft is in a scientists you’re talking to). A appearance of the very young wave Observatory (LIGO), hav- polar orbit, so images taken at galaxy 10 Mpc away would be universe. Based on our current ing proven its ability to detect high latitudes and near the moving at 730 (or 670) km/s. standard model, astronomers neutron star collisions, may poles are not foreshortened, as Two pieces of information can compare observations of give us a deciding vote. Or it they are in Earth-based images are needed to calculate the tiny temperature differences may give us a third contender. (and those from previous Hubble parameter: how far in the CMB with simulations Valerie Mikles spacecraft). That’s why the away an object is, and how fast using various values for the National Oceanic and Atmospheric turbulent regions at high lati- it appears to be moving. The Hubble parameter to find the Administration Contractor, tudes look so stunning. redshift of a galaxy — the best match. Quality Assurance, I.M. Systems Group, JunoCam’s images are also at a amount its light has been Ultimately, the difference College Park, Maryland higher resolution than those stretched due to its motion between 67 and 73 km/s/Mpc taken before. away from us — gives the lat- could come down to different Another reason is JunoCam’s ter. Measuring the former has assumptions made along the Q: THE PROCESSED wide field of view, which spans been notoriously harder way when arriving at a value. IMAGES FROM JUNO ARE 58°. So instead of capturing a because while an object’s Alternatively, each method SPECTACULAR, BUT THEY mere snippet of the planet, we brightness (or luminosity) looks at different epochs in our DON’T RESEMBLE PREVIOUS see an entire swath of swirling grows dimmer with distance in cosmic history, and could be IMAGES OF JUPITER. WHY clouds at high resolution in a predictable way, an astrono- telling us that our infant uni- ARE THEY SO DIFFERENT every image. mer must already know lumi- verse followed a set of rules FROM THOSE RETURNED Candice Hansen nosity to establish distance. different from the ones we live BY EARLIER MISSIONS? Senior Scientist, Planetary Science The key is to find a relation- by today. Calculations based Rick Kelley Institute, and Co-Principal Investigator ship between the luminosity of on the CMB favor a value of Hilo, Hawaii of JunoCam, Ivins, Utah

