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Home , 51 Eridani, 51 Eridani b, Antlia Dwarf, Aquarius Dwarf, Barred irregular galaxy, Daniel Pomarede

Travel through the whole in 30 pages! EXTRA! DECEMBER 2015

THE COSMOS TO SCALE

The world’s best-selling magazine SPECIAL ISSUE THE IMMENSITY OF THE

COSMOS p. 20 • Gas, ice, and rocks in the p. 24 • The ’s barred spiral p. 30 • Gravitational supremacy in the p. 44 • The of p. 50 • To the limits of the universe! p. 56

www.Astronomy.com

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For information on all of our products and services, or to find an authorized Sky-Watcher USA dealer near you, just visit www.skywatcherusa.com. Don’t forget to follow us on Facebook and Twitter for exclusive offers! Online Content Code: ASY1512 Enter this code at: www.astronomy.com/code DECEMBER 2015 to gain access to web-exclusive content VOL. 43, NO. 12 TONY HALLAS TONY ON THE COVER The universe, at least 93 billion light- across, is awash in some 100 billion galaxies like NGC 6946, a beautiful face-on spiral. CONTENTS 20 COLUMNS FEATURES Strange Universe 10 20 36 50 BOB BERMAN How immense is the The Sky this Month The : For Your Consideration 14 JEFF HESTER universe? A gemlike shower. Our 100,000 closest Unfettered by tales of science MARTIN RATCLIFFE AND galaxies Observing Basics 64 fiction, the cosmos is almost ALISTER LING Small groups combine with larger GLENN CHAPLE incomprehensibly large. clusters to form a vast network Secret Sky 66 DAVID J. EICHER 38 of interconnected galaxies that STEPHEN JAMES O’MEARA StarDome and spans 110 million light-years. 68 24 SARAH SCOLES Cosmic Imaging Path of the ADAM BLOCK Our solar system: RICHARD TALCOTT; Realms of fire and ice ILLUSTRATIONS BY ROEN KELLY 56 We start your tour of the cosmos Limits of the cosmos: QUANTUM with gas and ice giants, a lot of 44 The far reaches of space Snapshot 9 rocks, and the only known abode The Local Group: Everything astronomers have Astro News 12 for life. FRANCIS REDDY Our galactic learned points to a universe neighborhood populated by of gal- IN EVERY ISSUE 30 axies, incredibly long filaments, The Milky Way’s family of galax- The Milky Way: and colossal voids. LIZ KRUESI From the Editor 6 ies is locked in a multi-billion- ’s home battle for gravitational Letters 10, 14, 64 Giant clouds of gas and dust supremacy. KATHERINE KORNEI 62 Web Talk 65 sprinkled with splashy clus- Ask Astro New Products 67 ters adorn the Milky Way’s spiral Gathering the dark. arms, while the galaxy’s vast halo Advertiser Index 70 teems with darker matter. Reader Gallery 72 FRANCIS REDDY Breakthrough 74

Visit Astronomy.com/toc for bonus material — it’s ONLINE exclusive to Astronomy FAVORITES magazine subscribers. Astronomy (ISSN 0091-6358, USPS 531-350) is pub- Go to www.Astronomy.com Blogs from The Sky Reader Photo 50 Weirdest lished monthly by Kalmbach Publishing Co., 21027 Crossroads Circle, P. O. Box 1612, Waukesha, WI for info on the biggest news and the Local this Week Gallery Objects 53187–1612. Periodicals postage paid at Waukesha, observing events, stunning photos, Group A daily digest Share your Explore the WI, and additional offices. POSTMASTER: Send Insight from of celestial astroimaging most curious address changes to Astronomy, 21027 Crossroads informative videos, and more. Circle, P. O. Box 1612, Waukesha, WI 53187–1612. the editors events efforts cosmic oddities Canada Publication Mail Agreement #40010760.

4 ASTRONOMY • DECEMBER 2015 LARGE APERTURE VIEWS ROCK SOLID DESIGN NO COMPROMISE

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OPT Telescopes – 800.483.6287 – opttelescopes.com High Point Scientific – 800.266.9590 – highpointscientific.com B&H Photo – 800.947.9970 – bhphotovideo.com Optics – 800.504.5897 – opticsplanet.com Hands On Optics – 866.726.7371 –handsonoptics.com Focus Camera – 800.221.0828 – focuscamera.com Astronomics – 800.422.7876 – astronomics.com Woodland Hills – 888.427.8766 – telescopes.net Adorama – 800.223.2500 – adorama.com 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 Karri Ferron Associate Editors Eric Betz, Korey Haynes The end of Editorial Associate Valerie Penton ART Senior Graphic Designer Chuck Braasch Illustrator Roen Kelly Production Coordinator Jodi Jeranek everything CONTRIBUTING EDITORS Bob Berman, Adam Block, Glenn F. Chaple, Jr., Martin George, Tony Hallas, Phil Harrington, Jeff Hester, Liz Kruesi, Ray Jayawardhana, Alister Ling, Steve Nadis, Stephen James O’Meara, Tom Polakis, Martin Ratcliffe, Mike D. Reynolds, Sheldon Reynolds, Erika Rix, Raymond Shubinski EDITORIAL ADVISORY BOARD Buzz Aldrin, Marcia Bartusiak, Timothy Ferris, Alex Filippenko, Adam Frank, John S. Gallagher lll, Daniel W. E. Green, William K. Hartmann, Paul Hodge, Anne L. Kinney, Edward Kolb, Stephen P. Maran, Brian May, S. Alan Stern, James Trefil his special issue of cosmos will end? It is a stupe- reach a point of inactivity Astronomy has been fyingly difficult problem. called heat death. Kalmbach Publishing Co. a long time coming. Over the decades, astrono- The mileposts along the President Charles R. Croft Vice President, Editorial Kevin P. Keefe Delivering stories that mers have considered three way through a Big Freeze Senior Vice President, Sales & Marketing Daniel R. Lance address the cosmic leading possible outcomes for scenario give us a particu- Vice President, Consumer Marketing Nicole McGuire Editorial Director Diane Bacha Tdistance scale, the incredibly the distant future. The first larly fascinating glimpse of Corporate Art Director Maureen M. Schimmel large size of the universe, has says the universe has enough the probable future. The Art and Production Manager Michael Soliday Corporate Advertising Director Ann E. Smith been a topic our editors have to eventually halt its current Stelliferous Era, Single Copy Specialist Kim Redmond planned for some time. expansion and will fall back when normal and gal- ADVERTISING DEPARTMENT (888) 558-1544 While we can all appreci- on itself in a “Big Crunch.” axies are operating as they Advertising Sales Manager Steve Meni Advertising Sales Representative ate the immense size of it all, The second is a terrifying should be, has a long time to Dina Johnston, [email protected] cosmic evolution brings up a one, too: Some cosmological go. As we’ve seen, some Ad Services Representative Christa Burbank, [email protected] related topic: What is the scenarios suggest that gravity 4 billion years from now, the RETAIL TRADE ORDERS AND INQUIRIES fate of the universe? will become too weak to hold Milky Way and Andromeda Selling Astronomy magazine or products in your store: Phone (800) 558-1544, Press 3 This elegant question is not individual galaxies together, galaxies will merge into a Outside U.S. and Canada (262) 796-8776, ext. 818 an easy one to answer. We and the universe will experi- single galaxy we call Fax (262) 798-6592 Email [email protected] know that as we look out in ence a “Big Rip” in which Milkomeda. Some 100 bil- Website www.Retailers.Kalmbach.com space, we’re looking back in matter will be torn asunder. lion to 1 trillion years from CUSTOMER SALES AND SERVICE Phone (800) 533-6644; Fax (262) 796-1615 time. The distant universe is a The likeliest scenario, now, the Local Group will [email protected] snapshot of what existed bil- however, is the third, the merge into a giant galaxy. SPECIAL EMAIL ADDRESSES Ad Sales [email protected] lions of years ago, and we do “Big Freeze.” The cosmos may have Ask Astro [email protected] not have an accurate picture Most cosmologists believe started with a bang but will Books [email protected] Letters [email protected] of many of the objects we see the universe will expand most likely end with a whim- Products [email protected] as they really are now, at this forever and that a colder, per. Trillions of years from Reader Gallery [email protected] exact point in time. Knowing darker cosmos lies in the now, the universe likely will Editorial phone: (262) 796-8776; advertising: (888) 558-1544; customer service & sales: (800) 533-6644; outside the U.S. and Canada: (262) 796- the status of objects in the distant future. As billions undergo heat death, becom- 8776, ext. 421, Monday through Friday, 8:30 A.M. to 4:30 P.M. CT, Fax: (262) 796-1615; Email: [email protected]. Please include your universe in the “here and and even trillions of years ing not only dark and dead, name, mailing address, and telephone number with any correspondence. Copyright © 2015 Kalmbach Publishing Co., all rights reserved. This publi- now” only works well for our roll on, will stretch but also eternally cold. In this cation may not be reproduced in any form without permission. Printed in the U.S.A. Allow 6 to 8 weeks for new subscriptions and address changes. solar system — for Earth, the photons into undetectable state, no thermodynamically 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: Add $12.00 , and our family of plan- wavelengths, and eventually free energy would be avail- postage per year. Canadian price includes GST, payable in U.S. funds. All other international subscriptions: Add $16.00 postage per year, payable in ets, asteroids, and comets. the supply of gas that could able, so no more processes U.S. funds, drawn on a U.S. bank. Expedited delivery service surcharges: Domestic first class $30/yr; Canadian air $30/yr; International air $60/yr. As we look progressively make new stars will be could occur that would con- BN 12271 3209 RT. Not responsible for unsolicited materials. out even into our Milky Way exhausted. Ultimately, stellar sume energy. It would be the Galaxy, we begin to see remnants also will be gone, ultimate end for any activity things as they were, more so leaving behind only black in the universe. What an as distances increase. This holes, which themselves will anticlimax! makes interpretation diffi- ultimately disappear due to cult. How do we use what we Hawking radiation. An Yours truly, Follow Astronomy see well into the universe’s incredibly long way down history to predict how the the line, the universe will 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

6 ASTRONOMY • DECEMBER 2015 “When I went back to viewing, I wanted the best... 24” f/3.85 Slipstream telescope and Tele Vue eyepieces.” —Tony Hallas

M24 region imaged by Tony Hallas using a Tele Vue-NP101is refractor. Tony Hallas, Renowned Astrophotographer, Returns to the Eyepiece (from an unsolicited e-mail to David Nagler) Hi David and Al, Although I am still active in imaging, I have decided to go back to viewing and have taken possession of a new 24” f/3.85 Slipstream telescope from Tom Osypowski. You will be happy to know that I have acquired a treasure trove of TeleVue eyepieces to complement this telescope, specifically: 26 and 20mm Nagler Type 5, 17.3, 14, 10, 6, 4.5mm Delos, Paracorr Type 2, and 24mm Panoptics for binocular viewing. After using a Delos, “that was all she wrote;” you have created the perfect eyepiece. The Delos eyepieces are a joy to use and sharp, sharp, sharp! I wanted to thank you for continuing your quest to make the best eyepieces for the amateur community. I am very glad that you don’t compromise ... in this world there are many who appreciate this and appreciate what you and Al have done for our avocation. Hard Tony with his Tele Vue eyepiece collection awaits to imagine what viewing would be like without your creations. a night of great observing at his dark-sky site. Best, Tony Hallas

® Tele Vue 32 Elkay Dr., Chester, New York 10918 (845) 469-4551. televue.com Visionary The Search for : What Astronomers Know Taught by Professor Joshua N. Winn MASSACHUSETTS INSTITUTE OF TECHNOLOGY TIME ED O T FF LECTURE TITLES I E IM R L 1. Why Study Exoplanets? 70% 2. How to Find an 3. Doppler and Transit Planet-Finding Methods 3 O off 4. Pioneers of Planet Searching R R D E E B 5. The Misplaced Giant Planets R M BY DECE 6. Explaining the Misplaced Giant Planets 7. The Transits of Exoplanets 8. Sniffi ng Planetary 9. and Planetary Revolution 10. Super- or Mini-Neptunes? 11. Transiting Planets and the Kepler Mission 12. Compact Multiplanet Systems 13. Planets Circling Two Stars 14. Lava Worlds 15. Earthlike Planets 16. Living with a Dwarf Star 17. Living with a 18. Our Nearest Exoplanetary Neighbors 19. Finding Planets with Gravitational Lensing 20. Finding Planets with Direct Imaging 21. Near-Term Future Planet-Finding Projects 22. Long-Term Future Planet-Finding Projects 23. The Search for Life on Exoplanets 24. Coming Soon: Biosignatures, , and More!

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Embark on this visually stunning and unrivaled adventure in 24 lectures by veteran planet hunter and Associate Professor of at MIT, Dr. Joshua N. Winn. Designed for everyone from armchair explorers to serious skywatchers, The Search for Exoplanets follows the numerous twists and turns in the hunt for exoplanets - the false starts, the sudden Priority Code: 110121 breakthroughs, and the extraordinary discoveries. Exoplanets will be making big news for years to come, and this course will equip you to For 25 years, The Great Courses has brought the understand one of the most momentous developments of our time. world’s foremost educators to millions who want to go deeper into the subjects that matter most. No Off er expires 12/03/15 exams. No homework. Just a world of knowledge available anytime, anywhere. Download or stream THEGREATCOURSES.COM/8ASTR to your laptop or PC, or use our free mobile apps for iPad, iPhone, or Android. Over 550 courses 1-800-832-2412 available at www.TheGreatCourses.com. QUANTUM QG GRAVITY EVERYTHING YOU NEED TO KNOW ABOUT THE UNIVERSE THIS MONTH . . .

COSMIC SEEDS 10 TATOOINES LIFE FROM COMETS HOT BYTES >> Once life takes hold, it Astronomers have now Impacts don’t only cause may spread between star counted 10 planets extinctions. Experiments TRENDING systems via cosmic seeds known to binary show early Earth-comet TO THE TOP in a well-defined pattern stars, similar to Luke collisions could’ve made like an epidemic, say Skywalker’s home peptides, amino acid Harvard astronomers. planet in Star Wars. chains that are a key for life. NASA/JPL-CALTECH/SETI INSTITUTE (EUROPA); NASA/JPL-CALTECH/R. HURT (COSMIC SEEDS, 10 TATOOINES); LLNL (LIFE FROM COMETS) FROM (LIFE LLNL TATOOINES); 10 SEEDS, (COSMIC HURT NASA/JPL-CALTECH/R. (EUROPA); INSTITUTE NASA/JPL-CALTECH/SETI ’s large Europa shows a cracked icy surface in this Galileo spacecraft image; an extensive subsurface ocean likely exists below the ice, which could host microbial life.