68 ASTRONOMY • AUGUST 2018 Q: ONE NIGHT I NOTICED Second, localized atmo- A STAR CHANGING spheric disturbances can be Scrambled starlight COLORS AND MOVING created as heat dissipates from PERCEPTIBLY. BUT warm areas of the ground; look- Star THROUGH BINOCULARS, ing at the sky in the direction of Shorter path A FEW DIMMER STARS a concrete building or asphalt Less twinkling IN THE SAME FIELD parking lot may cause stars in APPEARED STEADY. THE that direction to twinkle more, Longer path BEHAVIOR CONTINUED while stars in other parts of the More twinkling THROUGHOUT THE NIGHT, sky might appear more stable. BUT NOT A COUPLE OF Finally, because the amount NIGHTS LATER. WHY? of scintillation depends so Allan Hawkinson much on what the atmosphere Atmosphere Carlsbad, New Mexico is doing at any particular moment, differences in wind A: You’re absolutely right that speed, humidity, temperature, stars twinkle — and sometimes and other factors from day to appear to move around — due day can affect how much stars Stars scintillate, or twinkle, more or less based on a few factors. These include stars’ apparent brightness and their altitude in the sky. Brighter to our atmosphere “scrambling” twinkle on different nights. stars appear more affected by this phenomenon, while stars closer to the their light as it travels from the Alison Klesman horizon appear to twinkle more often than stars overhead because you’re top of Earth’s atmosphere to Associate Editor observing them through more air. ASTRONOMY: ROEN KELLY the ground. This phenomenon, also called scintillation, tends from our point of view. So We can answer your second to occur more obviously in Q: HOW ARE SO MANY Venus typically misses crossing question by doing a back-of- bright stars. Your eye isn’t sen- EXOPLANETS DETECTED VIA the face of the Sun by a wide the-envelope calculation using sitive enough to pick up the TRANSITS, WHILE TRANSITS margin, making transits rare. planets discovered by a differ- changes as easily in a dim star, OF MERCURY AND VENUS As a result of its orbital tilt, ent method called radial veloc- even through binoculars or ARE SO RARE? DO ALL THE Venus can transit only when it ity (RV, which identifies your telescope. CHANCE TRANSIT ALIGN- is near one of two points in its planets by their gravitational A star’s brightness aside, MENTS FIT STATISTICALLY? orbit where it crosses Earth’s tug on a star), and then asking scintillation can actually be a Kenneth Caine orbital plane. Due to the ratio how many of them transit. localized effect in a few cases. Crafton, Pennsylvania of Earth’s orbital period com- About 10 out of 481 RV planets First, stars close to the horizon pared with Venus’ period, tran- transit, which is in line with twinkle more because their A: To answer your first ques- sits come in pairs separated by what one would expect from light must travel through more tion, let’s start with the transit eight years and then long gaps random orientations, based on air to reach your eye. As they of Venus, whose orbit is tilted on the order of 100 years. average values for the sizes of rise and the light travels with respect to Earth’s orbit by Now, imagine that Earth is stars and the distance of exo- through progressively less air, about 3.4°. This seems like a stationary where the plane of planets from them. the effect lessens, and stars at small amount, but it leads to a its orbit intersects with Venus’ Nathan De Lee the zenith (the overhead point) maximum separation of 9.6° orbital plane. We would get a Assistant Professor, Department of are affected least. between the Sun and Venus transit once every Venus orbit Physics, Geology and Engineering Tech, (225 days). This is essentially Northern Kentucky University, the situation we have with exo- Highland Heights, Kentucky planets. We are so far away from those stars that Earth’s orbital motion is insignificant. Send us your This means that the only thing questions controlling whether a transit Send your astronomy occurs is the orbital inclination questions via email to of the transiting system with [email protected], respect to our line of sight. We or write to Ask Astro, assume these orbits are ran- P. O. Box 1612, Waukesha, domly oriented in the sky, and WI 53187. Be sure to tell us from that, we can statistically your full name and where Venus crosses the face of the Sun in this ultra-high-definition image taken derive the number of planets you live. Unfortunately, we by NASA’s Solar Dynamics Observatory June 5-6, 2012. Transits of Venus are visible from Earth about twice a century. Similar transits allow astron- total in a given population of cannot answer all questions omers to identify planets in other solar systems by watching for dips in stars, based on the number of submitted. starlight associated with a planet crossing the face of its star. NASA/SDO, AIA transits that we observe.

WWW.ASTRONOMY.COM 69 READER GALLERY

1. SOUTHERN SWIRL NGC 289 is a large galaxy, although its surface brightness is low. This barred spiral’s main challenge for northern observers, however, is that it lies in the faint constellation Sculptor far to the south, so it’s relatively unknown. Jupiter NGC 289 is 62 million light-years away. • Adam Block/ChileScope The Moon

2. PLANET PARADE Mars Three planets are aligned with the waning gibbous Moon in this photo, Saturn which consists of four shots: two of the night sky and two of the foreground. The imager said it was a bit tricky to register the planets and several stars without blowing out the Moon. However, it worked out, and the photo even shows the nice orange glow of the clouds and the Sun’s first light behind the trees. • Ryan Imperio

70 ASTRONOMY • AUGUST 2018 3. GASSY CHANNEL The Lagoon Nebula (M8) is one of the great deep-sky objects visible in the sky. Classified as an emission nebula and a star-forming region, it lies in the constellation Sagittarius the Archer some 5,000 light-years away. • Gerald Rhemann

4. COLUMN OF FIRE This Sun pillar appeared at sunset in northwest Missouri on February 7, 2018. This phenomenon appears when tiny ice crystals in the atmosphere reflect the Sun’s reddened light. • Jared Bowens

5. WINTER STARSCAPE The winter sky shines brightly despite some low-altitude clouds near Masal in the Gilan Province of Iran. Orion the Hunter is visible at center. The three 3 stars of his belt point downward to Sirius (Alpha [α] Canis Majoris), the night sky’s brightest star. They point upward to the V of Taurus the Bull and then to the Pleiades star cluster (M45). • Amir Shahcheraghian

6. TOTAL SUCCESS To create this composite image, the photographer combined 28 exposures of the lunar eclipse January 31, 2018, taken at the Gardens by the Bay in Singapore. Shots of the total phase, which lasted 1 hour and 16 minutes, show a copper-colored Moon. • Chan Chee Guan

4 5

WWW.ASTRONOMY.COM 71 7 8

72 ASTRONOMY • AUGUST 2018 7. MONKEY SEE … Star-forming region NGC 2174, often called the Monkey Head Nebula, lies in the constellation Orion about 6,400 light-years from Earth. A number of stars already have been born within the cloud, and their ultraviolet radiation makes the gas glow while their high-speed stellar winds cause the nebula to disperse. • Chuck Ayoub

8. DAYLIGHT CRESCENT The Moon was only 2.1 percent illuminated when this imager shot it March 18, 2018 at 6:21 P.M. He captured our lone natural satellite through a 4-inch refractor, and said that he couldn’t see it at all with his naked eyes or through the finder scope. • Chuck Manges

9. TAKING STOCK Stock 21 is an open cluster some 3,600 light-years away in the constellation Cassiopeia the Queen. The cluster is the small grouping of stars between the top and center of this image. • Dan Crowson

10. DESERT DIPPER The northern sky is the target of this 13-second exposure captured at Iran’s Seqaleh Desert on a spring moonlit night. Rainfall the previous day created a small pond. In the sky, you can see the Big Dipper, whose pointer stars direct our gaze to Polaris, the North Star (Alpha [α] Ursae Minoris), just to the left of the foreground tree. • Amirreza Kamkar

11. SUNSET FLIGHT As this photographer was capturing a sunset January 9, 2018, over the Gulf of Mexico, a flock of seabirds flew just above the water. He even captured their reflections. • John Chumack

Send your images to: Astronomy Reader Gallery, P. O. Box 1612, Waukesha, WI 53187. Please include the date and location of the image and complete photo data: telescope, camera, filters, and exposures. Submit images by email 11 to [email protected].

WWW.ASTRONOMY.COM 73 BREAK THROUGH Face-on fireworks

Like a pearl at the center of a drab oyster, galaxy NGC 3344 overshadows its lackluster host constellation, Leo Minor. This barred spiral is roughly half the size of the Milky Way and lies some 20 million light-years from Earth, a mere stone’s throw in the universe of galaxies. NGC 3344’s proximity and face-on appearance give astronomers a grand view of the galaxy’s structure. Blue star clusters and the reddish clouds of hydrogen that will give birth to a new generation dot the spiral arms, while the yellowish glow of old suns dominates the galaxy’s nucleus and bar. NASA/ESA