SNAPSHOT Planetary scientists believe The extensive amounts of Further, in 2013, NASA Europa has an extensive liquid ice on Europa and tidal scientists detected phyllosili- water ocean beneath its icy flexing help to create the sub- cate clay minerals on Europa’s Could life exist crust. The sixth-largest moon surface ocean, the existence of surface — and these minerals in the solar system, Europa is which was bolstered in 2014 on Earth are often associated on Europa? rich in silicates, probably has with the detection of plate tec- with organic molecules. The an iron core, and possesses a tonics on the moon’s thick ice moon also displays evidence of A few places in our solar system tenuous and an crust, the first detection of this water vapor plumes similar to could host microbes, and Jupiter’s icy surface striated by cracks activity on a planetary body those of Saturn’s moon moon Europa is a prime candidate. and faults. other than Earth. Enceladus. — David J. Eicher

WWW.ASTRONOMY.COM 9 STRANGEUNIVERSE BY BOB BERMAN FROM OUR INBOX Do you question? Editor David Eicher’s opinion piece on p. 9 of the September issue, “Part-time believers not needed,” will come back to haunt him, if he lives long enough. He disses those who question what Toys and is now in the realm of science fiction. Let’s remember Star Trek “replicators,” “cloaking,” “communicators,” and “lasers.” Or better yet, recall what the chairman of IBM reportedly stated in compromises 1943, “There is a world market for maybe five computers.” Or the Digital Equipment president who commented, “There is no Why all astronomy enthusiasts will happily reason anyone would want a computer in his home.” Or even equipment-shop forever. Lord : “Heavier than air flying machines are impossible.” — Bob Found, Indian Harbour, Scotia

he holiday season because they’re lightweight We welcome your comments at Astronomy Letters, P. O. Box 1612, is here, and for us and have amazing optics. My Waukesha, WI 53187; or email to [email protected]. Please astro geeks that Canon 15x45 IS model is also include your name, city, state, and country. Letters may be edited for means toys. cool, but almost twice as heavy. space and clarity. In the quarter I also have 20x80s on a counter- Tcentury I’ve written these weighted swing arm binocular columns in Astronomy and tripod. I use them all. Still, no (110 kilograms) of instruments observatory, but it would be previously Discover, I’ve never stabilized model offers exit and counterweights, though the hard to duck out for a decent mentioned the equipment I use. pupils above 4.2 millimeters, motor’s clutch has a little back- sandwich. If you live in a city Since I love naked-eye astron- and in optimally dark condi- lash. Spectroscopes are part of but can travel to a dark site, omy so much, maybe I give the tions, if you’re young enough so every session. you could buy a big Dob or impression I’m not into optics. your eyes can adequately dilate, I haven’t done astrophotogra- Schmidt, but you must be will- So now for the first time let me the brightness of a 5mm to 7mm phy since the old film days, but ing to drag the heavy thing mention what I have, what I rec- exit pupil (as in a 7x50) makes it when I needed long-exposure around and periodically col- ommend, and how this whole an attractive choice. tracking with no periodic error, limate the optics. business is a bit complex. I also have solar binoculars my drive was excellent at deliv- For those who want “easy,” It’s true, naked-eye astron- on a portable Gemini mirror ering frustration. I’d labor 35 nothing’s more hassle-free than omy is awesome. Away from platform. This optically perfect, hours before decently capturing a refractor, and these days even city lights, what’s more inspir- flat, swiveling-mirror arrange- the Orion (M42). Hats 4-inch models are affordable. ing than the autumn Milky ment lets the “glasses” point off to all of you who take the But what will you do with it? Way splitting the sky? When We fanatics always find stuff. I you think about it, the four WHEN IT COMES TO TELESCOPES, once spent four hours staring at greatest spectacles are all won- only Saturn when the night air derful to the naked eye: total WE ENTER A WORLD OF COMPROMISE. was –13° F (–25° C), catching solar eclipses, major auroral moments of astonishing detail displays, great comets, and downward so that Sun viewing gorgeous shots featured in this unmatched by any photo. On brilliant meteor fireballs and never requires craning necks magazine. the other hand, astrophotog- bolides. None requires any upward. Toys. Anyway, astro toys depend raphers have their own sets of equipment. When it comes to telescopes, on what you like to observe, fussy requirements. But I’m a big fan of binocu- we enter a world of compromise. whether it involves photography, And if you get the 4-inch, lars. Of the displays just cited, There is no perfect telescope where you live, your budget, and the modest aperture precludes total eclipses and comets are design; each has its limitations. your degree of patience. When stunning looks at galaxies and usually enhanced through I own an old Celestron C5. price was no object, a wealthy globular clusters. The bottom them. So are the Milky Way and Despite its setbacks, how else can friend bought a huge Keck-type line is that all telescopes involve open clusters like the Pleiades I get a large motor-driven aper- dome and had a crane install a trade-offs. So here is Santa’s (M45). During the 12 years I ture in a portable configuration? professionally mounted 24-inch counsel: Make your decision, ran the astronomy program for I’ve lived in a dark rural PlaneWave, with a couple and then live with it serenely. the National Park Service at area for 44 years, and in 1982 I Takahashi refractors riding pig- Accept that no single instru- Yellowstone, the rangers and I built a 16-by-20-foot observa- gyback. The big scope’s optical ment can do everything. Then tried countless visitors’ pairs, tory with a motorized roll-off assembly cost $50,000 by itself. that great holiday gift — or the and we particularly adored the roof. Its pier-mounted 12.5- If only price were no obstacle goodie you’ve wanted for years Bausch & Lomb Audubons. inch f/6 Newtonian telescope for us all! And yet he overlooks — will give only pleasure. But then came image has flawless optics, with a a small city and has to endure a stabilization. That changed 5-inch Takahashi refractor rid- 4th-magnitude star limit. Contact me about the ball game. I love my ing piggyback. The massive If you have property in, say, my strange universe by visiting http://skymanbob.com. Canon 10x30 IS binoculars mount handles the 250 pounds Tibet, you could build a great

BROWSE THE “STRANGE UNIVERSE” ARCHIVE AT www.Astronomy.com/Berman.

10 ASTRONOMY • DECEMBER 2015 The world’s best amateur astrophotography

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www.fi refl ybooks.com At bookstores and online A SLOW DEATH. A section of the cosmos produces just half the energy it did 2 billion years ago, according to the ASTRONEWS multiwavelength Galaxy And Mass Assembly project, which surveyed 200,000 galaxies using many telescopes. NASA STUDIES TRIP TO AN BRIEFCASE Neptune’s moon Triton is an enticing DOUBLE option for many of the same reasons that Astronomers found that , the closest gal- axy to Earth that hosts a — an actively feeding prompted the Europa mission. black hole — actually sports twins. Research published Triton is nearly as big as Earth’s Moon August 20 in The Astrophysical Journal shows that a but has smoke-stack-like plumes from ice smaller black hole circles the supermassive central one, volcanoes that erupt nitrogen frost onto its carving out a doughnut hole around the galaxy’s core. Since most galaxies contain central black holes and gal- surface. For its part, Uranus has five moons axy mergers are common, astronomers suspect that big enough to be considered dwarf planets binary black holes may be as well. Like their hosts, the if they orbited the Sun on their own. black holes should eventually merge. Argo, the last proposed mission to • LADEE FINDS NEON Neptune, was grounded because NASA NASA’s Lunar Atmosphere and Dust Environment didn’t have enough plutonium to power all Explorer (LADEE) spacecraft confirmed the presence of of its spacecraft, according to one of its neon on the Moon, which has been suspected since the Apollo days, but only proven in a study published designers. Candice Hansen of the Jet May 28 in Geophysical Research Letters. The Moon’s NEXT VOYAGER. In 1989, the Voyager 2 space- Propulsion Laboratory says there was a atmosphere is extremely thin — 100 trillion times less craft became the first and last to see Neptune. NASA launch window from 2015 to 2020 that dense than the air we breathe on Earth. Most of it is would put Argo at Neptune in a decade via thought to come from the solar wind, but LADEE also revealed that lunar rocks outgas argon and . made its one and only visit to gravity assists from Jupiter and Saturn. • NASA Neptune more than a quarter- That timeline is now impossible to meet. ETERNAL RING century ago. And for years, planetary scien- But she says the Space Launch System A new study published in the August 4 issue of the tists have bemoaned a predicted 50-year rocket that NASA is currently building Proceedings of the National Academy of Sciences answers a long-standing question about the size of particles in gap between that Voyager 2 flyby and any could eliminate the need for gravity assists Saturn’s rings. Researchers proved that the distribution follow-up mission. and allow a mission to get to Uranus or of dust and pebbles is exactly what should occur in a Jim Green, NASA’s head of planetary Neptune much faster. system where collisions are common, so that small grains stick together and larger ones break apart at sciences, took a step toward narrowing that Another exciting potential mission steady rates. — Korey Haynes gap in August at the Outer Planets option announced in August is the Assessment Group meeting in Laurel, Enceladus Life Finder, or ELF, which Maryland, by announcing that NASA will would orbit Saturn and sample the moon’s study a potential flagship mission to Uranus curtain-like plumes of water that the and/or Neptune. If approved, it would be Cassini mission found streaming from its the next big mission following Mars2020 icy surface. If approved, ELF would catch and the Europa flyby. Past flagship missions the water and then directly sample it for include Cassini, Galileo, and Voyager. signs of life. — Eric Betz

Of the roughly 46,600 total person FAST HOW MANY YEARS days spent in orbit, Russians have logged more than 25,000 compared FACT IN SPACE? to just over 17,700 for Americans. WHERE IN THE BELT? Scientists used Earth-based observatories to study a passing asteroid and then compared it to meteorites that have plummeted to TOTAL TIME IN SPACE Earth in the past. NASA/ESA/STSCI

Gennady Padalka 878 DAYS* Sergei Krikalev 803 DAYS Mundane meteorites Aleksandr Kaleri 769 DAYS Sergei Avdeyev 747 DAYS have unexpected origin Valeri Polyakov 678 DAYS Astronomers have traditionally associated the most common type of meteorites, known as DAYS Anatoly Solovyev 651 H chondrites, with a main-belt asteroid called Yuri Malenchenko 641 DAYS 6 Hebe. This asteroid roughly three times Viktor Afanasyev 555 DAYS for every trip Jupiter takes around the Sun, Yury Usachev 552 DAYS placing it in a resonance orbit, which makes it a likely suspect to send accompanying rocky mis- Pavel Vinogradov 546 DAYS siles toward Earth. * Scott Kelly 522 DAYS However, spectral observations of a passing *UPON COMPLETION asteroid called 2007 PA8, published in the July 20 issue of The Astrophysical Journal, reveal SPACE STAMINA. Astronaut Scott Kelly is making headlines with his “Year in Space.” And when he returns in its remarkable similarities to H chondrites, March, Kelly will hold the record for the American with the most off-Earth time at 522 days. But that doesn’t even implying that some of these meteorites might

crack the top 10 list of Russians in space. In July, cosmonaut Gennady Padalka, orbiting alongside Kelly, broke his have originated with 2007 PA8’s family farther comrade Sergei Krikalev’s record of 803 days in orbit. ASTRONOMY: ERIC BETZ AND LUANN BELTER; NASA/BILL INGALLS (KELLY IMAGE) out in the main belt. — K. H.

12 ASTRONOMY • DECEMBER 2015 EIGHT NEW DWARFS. The Dark Energy Survey, which enlists one of the world’s most powerful digital cameras, turned up eight ASTRONEWS new faint dwarf satellite galaxies circling the Milky Way, bringing the team’s total to 17 for the year. TMT case advances to Hawaii’s Supreme Court QUICK TAKES The international astronomy col- GRAVITATIONAL laboration seeking to build the Thirty CONSTANT Meter Telescope (TMT) on Hawaii’s Gravity’s influence is constant Mauna Kea has seen progress grind throughout the universe, to a halt as protesters block construc- according to a decades-long tion physically on the mountain study of a distant pulsar — the access road and legally in courts. rapidly spinning remains of a The police have undertaken mul- massive star turned . tiple rounds of arrests and Hawaii’s • governor, after initially calling for a NEUTRINOS construction halt, later confirmed the CONFIRMED telescope’s right to proceed, though Antarctica’s IceCube Neutrino he advocated concessions that Observatory refined its 2013 include removing a quarter of the breakthrough Southern telescopes already present. The site is Hemisphere measurement of currently home to 13 observatories. neutrinos from outside our Protesters lost their initial court galaxy by pointing the instru- case and first appeal, with the courts PROTECTION VIA PROTEST. The Mauna Kea controversy’s complexities are made ment through Earth and con- ruling that the Board of Land and clear in this image of police joining hands in prayer with protesters before carrying out a firming the elusive particles Natural Resources (BLNR) had ful- round of arrests this spring. EVAN BORDESSA from the Northern filled its duties in evaluating TMT’s likening their actions to a judge pass- No court has issued a stay, so con- Hemisphere too. environmental impact and approving ing sentence before hearing a case. struction may legally proceed until and • construction. But the Supreme Court Justices also questioned the board’s unless the Supreme Court overturns CORONAL HEATING of Hawaii accepted protesters’ sec- reasoning that the “incremental” the initial case, though protests have Astronomers solved the ond appeal, hearing oral arguments damage projected by TMT’s environ- halted construction effectively thus far. 70-year mystery of how the August 27. As of press time, the court mental impact statement was enough Native Hawaiians are driving the Sun’s corona, or outer layer, had yet to issue a ruling, but during to exonerate it from the same docu- protests as they seek to protect a vital reaches a million degrees the hearing the justices rebuked the ment’s finding that “past construction and active cultural heritage site, while Celsius by catching magnetic BLNR for approving construction of these observatories has had cumu- TMT advocates point out Mauna Kea’s waves as they resonate before they held a contested (pub- lative impacts … that are substantial, exceptional and unique qualities as and one strengthens, lic) hearing, citing due process and significant, and adverse.” an observing site. — K. H. causing turbulence.

FULL HOUSE• International Space Station crew members from Russia, Kazakhstan, and Denmark makes first discovery launched in a Soyuz capsule Size of Saturn’s orbit September 2, boosting the around the Sun orbiting research lab’s staff to nine for the first time since 2013.

TEAM EUROPA• 2012 2015 The scientists who will lead NASA’s Europa exploration NIGHT LIGHTS. The Cities at Night project is using astronaut photos like these cap- mission a decade from now tured from the International Space Station to study global light pollution. Here Milan, met for the first time August b Italy, appears noticeably brighter after the city’s conversion to LED streetlights. NASA/ESA 10 at the Jet Propulsion Laboratory in Pasadena, 10 AU California. The all-star team

J. RAMEAU (UDEM) AND C. MAROIS (NRC HERZBERG) (NRC MAROIS C. AND (UDEM) RAMEAU J. Light pollution gets new measure includes veterans of New DIRECT DETECTION. Astronomers have For years, astronomers and night- warned about health impacts for Horizons, Cassini, Galileo, discovered thousands of exoplanets through sky advocates have warned about humans. And astronomers say their and Voyager. indirect methods, i.e., observing a planet’s the potential impact of new LED light is harder to filter. effects on the characteristics of its parent star, streetlights, which are being rapidly But the first real measure of SUPERNOVA• SPIN but they have only directly imaged a handful installed around the world in place increased LED light pollution came Shock waves from a nearby of worlds. Scientists using the new Gemini of the older, yellower high-pressure in August at the International supernova could have not only Observatory Planet Imager (GPI) hope to sodium vapor lights. LEDs, unlike Astronomical Union meeting in injected radioactive isotopes, change that because direct detection allows their predecessors, spread their light Hawaii. A group called Cities at but also set our solar system measurements of an exoplanet’s atmospheric across the electromagnetic spec- Night is using astronaut photos of composition and . In the August spinning when it was still a trum instead of confining their light Earth to create a Google Earth-style cloud, according to new 13 Science Express, they announced their first pollution to one small band. Los map of our world after dark. The GPI find: , an exoplanet 100 light- computer models. Angeles has already added 165,000 photos show how cities like Los years away with twice Jupiter’s mass orbiting • its young star at a distance a little farther LED streetlights. New York City says Angeles grow brighter after LED ENGINE TEST than Saturn is from the Sun. The exoplanet it will convert some 250,000. Smaller conversion projects. In August, NASA completed shows the strongest methane signature ever cities also are following suit. Cities at Night researchers now the first development testing detected on a world beyond our solar system Ecologists have discovered are enlisting the public’s help as on the R-25 rocket engines and has a temperature around 800° F (430° C). added negative consequences for they build up a database using that will one day power its Such characteristics point to a planet that wildlife associated with LEDs. The hundreds of photos from the Inter- Space Launch System — E. B. resembles an infant Jupiter. — Karri Ferron American Medical Association has national Space Station. — E. B.

WWW.ASTRONOMY.COM 13 FORYOURCONSIDERATION BY JEFF HESTER

Just as the mythical Ouroboros eats its own tail, the answers to questions at the The Ouroboros universe’s smallest scales may be Big meets small of today’s physics. found by studying the vast sprawl of the cosmos, and vice versa. © ISTOCK.COM/MARIAFLAYA

pace is big. Really big. To find a hundred billion tril- each direction, You just won’t believe lion atoms, look no further than the snake eats its how vastly, hugely, your thumb! own tail! “Smind-bogglingly big it Expanding our cosmic scope Fundamental is. I mean, you may think it’s a by four additional powers of 10 advances in science typi- long way down the road to the swallows the and cally do not happen because chemist, but that’s just peanuts many of the hundreds of billions of a theory’s continued success. are there; they just slosh around to space.” (Douglas Adams, The of comets that surround the solar Breakthroughs come when the among three different forms. Hitchhiker’s Guide to the Galaxy) system. Stepping in by four more predictions of an important The standard model says that This issue of Astronomy levels brings us to atomic nuclei, theory start to fail. Maxwell’s can’t happen! follows the journey taken in with densities a hundred billion theory of electromagnetic radia- Cosmology presents even the 1977 short film Powers of times that of water. tion predicted that objects of deeper challenges. The stan- Ten outward into the cosmos. Twenty-four powers of 10 out, even modest temperature should dard model is a resounding Twenty-six powers of 10 separate we find superclusters of galaxies. glow intensely with success but for the niggling the scale of the observable uni- Twenty-four powers of 10 inward, light. Sorting that out along with fact that between verse from the scale of the room we find the scale of the elusive another “oops!” or three pointed and dark energy, the standard that likely surrounds you as you neutrino. After 26 powers of 10, the way to quantum mechanics. model accounts for less than read these words. Yet the trip we have come to the end of the After 200 years, the success of 5 percent of the stuff of which outward is only part of the story. line in one direction, the scale of Newton’s physics had led some the universe is made. And Instead of adding zeros to the left the . But in to declare that nothing remained when particle physicists use of the decimal point, we can add the other direction, there is still a of science but mopping up a few quantum field theory to try to zeros to its right, taking us into long way to go. We have to shrink loose ends. All that gave way to explain dark energy, they get the an ever more microscopic realm. our gaze through 35 powers of relativity because a simple experi- wrong answer by 120 powers Each journey is as extraordinary 10 to finally arrive at the Planck ment failed to find expected of 10. That has to go down as as the other. length. This is the domain of variations in the . the worst prediction in history! Ten steps outward from the string theory and may represent The failed predictions of a Finally, it is cosmology center of the Sun, and we reach reality’s ultimate granularity. powerful, successful theory are that breaks our understanding the size of our local star and its The 61 powers of 10 spanned nature’s way of telling us where of gravity by squeezing it into extended corona. Something by today’s physics goes well there are new things to learn. So too small a quantum box. like a hundred billion trillion beyond those discussed in 1977. it can be really frustrating when Powers of Ten was made to (1023) stars inhabit the observ- One of the most profound dis- a theory stubbornly persists in highlight the vast range of phys- able universe. Ten steps inward, coveries of the last 40 years is making correct predictions. ical scales in the universe. But as and we reach the scale of atoms. that as we push to extremes in Such is the case with the stan- the frontiers of cosmology and dard model of particle physics. the frontiers of particle phys- For four decades physicists have ics merged, our perspective on FROM OUR INBOX pushed on their particle accel- scale has changed. Viewed from erators, trying to break through one direction, the structure and Science or science fiction? to physics beyond the standard fate of the universe hinge on as Bob Berman’s article “Multiverses: Science or science fiction?” model, and for four decades yet poorly understood particle (p. 28) in the September issue was exceptional. Instead of they have failed. physics beyond the standard defending the multiverse theory or debunking it out of hand, Yet there are cracks in the model. Viewed from the other he provided a cogent, balanced argument. The “theory of any- edifice of the standard model. direction, the Big Bang is the thing” approach promoted by adherents of the multiverse, by Those cracks were revealed not ultimate high-energy accelera- purporting to champion the multiverse idea, dilutes its impact by particle accelerators, but by tor, and the cosmos itself is the by its very nature — anything that can happen mathemati- observations of the cosmos. debris that we have to study cally does happen. And, without any substantive possibility of The first hole in the dike actu- from that most extreme of all experimentally verifying the theory or applying the falsifica- ally predates the standard model particle experiments. tion principle, it must remain problematic at best. I especially itself. In the 1960s, scientists appreciated the methodical approach that Berman brings to measuring neutrinos from the Jeff Hester is a keynote speaker, his analysis. We need more scientists dedicated to finding the Sun found only a third of the coach, and astrophysicist. truth, wherever it may lie. — Steven Cotton, Cottonwood, California expected number. We now know Follow his thoughts at jeff-hester.com. that all of the expected neutrinos

BROWSE THE “FOR YOUR CONSIDERATION” ARCHIVE AT www.Astronomy.com/Hester.

14 ASTRONOMY • DECEMBER 2015 STATION SUCCESS. A Japanese cargo ship reached the International ASTRONEWS Space Station on August 24 with supplies and a new electron telescope. ’ lonely mountain puzzles team

PUZZLING PYRAMID. Ceres’ home means it’s hit regularly by debris. However, astrono- mers don’t think an impact can explain this mountain.

The inner solar system’s only took astronomers by surprise when NASA’s Dawn spacecraft arrived early this year. Many expect- ed that impact craters on Texas-sized Ceres would “relax” due to abundant ice thought to lie just beneath its surface. Instead, they found a heavily cratered world with few outer signs of the hidden inner ice reservoir. But that’s not the weirdest part. Ceres has an enormous mountain — just one. At some 21,000 feet (6,400 meters) in elevation, the pyramid- PEAKABOO. Ceres has but one mountain. Mysteri- shaped peak towers higher than Alaska’s Denali, ously, it vaults 4 miles (6 kilometers) above the dwarf America’s highest mountain. planet’s plains. NASA/JPL-CALTECH/UCLA/MPS/DLR/IDA “The team is totally baffled by the mountain at the present time,” Dawn Principal Investigator come up mostly empty-handed. The mountain Chris Russell says. “We don’t really see how the does bear resemblance to some others — those mountain got made.” seen on . Some early theories suggested a foreign “The heights of [Pluto’s] mountains, and the object impact, but the mountain looks like it’s shape of the mountains, look very similar to the made from the same stuff as Ceres. Scientists shape of our mountains,” Russell says. “There is are also looking at volcanoes around the solar a story there, but we haven’t figured out what system in search of similarities but have so far the answer is.” — E. B.

SOLAR FLAIR. WHAT’S The Sun’s outer lay- ers sport a variety of THAT transient features to delight observers, THING ON all tied to the Sun’s tangled, twisted THE SUN? magnetic field. These phenomena appear at slightly different layers or stand out best at different wave- lengths, hence the variety of images com- bined here. ASTRONOMY: Sunspots usually FAST LUANN BELTER/KOREY HAYNES, occur in pairs or FACT AFTER NASA/SDO (FILAMENT, CORO- clusters of opposite NAL HOLE, FLARE, SUNSPOT); ESA/ magnetic polarity. FlareFilament Coronal Sunspot Prominence NASA/SOHO (PROMINENCE) hole

WWW.ASTRONOMY.COM 15 A COMPLICATED PICTURE OF PLUTO After six weeks of relative quiet Pluto’s atmosphere is so thin,” from scientists as says New Horizons team mem- they recovered from the probe’s ber William B. McKinnon of historic flyby of the Pluto sys- Washington University in St. tem July 14, they got back to Louis. “Either Pluto had a thicker work over Labor Day weekend. atmosphere in the past, or some The first images returned from a process we haven’t figured out scheduled yearlong downlink of is at work. It’s a head-scratcher.” data continue to show a more And even as New Horizons complex world than anyone continues to send back stored ever imagined. “Pluto is show- data from the flyby, the space- ing us a diversity of landforms craft is already preparing for and complexity of processes the next phase of its mission — that rival anything we’ve seen in assuming it gets NASA approval. the solar system,” says Principal The probe was designed to go OLD AND NEW. This 220-mile-wide (350 kilometers) view of Pluto’s surface shows many intriguing features, including dark ridges that resemble dunes (upper left and Investigator Alan Stern of the beyond Pluto to explore at least right) and ancient, heavily cratered terrain jutting up against smooth and therefore rela- Southwest Research Institute in one other icy body in the Kuiper tively young areas. NASA/JHUAPL/SWRI Boulder, Colorado. “If an artist Belt, but at the time of launch had painted this Pluto before in 2006, any future targets were miles beyond Pluto and likely actual science during a flyby. In our flyby, I probably would have a mystery. In 2014, the Hubble formed where it now orbits. the coming year, they will sub- called it over the top — but uncovered five Selecting New Horizons’ mit a proposal to NASA request- that’s what’s actually there.” potential Kuiper Belt objects next target right after the Pluto ing funding for this extended Among the most intriguing (KBOs) for New Horizons to flyby was necessary in order for mission as part of the space features revealed are some explore, and on August 28, the spacecraft to start making agency’s smooth, dark regions that NASA selected one, designated maneuvers before the end of portfolio. If after an indepen- could be wind-blown dunes. 2014 MU69, based on the mis- the year to put it on the right dent review NASA provides the “Seeing dunes on Pluto — if sion team’s recommendation. trajectory without wasting fuel. go-ahead, New Horizons will that is what they are — would The 30-mile-wide (45 kilome- Still, mission scientists need explore 2014 MU69 on January 1, be completely wild because ters) KBO lies nearly a billion further approval to conduct 2019. — K. F.