74 ASTRONOMY • AUGUST 2018

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As twilight fades in early pace than Venus. By the 31st, it Red Planet dims to magnitude separate the two, we always October, four naked-eye planets stands about 10° high 45 min- –0.6 by month’s end, you still have at least one on display. line up across the sky. Venus utes after sundown. won’t find a brighter object Although these two clusters anchors the dramatic scene. The best views of Jupiter closer than Jupiter, which lies often steal the show, another The brilliant world shines at through a telescope come early nearly halfway across the sky. delightful globular appears magnitude –4.8, some 16 times in the month when it remains Mars and its host constellation, prominent on October eve- brighter than its closest com- quite high in the west after Capricornus, pass nearly over- nings. M22 lies in northern petitor, Jupiter. On October 1, darkness falls. The gas giant’s head in early evening. Sagittarius, where our Venus stands 20° above the disk then measures 33" across Following late July’s excep- Northern Hemisphere friends western horizon an hour after and shows stunning detail. Any tional opposition, Mars appears also can enjoy it. You can find sunset. It lies in eastern Virgo, instrument reveals two dark noticeably smaller through a this 5th-magnitude cluster 2.4° close to that constellation’s bor- belts, one on either side of a telescope in October. It begins northeast of magnitude 2.8 der with Libra and Hydra. brighter zone that coincides the month with a disk that Lambda Sagittarii and just Although Venus crosses into with Jupiter’s equator. spans 16", and this shrinks to 0.6° east of the magnitude 5.5 Libra by the next evening, its You’ll have to search harder 12" by month’s end. Surface star 24 Sgr. eastward motion soon stalls out, to find our next target. Saturn features should still show up M22 shows up to the naked and it returns to Virgo on the glows at magnitude 0.5 — through most telescopes, but eye under a dark sky, and looks 9th. It picks up speed as it draws bright, but nowhere near as you’ll have to wait for moments impressive through 7x50 bin- closer to the Sun in our sky, and conspicuous as its planetary when Earth’s turbulent atmo- oculars. A telescope delivers disappears in the twilight glow siblings. The ringed world lies sphere steadies and details snap even better views. I especially at the end of October’s third some 40° east of Jupiter and into focus. enjoy observing it through a week. Venus passes between the stands some 60° above the west- While Venus disappears 20-centimeter instrument, Sun and Earth at inferior con- ern horizon as twilight ends in from view in October, Mercury which reveals the cluster’s junction on the 26th, and will early October. Even at the end takes its place. You can spot the slightly elliptical shape and a return to view before dawn by of the month, it’s more than 30° two worlds in evening twilight large number of its stars. mid-November. high as darkness falls. Saturn at midmonth. On the 15th, When German astronomer A small telescope delivers resides among the background Mercury appears 6° (about one Abraham Ihle discovered the wonderful views of the planet stars of Sagittarius, a few binocular field) to the lower cluster in August 1665, he in the weeks leading up to infe- degrees northwest of magnitude right of its brilliant neighbor. quickly recognized it as some- rior conjunction. On October 1, 2.8 Lambda (λ) Sagittarii, the Mercury shines at magnitude thing out of the ordinary. He Venus shows a disk that spans star that marks the lid of that –0.3 and should appear obvious found it while observing 46" and is 17 percent lit. The constellation’s Teapot asterism. within 45 minutes after sunset. Saturn, which at the time planet’s angular size swells and Saturn’s high altitude makes The innermost planet climbs was just a couple of degrees its crescent phase dwindles day it a great subject to observe higher with each passing day, northwest of the cluster. by day. On the 15th, the Sun through a telescope. The plan- sliding 3° south (upper left) of (Coincidentally, the ringed illuminates just 5 percent of the et’s globe measures 16" across Jupiter on October 29 and 30. planet lies 5° from the cluster inner world’s 57"-diameter disk. while the spectacular rings A telescope reveals Mercury’s this month.) Jupiter appears to Venus’ span 37" and tip 27° to our line small gibbous disk. M22 also holds the distinc- upper right on October eve- of sight. This large tilt affords tion of being one of only four nings. Although the giant excellent views of the dark The starry sky globular clusters known to planet shines three magnitudes Cassini Division that separates Southern Hemisphere observ- contain a planetary nebula. fainter than its neighbor, it’s the outer A ring from the ers should feel fortunate. Only Unfortunately, you won’t see still the sky’s second-brightest brighter B ring. people on our half of the globe the faintly glowing embers point of light. Jupiter lies As you scan farther east, you can enjoy great views of the of this dying Sun-like star against the backdrop of Libra, can’t help but see the orange- two brightest globular star through your telescope. The below the more conspicuous red glow of Mars. It shines clusters: Omega Centauri Infrared Astronomical Satellite stars of Scorpius. The planet at magnitude –1.3 in early (NGC 5139) and 47 Tucanae discovered the nebula in 1989; dips lower with each passing October, five times brighter (NGC 104). And because follow-up observations proved evening, though at a slower than Saturn. And though the 11 hours of right ascension that it belonged to the cluster. STAR S