VIEW FROM ABOVE. This perspective shows what it would be like to look down at Pluto from 1,100 miles (1,800 kilometers) above its surface. NASA/JHUAPL/SWRI NEWS Telescopes.net ASTRO OFFICINA STELLARE ORION Lunt Engineering 100mm Binoculars LUNT • MEADE • MOONLITE

25 years ago •

Hands down one of the best QSI • SBIG SKY-WATCHER USA • SOFTWARE BISQUE • STARLIGHT EXPRESS • TAKAHASHI • TELE VUE • THE IMAGING SOURCE • VIXEN PLANEWAV in Astronomy Giant binoculars in the world Astronomy’s December 1990 issue outlined the NASA/European Space Agency (ESA) joint Cassini- Huygens Saturn mission, as well as the Comet Now Rendezvous Asteroid Flyby (CRAF). Cassini’s suc- with a cessful four-year mission Fan. has lasted 11 years and will end in 2017. CRAF fell victim to congressional funding priorities, but subsequent spacecraft, including ESA’s mission, eventually COUNTING SUNSPOTS. NASA’s Solar Dynamics Observatory caught this accomplished its goals. nearly 80,000-mile-wide (130,000 kilometers) sunspot in 2014 during a high point of the recent solar cycle. NASA/SDO CGE Pro Sunspots can’t explain 1400 HD ongoing climate change Changes in our Sun’s activity But in the years since that low, levels can’t explain Earth’s cur- scientists have generally agreed rent climate change, according to that sunspots have increased, a corrected history of sunspots even though some disagreed 10 years ago announced at the International about what that implied. Astronomical Union meeting in However, astronomers have in Astronomy Hawaii on August 7. now released Sunspot Number For the December 2005 The Sunspot Number is the Version 2.0, which found a signifi- issue, Senior Editor longest ongoing science experi- cantly different sunspot count Richard Talcott teamed ment in the world and is used between 1885 and 1945. The up with master space art- 5348 Topanga Canyon Blvd. as a crucial tool for understand- international team of astrono- ist Adolf Schaller to pro- ing our Sun’s activity, as well mers said the previous count duce an illustrated spread Woodland Hills, CA 91364 as Earth’s climate. Its history contained a major calibration titled “A comet’s tale.” Mon-Sat: 9am-6pm (PST) includes the Maunder minimum, error that misled some theorists. The two-page infographic Toll Free: (888) 427-8766 a period between 1645 and 1715 The group says climate evolu- outlined astronomers’ Local: (818) 347-2270 where the Sun had a strong drop tion models now need to be best understandings of

Fax: (818) 992-4486 E in sunspot activity that coincided recalibrated using the updated comets and their impacts JMI • KENDRICK LUMENERA LUMICON • • FLI iNOVA • FARPOINT • CORONADO • CANON CELESTRON ATIK ASA • • IMAGING APOGEE with harsh winters. information. — E. B. on earthly life, including the possibility of supply- ing the ingredients for Hybrid RNA and DNA — a role ESA’s Rosetta mission CUSTOMER SERVICE CENTER 569 WHAT TYPE OF ECLIPSE Manage your subscription online 4.8% confirmed in July. — E. B. CAN WE EXPECT? (Registration Required) FAST www.Kalmbach.com/help SOLAR SURVEY. FACT In the 5,000 years 21 • Change Address • Renew .C. or E-mail Address Subscription Total Partial from 2,000 B 3,173 to A.D. 3,000, an A hybrid The number of ultra • Report Damaged • Pay Bill 4,200 eclipse is or Missing Issues 26.7% 35.3% astounding 11,898 high-energy detec- • Check Payment solar eclipses will annular • Check Expiration Status of Subscription occur. This chart along some tions by the IceCube • Newsletter shows how many of of its path observatory, confirm- • Subscribe Management Annular each type Earth will and total ing the existence of 3,956 experience. ASTRONOMY: along the neutrinos from our 33.2% MICHAEL E. BAKICH/ROEN KELLY; rest. MIKE REYNOLDS (ECLIPSE) galaxy and beyond. For all of our publications!

WWW.ASTRONOMY.COM 17 3 FEET DEEP. Decades of data from NASA spacecraft show Earth is locked into at least 3 feet (1 meter) of sea level rise, ASTRONEWS the agency said August 25. It’s less certain if that will happen within a century or take longer. Dark energy hides like a chameleon Physicists believe that the majority of the universe is composed of dark energy, which they infer from obser- vations but have so far been unable to measure directly. MODEL . Using the Cosmic Web One reason for this could be Imager, astronomers have discovered a distant pro- that dark energy particles togalaxy (circled) being fueled by a filament of cold “hide” like a chameleon, gas from the cosmic web (outlined). This is the first changing their mass depend- SCIENCE IN A SPHERE. Researchers looked for evidence of dark observational proof for the cold-flow model of galaxy ing on the density of mate- energy chameleon fields inside this vacuum chamber, a hollow space formation. C. MARTIN/PCWI/CALTECH rial around them. Thus in the with a trillionth the pressure of the air outside, which allowed them to empty depths of space, the search for the sneaky signals. HOLGER MÜLLER particles would have strong A distant clue to fields, able to push the cos- physicists searched for these no differences, but their work mos apart, as modern theory elusive particles by creating was still able to rule out a requires. But in a lab setting, an extremely low-density huge range of possibilities for galaxy formation they would be weak and environment and watching dark energy and limit the cha- almost impossible to detect. how individual atoms meleon fields to a million By peering deep into the cosmos to a time In a study appearing in the behaved closer and farther times weaker than those of when the universe was less than 4 billion years August 21 issue of Science, from a dense ball. They saw gravity. — K. H. old, scientists have uncovered key evidence to help support a model of galaxy formation that’s been hotly debated for over a decade. Astronomers led by Christopher Martin of the BEAMING DATA The two orbiters relay most of Curiosity’s FAST California Institute of Technology in Pasadena and Opportunity’s data. MRO’s total includes FACT have uncovered a swirling disk 400,000 light- BACK HOME about 500 megabits/sol and Odyssey’s some years across — what’s called a protogalaxy 200 megabits/sol from the rovers. — that is actively being fed by a filament con- nected to the cosmic web of gas that extends throughout the universe. “This is the first smoking-gun evidence for how galaxies form,” Martin says. Mars Reconnaissance 80,000 megabits/sol The discovery provides strong observational Orbiter support for the cold-flow model of galaxy formation. This model posits that relatively cold gas in the early cosmos from the interga- lactic medium that makes up the cosmic web is responsible for fueling rapid ol ts/s and therefore galaxy development. abi eg Its competing standard model of galaxy for- 0 m ,20 mation focuses on collapses 2 as the trigger, but this theory does not fit with ol Mars /s ts l the fast star formation recently discovered in i o Odyssey b s a / galaxies that existed just 2 billion years after g s e t i the Big Bang. The cold-flow model can pro- m 0 b 0 a duce such rapid star birth and enough spin 6 g e to form extended rotating disks, but obser- m

0 vational evidence had been lacking until this 0 1 protogalaxy discovery, which appeared in the Curiosity August 13 issue of Nature. Further, Martin and rover Opportunity rover his team already have found two additional swirling disks that could provide more support for the newer model. — K. F. 5,000 MARTIAN ARMADA. NASA currently has five spacecraft studying Mars close-up. But when it comes to the amount of information returned to Earth, the Mars Reconnaissance Orbiter (MRO) stands above all others. On The solar contained in NGC 1313, average, it returns about 80,000 megabits of data each martian day, or sol (one sol equals 1.03 Earth days). a newly identified intermediate mass This graphic compares MRO with the other three probes currently on extended missions. Because data rates vary with Mars’ distance from Earth — in the same way a wireless connection slows down with distance — it black hole. Astronomers say it’s just the does not include the recently arrived MAVEN orbiter. ASTRONOMY: RICHARD TALCOTT, ROEN KELLY AND KELLIE JAEGER seventh of its kind now known.

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WWW.ASTRONOMY.COM 19 THE COSMIC DISTANCE SCALE

Unfettered by tales of science fiction, the cosmos is almost incomprehensibly large. by David J. Eicher

20 ASTRONOMY • DECEMBER 2015

IT’S ABSOLUTELY AMAZING to know that shortly after the Big Bang, the universe was a relatively small, nearly infinitely dense place. It boggles the mind. But that was 13.8 billion years ago. The expanding universe means the entirety of what we know is now incredibly large — and is getting more immense every day. This is one area that two generations of science-fiction movies have seriously distorted in the minds of the public. The general feeling that technology is pretty good and will know almost no bounds, and that we can almost certainly one day travel between star systems, is pretty much taken on faith. But what the sci-fi movies have failed to communicate, among other things, is that the universe is an immensely large place. Even distances between the nearest objects are staggering, and the distances across the Milky Way Galaxy and certainly between galaxies in the universe are astonishingly huge to living beings stuck on a planet. A model of the Milky Way wherein the Sun is a grain of sand brings this home. On this scale, stars — sand grains — are 4 miles (6 kilometers) apart in the Milky Way’s disk and the disk is about 40,000 miles I/AURA)-ESA/HUBBLE COLLABORATION I/AURA)-ESA/HUBBLE (60,000km) across. Now who wants to go C traveling from grain to grain? The concept of the size of the universe has taken a huge stride forward in just the last few years. There was a time not too long ago when astronomers did not know even the approximate size of the cosmos with any degree of accuracy. We still don’t know with high precision. Incredible expansion NASA/ESA/THE TEAM (STS HUBBLE HERITAGE M74: The Big Bang theory tells us that once the But remember that the Big Bang was not diameter for the universe of approximately universe was very small. We know the fast- like an explosion that went off in a room. 93 billion light-years. est that radiation or any information can Following the Big Bang, space-time itself But there’s a major proviso to this result. travel is the speed of light, 186,000 miles expanded radially outward at all points, That diameter refers to the visible universe per second (300,000 km/s). We’re confident meaning all of space expanded too, not just we can see from Earth. Inflation theory, if that the universe is 13.8 billion years old. the stuff within it. (The term space-time correct — and it has widespread support We also know that a light-year is equal to refers to the mathematical model that com- among cosmologists — suggests the portion approximately 6 trillion miles (9.4 trillion bines space and time into a single, interwo- of the universe we can see is by no means km). In nearly 14 billion years, on first ven medium.) the entire cosmos. Some cosmologists pro- blush, we might expect radiation to expand As the expansion of the universe began, pose that the universe is infinite. But let’s radially outward to something like 30 bil- just 1 centimeter of “empty space” inter- work with what we really have and say the lion light-years across. stitially became 2 centimeters over time, cosmos, at least the part that we can observe, and so on. So the best ideas about the size is about 93 billion light-years across. David J. Eicher is editor of Astronomy maga- of the universe allowing for its expansion A thorough understanding of our zine. He has marveled at the cosmic distance over time point to a radius of slightly more neighborhood, our solar system, our area scale since the mid-1970s. than 46 billion light-years and therefore a of the Milky Way, our galaxy, and so on is

22 ASTRONOMY • DECEMBER 2015 THE NEW COSMOS: ANSWERING ASTRONOMY’S BIG QUESTIONS

This story is an excerpt are closing in on the answers from David J. Eicher’s new to — some of the biggest book, The New Cosmos (300 questions about the universe. pp., hardcover, Cambridge Eicher presents an exploration University Press, New York, of the cosmos that provides a with 100,000 words of text balanced and precise view of and 100 color illustrations, the latest discoveries. foreword by Alex Filippenko, Detailed and entertain- ISBN 978-1107068858). The ing narratives on compelling photos, maps, and explana- book is available at book- topics such as how the Sun tory diagrams. In each chap- stores and online retailers. will die, the end of life on ter, the author sets out the The New Cosmos seeks to Earth, why turned itself scientific history of a specific fill a major gap in the story of inside-out, the Big Bang the- question or problem before astronomy, planetary science, ory, the mysteries of dark mat- tracing the modern observa- and cosmology. Over the past ter and dark energy, and the tions and evidence in order to decade, astronomers, plan- meaning of life in the universe solve it. This fall you can join etary scientists, and cosmolo- are supported by numerous Eicher on this fascinating jour- gists have answered — or color illustrations including ney through the cosmos.

Nicolas Copernicus (1473–1543) proposed objects in the area to keep Pluto company the heliocentric model of the cosmos, and — Haumea at 40 centimeters, Makemake it was one of the last great visual astrono- at 45 centimeters, and Eris at 60 centime- mers, Danish nobleman Tycho Brahe ters. Now you can finish by indicating the (1546–1601), who made the first paral- region of the scattered disk, a sparse body lax measurements of comets and helped of energetically “spun up” icy asteroids, define a more modern distance scale to between 50 and 100 centimeters from the nearby objects. Sun. This gives you a complete scale model of the solar system in a region spanning 1 Starting close to home meter, or 3 feet, across. Let’s pause for a moment to appreciate Now appreciate that on this scale, the the physical scale of just our solar system inner edge of the , the vast halo — only the Sun, its attendant planets and of 2 trillion comets on the solar system’s debris, and our little island of life inside it. perimeter, is 100 meters (109 yards, more To envision our immediate vicinity a little than an American football field) farther better in your mind, imagine a scale solar away than the edge of your diagram. The system with the Sun on one end and 1 cen- outer edge of the Oort Cloud, on this scale, timeter representing the distance between is 1,000 meters (0.6 mile, more than 10 our star and Earth, called an astronomical football fields) away. unit (AU). That is, 1 AU = 1 centimeter. Yet as human astronaut-explorers, we You actually can draw this out on paper to only have traveled as far away as the Moon, critical to comprehending how the universe help crystallize it in your mind. Tape sev- about ⅓89 AU, or on our scale ⅓89 centi- works. And exploring the cosmic distance eral sheets of paper together and have at it. meter, from Earth, which on this scale is scale also unveils a slew of interesting With the Sun at one end, Earth is 1 centi- about the size of a human red blood cell. objects astronomers use to determine dis- meter away, and Mercury and Venus are in That distance is imperceptibly close to our tances to objects near and far. there too at 0.4 centimeter and 0.7 centime- planet’s “dot” on our scale drawing. The seeds of measuring the universe ter, respectively. Outward from Earth, we And yet the distances to the nearest stretch back in time all the way to the have Mars at 1.5 centimeters, the main-belt stars are larger than our imagined scale of Greek astronomer Aristarchus of Samos asteroids centered around 2.5 centimeters, the Oort Cloud. And then come perhaps (ca. 310–230 b.c.), who had correct notions Jupiter at 5 centimeters, Saturn at 9.5 cen- 400 billion stars scattered across the bright of parallax in mind with regard to dis- timeters, Uranus at 19 centimeters, and disk of our Milky Way Galaxy, 150,000 tances of the Sun and Moon. Parallax is the Neptune at 30 centimeters. Pluto can be light-years across, and a hundred billion technique of measuring the offset of nearer placed at 40 centimeters. more galaxies spread across a vast cosmos. bodies to the distant background of stars The outer solar system is sparse, con- The next time you’re out under the and geometrically calculating a distance. sisting of the Kuiper Belt region from 30 to stars, look up and think carefully about the Little progress took place after 50 centimeters from the Sun, and you can enormity of the universe. It is one of the

PREVIOUS PAGE: SDSS ESA/NASA/K. J1004+4112: SHARON (TEL AVIV UNIV.)/E. OFEK (CALTECH) Aristarchus until Polish astronomer even indicate some of the more interesting great humbling feelings of humanity.

WWW.ASTRONOMY.COM 23 OUR SOLAR SYSTEM Realms of fire and ice We start your tour of the cosmos with gas and ice giants, a lot of rocks, and the only known abode for life. by Francis Reddy

cosmic perspective is always a correctly describe our The cosmic distance scale little unnerving. For example, we as consisting of Jupiter plus debris. It’s hard to imagine just how big occupy the third large rock from The star that brightens our days, the our universe is. To give a sense of its Aa middle-aged dwarf star we Sun, is the solar system’s source of heat vast scale, we’ve devoted the bot- call the Sun, which resides in a and light as well as its central mass, a tom of this and the next four stories quiet backwater of a barred gravitational anchor holding everything to a linear scale of the cosmos. The known as the Milky Way, itself one of bil- together as we travel around the galaxy. distance to each object represents the amount of space its light has lions of galaxies. Yet at the same time, we Its warmth naturally divides the planetary traversed to reach Earth. Because can take heart in knowing that our little system into two zones of disparate size: the universe is expanding, a distant tract of the universe remains exceptional one hot, bright, and compact, and the body will have moved farther away as the only place where we know life other cold, dark, and sprawling. Here’s during the time its light has trav- exists. Our solar system hosts one abode how it all fits together. eled. To show the entire cosmos in 30 pages, we had to use a scale of for life, Earth, but in decades to come we 1 inch = 55.3 million light-years, or may learn of others — perhaps biologi- The hot zone 1 millimeter = 2.18 million light-years. cal enclaves on Mars or Jupiter’s moon Light takes eight minutes, give or take a Europa. Although our solar system likely few seconds, to reach Earth from the Sun’s OBJECT DISTANCE is not unique in this respect, definitive surface. The average distance is 92.96 mil- Sun 8.3 light-minutes evidence for life on worlds orbiting other lion miles (149.6 million kilometers). We’ve stars will be harder to find. barely begun our tour of the solar system, Jupiter 43.3 light-minutes All told, the solar system contains and numbers using familiar units already Saturn 79.3 light-minutes about 4 million trillion trillion pounds of are cumbersome. So astronomers devised Pluto 329 light-minutes material, or about 1.0013 solar masses. the (AU; see “Solar sys- tem yardstick,” p. 26) as a simpler way to Oort Cloud ~1 light-year The number on the left side of the decimal is the mass of the Sun itself, and express distances at the scale of planetary Orion Nebula 1,350 light-years about 73 percent of what’s on the right orbits. The average distance from Earth to 7,500 light-years side is held in the Jupiter. The the Sun represents 1 AU. Omega Centauri 17,000 light-years remainder includes everything else: Earth Mercury lies closest to the Sun, at an average distance of 39 percent of Earth’s, Milky Way’s center 27,200 and the other planets, moons, dwarf light-years planets, asteroids and comets of all sizes, but its eccentric orbit — the most elongat- as well as dust and icy grains. One could ed of all the eight major planets — means 160,000 light-years this number varies. At its greatest distance, Francis Reddy is the senior science writer for the called aphelion, Mercury is 200,000 light-years Astrophysics Science Division at NASA’s Goddard 0.467 AU from the Sun, Space Flight Center in Greenbelt, Maryland. and at its closest point, (M31) 2.5 million light-years A 12 million light-years (M51) 26 million Great light-years Attractor Cluster 218 million 248 million 55 million light-years light-years light-years 319 million light-years

million light-years The cosmic 100 200 300 400 distance scale called perihelion, it’s just 0.308 AU away. Standing on Mercury at this time, the Sun would appear 3.2 times larger than it ever Spacecraft have now appears from Earth. Surface temperatures visited all the planets, reach 800° F (400° C), hot enough to melt plus members of the lead. Baked by sunlight, pummeled by asteroid and Kuiper belts, as well as sev- charged particles streaming from the Sun, eral comets. The solar and with just 38 percent of Earth’s diameter, system family portrait Mercury maintains only a wisp of an atmo- grows ever more complete. sphere. But a growing body of evidence suggests that despite its front-row seat in the solar system’s hot zone, Mercury has ice. In 1992, astronomers at Arecibo Observatory in Puerto Rico bounced radar off the planet’s polar regions, revealing radar-reflective deposits on the floors of a few craters where daylight never shines. Decades earlier, astronomers had theo- rized that ice from comets and asteroids crashing on Mercury could find their way to “cold traps” in permanently shadowed polar craters. Data collected by NASA’s Mercury-orbiting MESSENGER spacecraft in 2012 confirmed that these craters con- tain -rich layers consistent with the presence of water ice. A similar process appears to have preserved subsurface ice in the Moon’s polar areas. Next out is Venus, a planet 95 percent NASA (EARTH, JUPITER, NEPTUNE); NASA/SDO (SUN); NASA/ of Earth’s size and likewise a good match JHUAPL/CIW (MERCURY); NASA/JPL (VENUS); NASA/USGS: JODY SWANN, TAMMY BECKER, & ALFRED MCEWEN (MARS); NASA/JPL- in properties like mass, density, gravity, CALTECH/UCLA/MPS/DLR/IDA (CERES); NASA/JPL/SSI (SATURN); UNIV. OF WI-MADISON: LAWRENCE SROMOVSKY/W.M. KECK OBSERVATORY (URANUS); NASA/JHUAPL/SWRI (PLUTO); ESA/ ROSETTA/MPS FOR OSIRIS TEAM MPS/UPD/LAM/IAA/SSO/INTA/ UPM/DASP/IDA (COMET 67P)

ESO (); NASA/ESA/THE HUBBLE HERITAGE TEAM (STSCI/AURA) (COMA CLUSTER); ESO/INAF-VST/OMEGACAM ( CLUSTER); NASA/ESA/J. BLAKESLEE (WASHINGTON STATE UNIVERSITY) ()

Hercules Cluster Shapley Supercluster 501 million light-years 586 million light-years

500 600 700 800 900 WWW.ASTRONOMY.COM 25 The Soviet probe Venera 13 holds the record at 2 hours, 7 minutes for the longest spacecraft to survive on the surface of Venus. It sent back pictures of its basalt-like surroundings, with the spacecraft itself partially visible at the bottom of the image. NASA HISTORY OFFICE

define a star’s “habitable zone” — a range SOLAR SYSTEM YARDSTICK of orbital distances where liquid water potentially could exist — as a way to iden- After Copernicus placed the Mars at opposition, produced the results of Venus transits in tify exoplanets that may be capable of sup- Sun at its center, astronomers inconsistent results. 1761 and 1769 — though this porting life as we understand it. While we had a solid sense of the rela- Edmond Halley, famous was still a huge improvement tive dimensions of the solar today as the first to predict in accuracy. By the time the can quibble with the definition — perhaps system. They knew, for exam- the return of a comet, next transit rolled around, in there’s a biology that uses solvents other ple, that Venus orbited 30 showed in 1716 that Mercury 1874, astronomers had begun than water or maybe life can develop entire- percent closer to the Sun was too far from Earth for exploring more promising ly beneath the surface — it’s a place to start. than Earth. But they had no transit measurements to be techniques, such as photo- For the solar system, conservative values way of putting everyday effective, but Venus was graphic observations of Mars. place the habitable zone between 0.99 and measurements to these dis- ideal. He proposed a plan for We now can measure the 1.69 AU. More optimistic values extend it in tances. In effect, the astro- observations of the 1761 distances to Venus, Mars, nomical unit (AU; average Venus transit and suggested asteroids, and many other both directions, from 0.75 to 1.84 AU. Either Earth-Sun distance) was a that astronomers be dis- solar system objects by ping- way, the zone excludes desiccated Venus but yardstick with no markings. patched across the globe to ing them with radar. By send- includes Mars, thought to be warmer and In 1663, the Scottish math- observe and time the event. ing a pulse of energy from a wetter in the distant past. ematician James Gregory He believed the technique radio telescope and knowing Located about 1.5 AU out, Mars long has described a way to use transits could establish the length of the speed of light, the time been viewed as the best bet for finding life in — the apparent passage of the astronomical yardstick to taken for the signal to return the solar system. But with half Earth’s size Mercury or Venus across the an accuracy of 0.2 percent. provides the distance. Today Sun’s face — to determine the It proved far more difficult astronomers define the AU and only 38 percent of its , the AU’s value. But efforts to use in practice. To reach even 4 as equal to 92,955,807.273 Red Planet was crippled in its ability to hold Mercury transits and other percent of the current value, miles (149,597,870.700 kilo- onto the thick atmosphere needed to main- techniques, such as observing astronomers had to combine meters). — F. R. tain surface water. In its first billion years, the upper layers of the atmosphere slowly bled into space, and occasional asteroid and composition. Yet with an average sur- spacecraft shows a world of fascinating impacts drove away great masses of martian face temperature of 864° F (462° C) day and geography. It is the only other planet in night, Venus stays hotter than Mercury gets the solar system known to host active vol- on its worst day despite its greater distance canoes. In 2015, scientists studying ther- of 0.72 AU. The reason is the planet’s dense mal imaging by the European Space atmosphere, which is composed almost Agency’s (ESA) orbiter entirely of heat-trapping carbon dioxide reported 1,530° F (830° C) hot spots along gas. Compounding the lack of hospitality, the planet’s Ganiki Chasma rift zone, a the surface pressure of the atmosphere is type of feature associated with terrestrial about 92 times sea-level pressure on Earth, volcanism. Researchers have observed epi- Kandinsky the same we would experience at a depth of sodes where the temperatures of these

3,000 feet (1,000 meters) under the ocean. spots abruptly increase and then cool Prokofiev Unsurprisingly, no spacecraft down, suggesting ongoing eruptions. landing on our inner neighbor has continued Goldilocks zone to transmit for much Next out is Earth, home sweet home and the

more than two hours. only planet in the solar system where liquid NASA/JHUAPL/CIW Mercury holds highly reflective material in its Radar mapping of water freely exists on the surface. As far as permanently shadowed craters that could be Venus from Earth we know, the presence of water is a necessity water ice, as revealed here by NASA’s MESSENGER

Boötes Supercluster and from orbiting for life. Astronomers extend this concept to spacecraft and Earth-based radar mapping. (BOÖTES SUPERCLUSTER) NASA 936 million light-years

billion light-years 11.25 26 ASTRONOMY • DECEMBER 2015 The bases of mesas in Deuteronilus Mensae on Mars exhibit strange textures. Astronomers think they could have formed from the rapid melting of ice that lurks under the dust and rock currently overlaying the features. ESA/DLR/FU BERLIN (G. NEUKUM)

would have formed a global water layer at least 6.5 times deeper than today’s polar deposits could provide. Mars may yet have its day in the Sun. Our star’s luminosity is gradually increas- ing, which pushes the habitable zone out- ward. In about a billion years, the habitable Our planet Earth is now under observation by DSCOVR, a joint project by NASA, the U.S. Air Force, and the National Oceanic and Atmospheric Administration, designed to monitor the solar wind in real zone will leave Earth roasting outside its time. Visually stunning images like this one, taken from a million miles away, are a bonus. NASA inner edge. But the Red Planet will experi- ence a more temperate period lasting a few billion years as the Sun transitions into its Venusian hot spot phase. For Mars, summer is com- ing — though since much of its water has June 22, 2008 June 24, 2008 Oct. 13, 2008 been lost to space through the years, it may not grow much more hospitable. 20° The rocks Next comes the asteroid belt, which, along Object A with the Kuiper Belt beyond Neptune, is a 10° remnant of the disk of rocky and icy debris that gave rise to our planetary system. abc Contrary to Hollywood imaginings, the 185° 195° 185° 195° 185° 195° asteroid belt is mainly space. The main Astronomers recently spotted regions on belt totals less than 5 percent the mass of 0.75 0.85 0.95 1.05 1.15 1.25 Venus that grow rapidly hotter and then cool Earth’s Moon, and about a third of this is Relative brightness again over short time periods. The most likely contained in Ceres, the largest object and explanation is volcanic eruptions on the the only dwarf planet in the asteroid belt. planet’s surface. E. SHALYGIN, ET AL. (2015) Add in Vesta, Pallas, and Hygiea, the next three largest asteroids, and half the belt’s air. Once the atmosphere was thin enough, a permanent dry ice component, water ice mass is accounted for. Mars cooled, and its water froze into the resides in both poles in the form of polar The main belt starts at 2.06 AU, where glaciers and ice caps we see there now. layered deposits, which, thanks to extensive objects make four orbits around the Today, the average atmospheric pressure is spacecraft mapping, provide a minimum Sun in the time it takes Jupiter to make 0.6 percent that at Earth’s mean sea level. inventory of the planet’s water ice content. one. Astronomers call this a 4:1 orbital Near the winter pole, temperatures reach If melted, the water in these deposits would resonance. Whenever an asteroid’s orbital –195° F (–126° C) — so cold that roughly cover an idealized smooth Mars in a liquid period is a whole number fraction of 30 percent of the carbon dioxide atmosphere layer 69 feet (21 meters) deep. According to Jupiter’s , the giant planet snows out there, covering the surface with a recent studies, the planet may have hosted a easily destabilizes the tiny rock, quickly — dry ice veneer. While only sea containing more water than the Arctic on astronomical timescales — clearing it Supercluster the south pole contains Ocean about 4.3 billion years ago. This out of its original orbit. The 2:1 resonance 1.37 billion light-years

1.5 1.75 WWW.ASTRONOMY.COM 27 The disk around the star Beta Pictoris is similar to our own Kuiper Belt, rich in dust and debris left over from the solar system’s birth. NASA/ESA/D. GOLIMOWSKI AND H. FORD (JHU)/D. ARDILA (IPAC)/J. KRIST (JPL)/M. CLAMPIN (GSFC)/G. ILLINGWORTH (UCO/LICK)/ACS SCIENCE TEAM

ACTIVE ASTEROIDS In the past decade, astronomers collision with a small asteroid tens have recognized a population of of meters across is the best expla- asteroids that are losing mass, nation for Scheila’s outburst in sometimes looking distinctly 2010. Astronomers have reported like comets. They include Ceres, water vapor in the spectrum of Scheila, and Phaethon, the Ceres, where true comet-like source of particles producing activity may be occurring. The the Geminid meteor shower 2009 outburst of Phaethon was each December. likely an ejection of dust, possibly According to David Jewitt at in response to the breakdown of the University of California, Los surface rocks under intense heat- Angeles, who has been studying ing at perihelion (just 0.14 AU), Neptune’s largest moon, Triton, may be a captured Kuiper Belt object. Here, these objects, different processes when temperatures approached Neptune is shown in the background as an orbiting spacecraft might see it. may be responsible in each. A 1,400° F (760° C). — F. R. Triton hosts ice volcanoes and streaks caused by winds blowing nitrogen frost across the rough surface. NASA/JPL/USGS