DOME

MUSCA NGC 2516 NGC

THE ALLSKY MAP CENTAURUS SHOWS HOW THE `

SKY LOOKS AT: VOLANS

CIRCINUS 10 P.M. October 1 _

9 P.M. October 15

PICTOR AUSTRALE

8 P.M. October 31 CHAMAELEON TRIANGULUM

NGC 2070 NGC Planets are shown MENSA

at midmonth

SCP LMC

LUPUS

APUS

RADO

O HYDRUS

NORMA

RETICULUM

ARA

NGC 6231 NGC

SMC

OCTANS

NGC 6397 NGC

PAVO

NGC 104 NGC

SCORPIUS

HOROLOGIUM

TELESCOPIUM

TUCANA

Achernar Antares

AUSTRALIS

CORONA

INDUS

MICROSCOPIUM

SAGITTARIUS

Saturn

GRUS

M20

M22

PHOENIX M17

OPHIUCHUS

M16

PISCIS SCULPTOR SGP W SCUTUM AUSTRINUS

Mars NGC 253 NGC Fomalhaut CAPRICORNUS

SERPENS CAUDA

M11 AQUILA

AQUARIUS

EQUULEUS

Altair Enif VULPECULA SAGITTA DELPHINUS M15 PISCES

PEGASUS

CYGNUS MAGNITUDES LACERTA Sirius Open cluster

0.0 Globular cluster M31 1.0 Diffuse nebula 2.0 Deneb 3.0 Planetary nebula ANDROMEDA 4.0 5.0 Galaxy

N HOW TO USE THIS MAP: This map portrays the sky as seen near 30° south latitude. OCTOBER 2018 Located inside the border are the four directions: north, south, east, and west. To find stars, hold the map Calendar of events overhead and orient it so a

Canopus direction label matches the 2 Last Quarter Moon occurs at 17 The Moon is at apogee direction you’re facing. 9h45m UT (404,227 kilometers from Earth), The stars above the 19h16m UT 5 map’s horizon now Venus is stationary, 4h UT

18 The Moon passes 1.9° north of

COLUMBA match what’s DO Mercury passes 2° north of Spica, in the sky. Mars, 13h UT 18h UT 20 The Moon passes 3° south of

The Moon is at perigee Neptune, 22h UT CAELUM (366,392 kilometers from Earth), 22h27m UT 21 Orionid meteor shower peaks 7 Dwarf planet Ceres is in 24 Uranus is at opposition, 1h UT conjunction with the Sun, 10h UT The Moon passes 5° south of 9 New Moon occurs at 3h47m UT Uranus, 13h UT 10 The Moon passes 13° north of Full Moon occurs at 16h45m UT Venus, 15h UT 26 Venus is in inferior conjunction, 11 The Moon passes 4° north of 14h UT Jupiter, 21h UT

30 Mercury passes 3° south of ERIDANUS 14 Mercury passes 7° north of Jupiter, 4h UT Venus, 15h UT 31 Last Quarter Moon occurs at E 15 The Moon passes 1.8° north of 16h40m UT Saturn, 3h UT The Moon is at perigee 16 Asteroid Juno is stationary, (370,204 kilometers from Earth), 18h UT 20h23m UT First Quarter Moon occurs at Mira 18h02m UT

CETUS

Uranus Path o f the Sun (ecliptic)

ARIES STAR COLORS: Stars’ true colors depend on surface temperature. Hot stars glow blue; slight- M33 ly cooler ones, white; intermediate stars (like the Sun), yellow; followed TRIANGULUM by orange and, ulti mately, red. Fainter stars can’t excite our eyes’ color receptors, and so appear white without optical aid.

Illustrations by Astronomy: Roen Kelly

BEGINNERS: WATCH A VIDEO ABOUT HOW TO READ A STAR CHART AT www.Astronomy.com/starchart. SHOP NOW FOR SCIENCE & ASTRONOMY PRODUCTS

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