— twice around for every Jupiter orbit — Beyond the wall Hydrogen makes up more than 90 per- marks the distant edge of the main belt, When comets head toward the Sun on cent of both planets by volume. Both plan- located at 3.27 AU. While the main belt the inbound legs of their elongated orbits, ets also give off more heat than they receive accounts for the greatest density of aster- astronomers usually start seeing enhanced from the Sun as they continue their gravi- oids, many follow orbits that stray outside activity as they approach within 3 AU. tational contraction and cooling billions of it thanks to Jupiter’s work. In the past This is the distance where exposed water years after they formed. Their atmospheres decade, a dozen or so asteroids have caught ice rapidly begins sublimation, turning are essentially bottomless, transitioning the eye of astronomers for their peculiar directly to a gas and powering jets that from gaseous to liquid and even electrically activity (see “Active asteroids” above), eject sunlight-reflecting dust into space. conducting liquid forms of hydrogen with showing we still have much to learn about This seems as good a place as any to draw increasing depth. Depending on the details the solar system’s “rock belt.” the line between the solar system’s inner of how they formed, there may or may not warmth and outer cold. be a solid core roughly the size of Earth. From here on out, the planets are giant NASA’s Juno orbiter, scheduled to arrive at worlds quite different in mass, density, and Jupiter in July 2016, will measure the plan- chemical composition from their siblings et’s gravitational field with sufficient accu- closer to the Sun, where warmer tempera- racy to determine if a core exists. tures cooked away more volatile sub- A pair of slightly different giants lies stances. Where the inner planets are built farther out. Uranus, located at 19 AU, and of rock and metal, the outer giants are Neptune, at 30 AU, have smaller hydro- massive worlds composed mainly of hydro- gen atmospheres making up less than gen, the lightest element. Jupiter, puppet 20 percent of their masses, which are 15 master of the asteroid belt and the largest and 17 times Earth’s, respectively. Instead, planet in the Sun’s retinue, orbits at a dis- heavier elements dominate their bulk, tance of 5.2 AU. It holds 318 with carbon, oxygen, nitrogen, and times Earth’s mass and sulfur being likely candidates. has 11 times its width. Because scientists think the

NASA/JPL-CALTECH/UCLA/MPS/DLR/IDA Saturn, not quite twice planets incorporated these NASA’s Dawn spacecraft observed bright spots as distant at 9.6 AU, chemicals as they accumu- on the dwarf planet Ceres, orbiting in the aster- oid belt. These spots of salt or water ice have carries 95 times Earth’s lated frozen debris, Uranus fascinated astronomers since the spacecraft first mass and is 9.5 and Neptune are spotted them. 3C 273 (quasar) times as wide. sometimes called () 1.99 billion light-years 2.27 billion light-years

billion light-years 22.25 28 ASTRONOMY • DECEMBER 2015 With New Horizons’ Pluto flyby in July, humans have now explored every object in the solar system that has, at least at some point, been classified as a planet. Pluto’s moon Charon is shown as well, each of them to scale in both size and separation. NASA/JHUAPL/SWRI ESA/ROSETTA/NAVCAM Comets like 67P/Churyumov-Gerasimenko, now under careful investigation by the European Space Agency’s Rosetta spacecraft, might be swept into their extreme orbits after gravitation- al nudges from Jupiter or another large planet.

take millennia to coast through the solar system’s biggest structural component. This is the Oort Cloud, where per- haps a trillion comets randomly orbit in a spherical shell centered on the Sun. It extends from 5,000 to 100,000 AU — that’s 1.6 light-years, about 40 percent of the way to Proxima Centauri, the nearest star. NASA’s Solar Dynamics Observatory captured the June 5, 2012, transit of Venus more than 250 years Astronomers think the Oort Cloud formed after astronomers’ first attempt to use such a transit to measure the absolute distance scale of the early in the solar system’s history, when icy solar system. NASA/GSFC/SDO objects much closer to the Sun were hurled outward by gravitational interactions with the planets. They now may take as long as “ice giants.” Both planets are about four that are unfavorable are swept out of the 30 million years to complete an orbit. These times the size of Earth. belt, scattered by Neptune inward and comets are so feebly gripped by the Sun outward onto more tilted and elongated that other forces, such as the overall gravi- Icing over orbits. Comets like 2P/Encke and 67P/ tational field of the galaxy’s irregular mass The Kuiper Belt is a doughnut-shaped Churyumov-Gerasimenko, subject of distribution (known as the ) region located 30 to 50 AU from the Sun, ESA’s Rosetta orbiter, may have originated along with passing stars and massive molec- and Neptune plays a key role in shaping it. as fragments of scattered KBOs whose ular clouds, strongly affect them. Eventually In fact, astronomers think that Neptune’s orbits were tightened up by encounters these gravitational tugs can alter a comet’s largest moon, Triton, may be a captured with Jupiter. path so that it starts the long fall toward the Kuiper Belt object (KBO). Like its rocky Now that its flyby of Pluto is complete, Sun for the first time since it was cast out. counterpart between Mars and Jupiter, scientists hope NASA’s New Horizons These “dynamically new” comets travel on the Kuiper Belt is a faint echo of the Sun’s spacecraft will be able to provide close- extremely elongated and randomly oriented vast , the construction zone up information on additional KBOs as paths. Gravitational interactions with the of our planetary system. In this region, it heads through the belt and out of the planets can divert them into shorter orbits, orbits in favorable resonances can protect solar system. NASA’s Voyager 1 spacecraft, and astronomers think the famous Halley’s KBOs from disruptive encounters with now 132 AU out, has already done so in Comet is one example. Neptune. Pluto, at 39 AU, is the brightest one sense. It has left the heliosphere, the At the fringes of the cloud, comets eas- of a KBO family locked in a magnetic bubble shaped by the outflow of ily escape into interstellar space via the 2:3 resonance, complet- charged particles from the Sun, and most of same tugs that nudge them sunward. ing two orbits for every the particles now detected by the spacecraft Comets from our solar system already may three trips Neptune have traveled to it from interstellar space. have raced around another star. Might we takes around the Sun; From Voyager 1’s location, the Sun is a bril- one day see a comet from an alien Oort stable groups also liant point about 24 times brighter than Cloud, another star’s realm of ice? As far as occupy other resonances. a Full Moon as seen from astronomers know, it hasn’t happened yet,

NASA/ESA/HUBBLE ( 3C 273); NASA/ESA/N. BENITEZ (JHU), ET AL./ACS SCIENCE TEAM (ABELL 1689); DSS (ABELL 1687) (ABELL DSS 1689); TEAM SCIENCE (ABELL NASA/ESA/N. AL./ACS ET BENITEZ (JHU), 273); NASA/ESA/HUBBLE3C ( KBOs in resonances Abell 1687 (galaxy cluster) Earth. Still, the probe will but it’s definitely possible. 2.40 billion light-years

2.5 WWW.ASTRONOMY.COM 29 THE MILKY WAY Earth’s home galaxy Giant clouds of gas and dust sprinkled with splashy star clusters adorn the Milky Way’s spiral arms, while the galaxy’s vast halo teems with darker matter. by Francis Reddy

SOONER OR LATER ON ANY inside a disk where dust clouds dim and CLEAR, DARK NIGHT, AN ETHE- block starlight. REAL BAND CALLED THE MILKY The true scale of the Milky Way Galaxy — and, indeed, the universe as a whole — WAY ARCHES ACROSS THE SKY. became dramatically clearer in the 1920s. Although recognized since antiquity, phi- That’s when a new generation of large tele- losophers and scientists could only guess at scopes coupled with photography revealed what it represented until fairly recently (see that “spiral nebulae” were actually entire “How the Milky Way Galaxy got its name,” galaxies like our own — “island ” p. 33). With the invention of the telescope, in the evocative parlance of the time. Sur- it became clear that the Milky Way was the veys showed that most disk-shaped galaxies collective glow of stars too faint to be seen possessed winding spiral arms where young by the naked eye. More than a century later, stars, gas, and dust were concentrated. English astronomer Thomas Wright sug- Astronomers assumed our galaxy was a gested that this glowing band was precisely spiral too. In the 1950s, radio telescopes what one would expect to see if the Sun were produced the first crude maps of the Milky embedded in a flat disk of stars. Way’s spiral arms by tracking how gas We now know that the Milky Way is the clouds moved around the galaxy. primary structure of our galaxy seen edge- Over the past two decades, surveys using wise. Additional detail and especially the dust-penetrating infrared light have brought physical scale of the galaxy took another two the general picture of our galaxy into better centuries to work out. The process continues focus. These projects include the ground- today as astronomers wrestle with conflict- based Two Micron All-Sky Survey and Sloan ing evidence and make new discoveries. Digital Sky Survey (SDSS) as well as two Much like mapping a fogbound city from a NASA spacecraft, the Wide-field Infrared single intersection, scientists must decipher Survey Explorer (WISE) and the the galaxy’s structure while viewing it from . These FAST FACT observations have helped astron- Francis Reddy is the senior science writer for the omers better define our galaxy’s Astrophysics Science Division at NASA’s Goddard spiral arms, take a census of star 27,200 Space Flight Center in Greenbelt, Maryland. clusters and other phenomena in LIGHT-YEARS The Sun’s distance from the galaxy’s billion light-years center 2.75 3 Scientists think our gal- axy has four major spiral arms that wind their way out from a central bar. The Sun lies approximately S 27,200 light-years from cu the center. NASA/JPL-CALTECH/R. t HURT (SSC-CALTECH) um -C e n ta r u s A r

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No rm a-O ute r Arm I)/J. MERTEN (HEIDELBERG/BOLOGNA) I)/J. C ESO/D. COE (STS COE ESO/D. Pandora’s Cluster (PANDORA’S CLUSTER) 3.52 billion light-years

3.25 3.5 WWW.ASTRONOMY.COM 31 The Nebula (NGC 3372) ranks among the Milky Way’s biggest stellar nurseries. It lies about 7,500 light-years from Earth and burst to life when its first stars ignited some 3 million years ago. Today, it holds nine stars with lumi- nosities at least a million times that of the Sun. NASA/ESA/N. SMITH (UCB)/THE HUBBLE HERITAGE TEAM (STSCI/AURA)

Exploration of Radio (VERA), are using this capability to pinpoint the locations and motions of regions where new stars are forming in order to trace our galaxy’s spiral structure. Movin’ out The frontier of the galaxy lies at the outer fringe of the Oort Cloud of comets, about 100,000 astronomical units (AU; the aver- age Earth-Sun distance) or 1.6 light-years away (see “From AU to light-year,” p. 34). The Pleiades (M45) resides 440 light-years from Earth in the constellation . A proto- Here, the Sun’s gravitational pull weakens typical , it spans about 15 light-years and holds some 500 stars. These luminaries will to the level of nearby stars, and comets disperse over the next few hundred million years. NASA/ESA AND AURA/CALTECH whose orbits take them this far may drift out of the Sun’s grasp entirely. Although the nearest star today is Proxima Centauri, dust-obscured regions of the disk, and But Gaia largely covers optical wave- 4.22 light-years away, other stars played this reveal that the central component of our lengths, which means that intervening dust role in the past and will do so in the future. galaxy, called its “bulge,” is really a vast clouds limit how deeply it can probe into All stars orbit the center of the galaxy, football-shaped star cloud seen nearly end the galaxy’s disk. Dust doesn’t affect radio but these orbits are more elliptical and on. This discovery resulted in a classifica- wavelengths, and a facility called the Very more tilted than planetary orbits in the tion change for the Milky Way from spiral Long Baseline Array (VLBA) can measure solar system. The Sun now lies about galaxy to . distances and motions to a small number 27,200 light-years from the Milky Several ambitious and complementary of sources more accurately than Gaia. Way’s center — more than projects now aim to provide a true 3-D por- By linking 10 radio dishes located from one-third of the way into trait of our galactic home. The European Hawaii to St. Croix so they function as a the disk — and roughly 90 Space Agency’s Gaia spacecraft, which was single telescope, the VLBA has the greatest light-years above the gal- launched in 2013, should return position resolving power available to astronomy. axy’s midplane. During and motion information of unprecedented Two projects, the Bar and Spiral Structure each orbit, which takes accuracy for roughly a billion stars. Legacy (BESSEL) survey and the VLBI about 240 million years to 3C 48 (quasar) 4.05 billion light-years

billion light-years 3.75 4 32 ASTRONOMY • DECEMBER 2015 HOW THE MILKY WAY GALAXY GOT ITS NAME Spend some time under the stars any clear night far from city lights, and a ghostly band called the Milky Way eventually will come into view. Flecked with some of the brightest stars in our galaxy and cleaved by intervening dust clouds for about a third of its extent, the Milky Way has been recog- nized since antiquity. In mythology, it was frequently associated with a cosmic path- way or heavenly stream. The ancient Greeks called it galaxías kýklos, the “milky circle,” a description that At a distance of 1,350 light-years, the Orion Massive stars live fast and die young, exploding also gave rise to our word “galaxy.” The Nebula (M42) is the nearest large star-forming as supernovae and leaving behind remnants like Romans lifted the concept but gave it a region. Our Sun likely formed in a cloud like this, the Crab Nebula (M1). Such star death often trig- twist appropriate for a civilization fond of one capable of producing 1,000 to 10,000 stars. gers future star formation as shock waves com- road construction, calling it via lactea, the NASA/ESA/M. ROBBERTO (STSCI/ESA)/THE HST ORION TREASURY PROJECT TEAM press surrounding gas. NASA/ESA/J. HESTER AND A. LOLL (ASU) “milky way.” Galileo Galilei took the first step in understanding what it actually represented in 1610, when his new and passage to reach planetary orbits, but the The night sky distorts our picture of the improved spyglass revealed that the pale light came from individual faint stars “so system’s low mass — just one-sixth the galaxy’s stars in another way, too. Of the numerous as almost to surpass belief.” Sun’s — and its path through the outer 100 brightest stars in the sky, a third lie Over the next two centuries, as astrono- Oort Cloud argue against any significant within 100 light-years. These include Sirius, mers began to understand that the Milky comet enhancement. the night sky’s brightest, 8.6 light-years Way was part of an “island universe” that Dim, low-mass M dwarfs like Scholz’s away; Procyon, 11 light-years away; Vega included the Sun and other visible stars, the Star and Proxima Centauri actually typify and Fomalhaut, both 25 light-years off; name for a mythical cosmic pathway was the Milky Way’s . Most Castor (52); Aldebaran (65); and Regulus transferred to our galactic home. — F. R. of the galaxy’s roughly 400 billion stars (77). But another third lie more than 400 are likely M dwarfs, but because they emit light-years away, including Polaris (430), little visible light, we’re still finding those Antares (600), (640), Rigel (860), complete, the Sun’s position oscillates close to the solar system through infrared and Deneb (2,600). All these stars have above and below the . Other surveys like WISE. For stars, mass is des- masses more than seven times the Sun’s and stars in our vicinity follow slightly different tiny. M dwarfs may be dim, but their low are tens of thousands of times more lumi- paths, which means that the distribution masses mean they burn their nous. Consequently, they burn and composition of our stellar neighbor- nuclear fuel sparingly and will through their hydrogen fuel at hood gradually changes. Stars routinely keep shining billions of FAST FACT a faster clip. Long before our pass much closer to the Sun than Proxima years after the Sun dies. Sun’s fires quench, these Centauri is now. Some stars barely stars will end their days For example, in 2014, astronomer Ralf- shine at all. They never 150,000 in spectacular supernova Dieter Scholz of the Leibniz Institute for generate energy in their LIGHT-YEARS explosions. Astrophysics in Potsdam, Germany, dis- cores though true hydro- covered that a faint M dwarf star detected gen fusion, the power Diameter of the From stars to by WISE was about 20 light-years away, source that heats stars galaxy’s disk clusters making it a previously unknown close for most of their lives, but Going further up the neighbor. A team led by Eric Mamajek at when young they can produce mass scale results in an ever- the University of Rochester in New York energy by fusing a rare form of dwindling number of stars, and noticed that “Scholz’s Star,” which is actu- hydrogen, deuterium. Called brown not just because the most massive ones ally a binary, shows little motion across dwarfs, they measure between 1.2 and are so short-lived. Stars are born in dense, the sky but rapid motion directly away 7 percent of the Sun’s mass. The compan- cold molecular clouds. Once a massive from us, suggesting it might have grazed ion to Scholz’s Star is a member of this star forms, its intense ultraviolet light and the solar system. class. With surface temperatures as cool a powerful outflow called a The study reveals that the binary passed as one-tenth the Sun’s, brown dwarfs are start to disperse the birth cloud, limiting well inside the Oort Cloud, coming within marginal stars that may be as numerous as the number of other massive stars able to 52,000 AU about 70,000 years ago, and the real things. More than 50 known stars form nearby. Only a few dozen stars in the now holds the record for the closest flyby and brown dwarfs reside within 16 light- Milky Way have energy outputs exceeding of any known star. It will take about 2 mil- years of the Sun, but only 10 of them are a million times the Sun’s. Topping the list

NASA/ESA/NICMOS48) (3C lion years for any comets dislodged by this visible to the naked eye. are WR 25 and Eta Carinae, two massive

4.25 4.5 WWW.ASTRONOMY.COM 33 FROM AU TO LIGHT-YEAR Where the solar system ends, interstellar space and the galactic fron- tier begin. At the fringe of the Oort Cloud, perhaps 100,000 astronom- ical units (AU; the average Earth-Sun distance) away, comets are about as likely to be dislodged from the solar system as to continue in their slow orbits. From here on out, expressing distances in the manner commonly used within the solar system rapidly becomes unwieldy. It turns out that 63,241 AU equals the distance light travels in a vacuum over the course of one year: a light-year. The fringe of the Oort Cloud is about 1.6 light-years away. The closest star, Proxima Centauri, is a mere 4.22 light-years distant. And the Orion Nebula, the closest large star-forming region, is about 1,350 light-years off. According to relativity, no matter or information can travel faster than the speed of light in a vacuum. But there is a consequence to thinking about distance in terms of light travel time. The farther away we look, the longer light takes to reach us. At any given moment, we see Proxima Centauri as it looked 4.22 years ago and young stars in the Orion Nebula as they appeared more than a millennium ago. Applied *, the bright spot at the heart of the luminous cloud at center, to large numbers of galaxies at different distances, this time-machine glows strongly in X-rays as matter swirls into the maw of a 4-million-solar- effect gives astronomers a powerful tool for understanding how gal- mass black hole. This supermassive object lies 27,200 light-years from Earth axies like our own developed and evolved over billions of years. — F. R. and is the gravitational hub of our galaxy. NASA/CXC/UNIVERSITY OF WISCONSIN/Y. BAI, ET AL. binary systems located about 7,500 light- Open star clusters, such as the Hyades 300 light-years across. Our galaxy has years away and shining with 6.3 million and Pleiades in Taurus (150 and 440 light- fewer than 200, and all are more than 10 and 5 million solar , respec- years away, respectively) and the Beehive in billion years old. Globular clusters orbit tively. Another eight stars in the Carina Cancer (about 580 light-years distant), are the galaxy’s center, but they follow wildly Nebula make the cut as well, and seven relatively compact collections of stars that inclined paths that take them far above and more occur in a stellar grouping called formed together within the same molecular below the disk. Researchers now know that the Cygnus OB2 association. cloud. These clusters contain anywhere the Milky Way has pilfered at least some Massive stars play a powerful role in from dozens to hundreds of stars in a globular clusters, but more on that later. mapping out our galaxy’s spiral arms. They region less than about 50 light-years wide, can be seen across great distances, they and they will gradually disperse over a few Galactic architecture explode before wandering too far from hundred million years. Astron omers have Early in the last century, the differences their stellar nurseries, and they can light cataloged about 1,200, though the Milky between open and globular star clusters up their dissolving birth clouds and excite Way may contain as many as 100,000. guided astronomers into an overview of the molecules within them, like water and In places like the Orion Nebula and the Milky Way. Open clusters orbit in a disk- methanol. Under the right conditions, Eagle Nebula (1,350 and 7,000 light-years shaped volume that also contains nearly which are common in star-forming away, respectively), where young stars have all of the galaxy’s gas and dust, the seed for regions, these molecules can become emerged from their birth clouds and set new stars. This disk is some 1,000 light- masers — the microwave equivalent of them aglow, scientists are seeing the years thick and extends probably 75,000 lasers — and beam amplified radio waves creation of new open clusters less than light-years from the , placing our way, creating beacons that cut through 2 million years old. Young clusters can the solar system a little more than a third starlight-blocking dust clouds. be detected in the infrared even before of the way out in the disk. Star groupings of various types also they break out of their natal cloud. Using In the disk’s center lies a football- help trace galactic structure. OB associa- data from WISE, a team led by Denilso shaped concentration of mainly old stars tions are loose collections of between 10 Camargo at the Federal University of Rio called the , which is about and several hundred hot, young, and mas- Grande do Sul in Brazil reported in 2015 12,000 light-years long. Although its exact sive O- and B-type stars spanning up to the discovery of hundreds of dust-shrouded size, shape, and viewing angle remain a few hundred light-years. The nearest is clusters embedded deeply in their host somewhat unclear, we see the bulge the Scorpius-Centaurus OB2 association, molecular clouds. obliquely not too far from end on. Until which lies about 470 light-years away and OB associations, open clusters, and recently, astronomers regarded the bulge boasts the red supergiant Antares among embedded clusters all reside in the Milky as sort of a senior center for aging stars, its members. Its oldest subgroup was born Way’s disk. But globular clusters form a a population that formed rapidly as the roughly 15 million years ago, and shock radically different kind of galactic tracer. galaxy assembled through mergers with waves from a supernova helped trigger star Essentially giant star balls, these clusters smaller galaxies some 10 billion years ago. formation in a neighboring cloud about pack tens of thousands to perhaps a mil- Predominantly older stars do occupy parts 5 million years ago. lion stellar siblings into spheres less than of the bulge well above and below the disk,

billion light-years 4.75 5 34 ASTRONOMY • DECEMBER 2015 but recent studies show a wide range of stellar ages, from 3 to 12 billion years, closer to the midplane. Various lines of evidence suggest the bulge population formed largely as a result of natural insta- bilities in the evolving disk. At the center of the bulge is the galaxy’s anchor, the object everything else orbits — a weighing about 4 million solar masses. Regular monitoring of the galactic center shows that it often flares in X-rays — the signa- ture of matter falling toward its doom — but this pales in comparison to what we Two million stars glow in the core of Omega What does the Milky Way look like from afar? know a monster black hole can do, and Centauri, though this is just 20 percent of the From what we know, it resembles this barred ’s total. Omega lies 17,000 light- spiral galaxy, UGC 12158. This island universe there is evidence it has been more active years away and likely is the bulge of a disrupted spans about 140,000 light-years and lies some in the past. In 2010, data from NASA’s . NASA/ESA/THE HUBBLE HERITAGE TEAM (STSCI/AURA) 400 million light-years away. ESA/HUBBLE AND NASA Fermi Gamma-ray Space Telescope revealed ginormous gamma-ray-emitting bubbles reaching 25,000 light-years above the Chinese Academy of Sciences in globular clusters and satellite galaxies, as and below the galactic center, likely the Beijing has extended its size by about 50 well as strewn with stars stripped from smoking gun of a powerful outburst percent over previous values. The number them. Our galaxy — indeed, most galaxies millions of years ago. of stars in the disk had seemed to drop off — may have been built by gobbling up The precise structure of the disk around 50,000 light-years from the center, many smaller galaxies. Today we see remains poorly known, including the but then SDSS found what appeared to be streams of stars linked to several small number and position of its spiral arms. a vast ring of stars about 10,000 light-years satellites, and the Milky Way appears to Recent radio studies of thousands of farther out. The new study shows this is an have swiped several globular clusters from sources — stars in embedded clusters illusion caused by at least four ripples that the Sagittarius Dwarf Spheroidal. The larg- detected in the infrared, nebulae set aglow displace stars in the disk above and below est and brightest globular cluster, named by young stars, giant molecular clouds, the galactic plane. When we look out of the Omega Centauri and located about 17,000 and water and methanol masers — seem to galaxy from the solar system, the disk is light-years away, has a more complex stellar show that the Milky Way has four major perturbed up a few hundred light-years, makeup than others. Researchers suspect it spiral arms that originate near the galac- then down, then up, and then down again, is the leftover bulge of a dwarf galaxy long tic center and wind outward. In starting about 6,500 light-years ago shredded by our own. order from the center moving from the Sun and reaching at Yet most of the Milky Way’s mass toward the Sun, they are FAST FACT least 50,000 light-years remains unseen. The motions of stars the Norma-Outer Arm, away. Additional ripples around our galaxy and others reveal a the Scutum-Centaurus may yet be found. gravitational influence extending far Arm, and the Carina- 1.5 to 2 Small galaxies orbit- beyond the structures we can see. Studies Sagittarius Arm. Farther TRILLION ing our own may have show that the Milky Way resides in a out, we encounter the produced the ripples. roughly spherical halo of invisible material , and farther Galaxy’s total mass One in particular, — called dark matter — reaching 900,000 still, the outer arc of the (including dark known as the Sagittarius light-years across, or about six times the Norma-Outer Arm. matter) Dwarf Spheroidal, has disk’s diameter. This stuff makes up Astronomers long thought passed through the disk mul- 27 percent of the cosmos and created the that the solar system resided in a tiple times and is gradually dis- gravitational infrastructure that coaxed starry spur located near the inner edge of solving into streams of stars as it orbits ordinary matter into structures that even- the Perseus Arm. Yet one of the first sur- the Milky Way. Like a stone tossed into tually built galaxies like our own. prising results from the BESSEL and still water, the gravitational effect of a sat- The current phase of galactic exploration VERA projects is that our “spur” is a sig- ellite galaxy plunging through the disk already has provided major new insights, nificant structure, sporting as much mas- could produce ripples. Simulations sug- but many questions remain. As astronomers sive star formation as the adjacent major gest that satellite galaxies tearing through consolidate the results of this research over arms. At this point, astronomers aren’t the disk can play a role in creating spiral the next decade, an accurate 3-D portrait of sure whether to classify our local patch of structure. And intriguingly, the newfound the Milky Way will emerge, enabling us for the galaxy as a branch of the Perseus Arm ripples align closely with the Milky Way’s the first time to view our island universe in or an independent segment. spiral arms. the same way we see other galaxies: as a And the disk holds more surprises. A The disk sits within a spherical volume complete cosmic object — a whole greater 2015 study of SDSS data led by Yan Xu at called the , a place ruled by than the sum of its parts.

— The Cosmic Distance Scale continues on page 44

5.25 WWW.ASTRONOMY.COM 35 SKYTHIS MARTIN RATCLIFFE ALISTER LING Visible to the naked eye and describe the Visible with binoculars MONTH solar system’s changing landscape as it appears in Earth’s sky. Visible with a telescope December 2015: A gemlike shower

thus is closer to setting, so target it first. The ice giant planet lies in central Aquar- ius, a large and relatively inconspicuous constellation that appears nearly halfway from the southern horizon to the zenith as night falls. At midmonth, it sets after 10 p.m. local time. The best guide star for finding Neptune is magnitude 4.8 Sigma (σ) Aquarii. If your sky is less than perfect, first locate magnitude 3.8 Lambda (λ) Aqr; Sigma lies 6° south- west of Lambda. Neptune appears in the same binocular field as Sigma all month. On the 1st, the planet resides 1.5° northeast of the star. The gap A Geminid meteor shoots past Orion (lower center) shortly before dawn at the peak of the 2013 shower. Similarly grows to 2.0° by month’s end. Moon-free conditions bode well for this year’s shower. TONY ROWELL Neptune glows at magni- tude 7.9, nearly 20 times dim- he morning sky sees Let’s begin our tour of the disk that measures 7" across mer than Sigma. You may most of the planetary solar system shortly after the and appears close to half-lit. have a hard time seeing this action in December, Sun goes down in the latter Keep your binoculars faint with hand-held binocu- with Venus, Mars, and half of December. Mercury handy as the sky darkens, and lars. Mount them on a tripod, Jupiter all prominently appears above the southwest- you can put them to work however, and you can easily Tdisplayed. A highlight of the ern horizon starting about tracking down the outer two view objects a full magnitude month occurs during daylight 30 minutes after sunset. On major planets. Neptune lies fainter. Although binoculars on the 7th when the Moon the 15th, it lies only 4° high farther west than Uranus and certainly can reveal Neptune, passes directly in front of but should show up because Venus and hides it from view. it shines so brightly, at mag- A planetary lineup The evening sky possesses nitude –0.6. (Binoculars will more subtle attractions. Ura- help you to spot it in the twi- LEO nus and Neptune remain bin- light glow.) Alphard ocular targets all month, while The innermost planet Mercury climbs into view remains within 0.1 magnitude Jupiter during twilight after mid- of this benchmark for the rest BOÖTES HYDRA December. But perhaps the of December but becomes month’s biggest event comes significantly easier to see as on the 14th when the annual it climbs away from the hori- VIRGO Mars Geminid meteor shower peaks zon. By the time it reaches CORVUS under a Moon-free sky. greatest elongation on the Venus Spica 28th, when it lies 20° east of 10° Martin Ratcliffe provides plane- the Sun, Mercury stands 9° tarium development for Sky-Skan, high a half-hour after sun- Inc., from his home in Wichita, down and doesn’t set until December 1, 5 A.M. Looking southeast Kansas. Meteorologist Alister an hour later. When viewed Ling works for Environment through a telescope in late Venus, Mars, and Jupiter stretch across the predawn sky on chilly Canada in Edmonton, Alberta. December, the world shows a December mornings. ALL ILLUSTRATIONS: ASTRONOMY: ROEN KELLY

36 ASTRONOMY • DECEMBER 2015 RISINGMOON A Christmas present at Luna’s north pole Hop aboard the polar express

Whether Santa Claus delivers a If you’re up for a little adven- Peary new telescope or you use just ture, let’s embark on a trek to your naked eyes, marvel at the the north pole. Full Moon high in a late evening Lunar cartographers named sky December 25. Chances are many features close to the pole Byrd you’ll feel there’s something after great polar explorers of unusual about it. You’re right. past centuries. Two major cra- Mare Crisium on the eastern ters honor the American adven- N limb looks like it’s nearly on turers Richard Byrd and Robert the equator, and if you view Peary. Like those pioneers, avoid E through binoculars, the rays eyestrain by using sunglasses or pointing back to Tycho reach a filter to cut down on the Full A favorable libration at Full Moon brings normally hidden craters at the near the southern limb. Plenty Moon’s glare. Look just below lunar north pole into view. ALISTER LING, BASED ON CLEMENTINE DATA; INSET: NASA/GSFC/ASU of “extra” light-hued terrain also the northern limb for Byrd, appears near the north pole. which appears as a large, flat, past Peary’s parapets into its named for Soviet physicist On the 25th, Luna lies about and fully illuminated oval. partially shadowed floor right Dmitriy Rozhdestvenskiy. This as far south of the ecliptic — On the 25th, sunlight nicely up to the back wall. Immediately feature lies at 87° north latitude Earth’s orbital plane around the illuminates this crater’s back wall, behind that lies the north pole on the lunar farside. Off to the Sun — as it can get. From our which is almost indistinguishable itself, which remains in shadow. east and down the limb a little earthbound perspective, we see from Peary’s southern flank. The If you look carefully beyond ways, you’ll find the crater walls a little past the north pole and shadows help our minds gener- Peary, you’ll see another lit of Nansen, named for the down the farside in what astron- ate a 3-D view of the polar wall. This is the outer rim of Norwegian polar explorer omers call a northern libration. regions. We can see over and Rozhdestvenskiy, a large crater Fridtjof Nansen. you might not be able to iden- METEOR tify it unambiguously. To con- WATCH Geminid meteors firm a sighting, point your Active dates: Dec. 4–17 telescope at the suspected Absent Moon means magical meteor show Peak: December 14 planet. Neptune will show a Moon at peak: Waxing crescent Maximum rate at peak: disk that measures 2.3" across Meteor observers always look at Geminid meteor shower 120 meteors/hour and sports a blue-gray color. the calendar to see which showers Head 40° east of Neptune will occur near New Moon. Bright Capella along the ecliptic — the moonlight drowns out fainter apparent path of the Sun meteors and renders the brighter URSA AURIGA TAURUS MAJOR across the sky that the planets ones less impressive. But no wor- follow closely — and you’ll ries this month — the prolific reach Uranus. This ice giant Geminid shower peaks December Aldebaran 14, just three days after New Moon. Radiant world, a kissing cousin to Castor Neptune, resides among the The “shooting stars” appear to Pollux GEMINI faint stars of Pisces the Fish. radiate from a point near Castor in Uranus appears slightly more Gemini. The radiant climbs higher ORION in the east throughout the evening Betelgeuse than halfway to the zenith in LEO the southeast as darkness and passes nearly overhead around 2 A.M. local time. Don’t stare at the Rigel falls and remains on view all Regulus Procyon evening. It doesn’t set until radiant, however — you’ll see lon- ger streaks if you look 40° to 60° around 1 a.m. local time even away. For the best views, find an December 14, 1 A.M. as the month comes to a close. Looking high in the south 10° observing site far from the city, The seventh planet shows where you can expect to see up up easily through binoculars. One of this year’s most prolific meteor showers could deliver up to 120 to 120 meteors per hour. “shooting stars” per hour under ideal conditions. Glowing at magnitude 5.8, it is technically visible with naked eyes under a dark sky, OBSERVING A waning crescent Moon passes directly in front of Venus during HIGHLIGHT — Continued on page 42 daylight hours December 7 for observers across North America.

WWW.ASTRONOMY.COM 37

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38 ASTRONOMY • DECEMBER 2015 S Note: Moon phases in the calendar vary in size due to the distance from Earth DECEMBER 2015 and are shown at 0h Universal Time.

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

Open cluster ` 12345

O Globular cluster i DRAC

Diffuse nebula a 6 7 8 9 10 11 12

NW

` Galaxy

Vega 13 14 15 16 17 18 19 _

: ROEN KELLY ROEN : ¡ LYRA

20 21 22 23 24 25 26 ASTRONOMY

M57 a

d b

27 28 29 30 31

ILLUSTRATIONS BY BY ILLUSTRATIONS

`

CYGNUS

Deneb

_

d r a

_ Calendar of events

VULPECULA A 3 Last Quarter Moon 19 The Moon passes 1.2° south of

¡ occurs at 2:40 A.M. EST Uranus, 8 P.M. EST

M27 LACERT a 4 The Moon passes 1.8° south of 21 The Moon is at perigee

A.M.

c a Jupiter, 1 EST (228,924 miles from Earth),

4:00 A.M. EST SAGITTA

_ 5 The Moon is at apogee (251,531 _

Altair miles from Earth), 9:56 A.M. EST Mars passes 4° north of Spica,

d

` AQUILA ` 7 A.M. EST ` W The Moon passes 0.1° south of

+ Mars, 10 P.M. EST Winter solstice occurs at DELPHINUS M15 11:48 P.M. EST 7 The Moon passes 0.7° north of Venus, noon EST 23 The Moon passes 0.7° north of

Enif Aldebaran, 3 P.M. EST

PEGASUS _ _ 9 Asteroid Psyche is at

c EQUULEUS opposition, 9 A.M. EST 24 Asteroid Euterpe is at e opposition, midnight EST

_ 11 New Moon occurs at A.M. ` 5:29 EST 25 Full Moon occurs at a 6:11 A.M. EST SPECIAL OBSERVING DATE 14 The annual Geminid 26 Uranus is stationary, 6 A.M. EST meteor shower peaks Path of the Sun (ecliptic) under a Moon-free sky. 28 Mercury is at greatest eastern UARIUS elongation (20°), 10 P.M. EST Q a

b b 17 The Moon passes 3° north of 31 The Moon passes 1.5° south of P.M. Neptune, 3 A.M. EST Jupiter, 1 EST

18 First Quarter Moon PISCIS occurs at 10:14 A.M. EST AUSTRINUS

Fomalhaut _ SW TOR

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 December 2015

DRA Objects visible before dawn UMa LYN

HER CVn LMi PER Asteroid Euterpe reaches AUR LYR BOÖ opposition December 24/25 Nausikaa CrB COM LEO ic) eclipt Sun ( the h of Pat SER Comet CNC Asteroid Psyche reaches TAU OPH Catalina Jupiter opposition December 9 SER (C/2013 US10) Celestial equator CMi ORI VIR SEX LIB Mars MON SCT Saturn Venus CRT ERI HYA The Moon occults Venus Sun CMa in daylight December 7 LEP

PYX ANT COL LUP SCO PUP CAE CEN TEL VEL

Moon phases Dawn Midnight

13 12 11 10 89 7 6 345 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 in the sky for the dates in the data table at bottom. South is at the top to match the view through a telescope.

Mercury Mars Uranus

S

WE Saturn Pluto N Venus Ceres Neptune 10" Jupiter

Planets MERCURY VENUS MARS CERES JUPITER SATURN URANUS NEPTUNE PLUTO Date Dec. 31 Dec. 15 Dec. 15 Dec. 15 Dec. 15 Dec. 31 Dec. 15 Dec. 15 Dec. 15 Magnitude –0.5 –4.1 1.4 9.3 –2.1 0.5 5.8 7.9 14.2 Angular size 7.1" 15.8" 5.1" 0.4" 37.1" 15.3" 3.6" 2.3" 0.1" Illumination 53% 72% 92% 98% 99% 100% 100% 100% 100% Distance (AU) from Earth 0.947 1.054 1.845 3.489 5.316 10.868 19.557 30.207 33.915 Distance (AU) from Sun 0.330 0.719 1.663 2.977 5.413 10.011 19.977 29.960 33.003 (2000.0) 20h02.4m 14h38.7m 13h11.3m 21h14.2m 11h32.8m 16h37.9m 1h01.4m 22h36.1m 19h01.2m (2000.0) –21°28' –13°04' –5°57' –24°49' 4°12' –20°27' 5°51' –9°41' –21°04'

40 ASTRONOMY • DECEMBER 2015 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.

CAS DRA Objects visible in the evening Jupiter’s moons

AND LAC Dots display positions Io CYG HER of Galilean satellites at 5 A.M. EST on the date Europa TRI LYR shown. South is at the top to match ARI VUL S Eunomia PEG the view Ganymede through a WE Amphitrite SGE telescope. N Callisto Uranus PSC EQU OPH AQL SER 1

Vesta AQR 2 Laetitia SCT 3 CET Path of th Neptune e Moon 4 CAP Pluto Sun SCL PsA Ceres 5 FOR SGR 6 Mercury appears bright in the PHE evening sky in late December CrA SCO 7 GRU TEL 8 Early evening 9 To locate the Moon in the sky, draw a line from the phase shown for the day straight up to the curved blue line. Note: Moons vary in size due to the distance from Earth and are shown at 0h Universal Time. 10 11 Jupiter

22 21 20 19 18 17 16 15 14 13 12 11 12 Ganymede

13 Callisto

14

Earth 15 Winter solstice is December 21/22 16

17

Jupiter 18 Europa Mars Mercury Greatest eastern elongation 19 Venus is December 28 20

Ceres 21 22 Io

23

24

25

26

The planets Jupiter 27 Uranus in their orbits 28 : ROEN KELLY ROEN : Arrows show the inner planets’ 29 monthly motions and dots depict Neptune

Saturn ASTRONOMY the outer planets’ positions at mid- 30 month from high above their orbits. 31 Pluto ILLUSTRATIONS BY BY ILLUSTRATIONS

WWW.ASTRONOMY.COM 41 — Continued from page 37 Shadow play on Jupiter WHEN TO VIEW THE PLANETS S Io Callisto EVENING SKY MIDNIGHT MORNING SKY Jupiter Mercury (southwest) Jupiter (east) Venus (southeast) Uranus (southeast) Uranus (west) Mars (southeast) W Neptune (south) Jupiter (south) Callisto’s shadow Saturn (southeast) though even a little optical aid due north, and Uranus will 30" makes the task much simpler. come into view. Ganymede Uranus moves glacially (appro- Just as Uranus sets in the December 8, 5:30 A.M. EST priate for cold December west, the eastern sky lights up On December 8, Callisto lies well east of Jupiter yet still manages to cast a nights) relative to the back- with the arrival of brilliant shadow onto the ’s cloud tops. ground stars in December, Jupiter. The giant planet remaining 2° due south of rises around 12:30 a.m. local 10 percent during the month familiar with Jupiter and the magnitude 4.3 Epsilon (ε) time in early December and as Earth’s orbit brings the two details visible in its massive Piscium. To quickly find its more than an hour earlier by worlds closer together. atmosphere. This practice general location, run an imagi- month’s end. Jupiter brightens This large size means you will prepare you to see the nary line diagonally across the from magnitude –2.0 to –2.2 should get exquisite views of most when the world reaches Great Square of from during December against the jovian cloud features. Most opposition and peak visibility its northwestern corner (Beta backdrop of much fainter obvious are two relatively dark in March. [β] Pegasi) to the southeast stars in southeastern Leo. belts, one on either side of a Any telescope also will (Gamma [γ] Peg) and extend Three hours after rising, bright zone that coincides with show Jupiter’s four bright it an equivalent distance. Jupiter has reached an altitude the planet’s equator. The best moons. Io, Europa, Gany- If you swing your telescope of 30°. This provides three views come during moments mede, and Callisto orbit above toward Uranus, you’ll see a solid hours of observing time of steady seeing when the nor- the planet’s equator and thus blue-green disk that spans 3.6". before twilight starts to paint mally turbulent air flowing wander east and west of the If you’re observing the slightly the sky. The planet’s disk above your head (and tele- gas giant. When one of them gibbous Moon on December spans a healthy 36" in early scope) calms for a few seconds. passes in front of Jupiter, you 19, slew your scope about 1.5° December and grows nearly Take some time now to get can watch as its dark shadow COMETSEARCH

Tails of dust and gas at dawn Comet Catalina (C/2013 US10) N j d With bated breath, observers But don’t let that discourage Arcturus across the Northern Hemisphere you. The comet should be a k 31 o / have been waiting for Comet pretty sight through binoculars BOÖTES Catalina (C/2013 US10) to pop or a telescope at low power. c into the predawn sky. If all goes Imagers should be prepared the according to plan, it could shine morning of the 7th to capture Path of Comet Catalina 26 as bright as 4th magnitude in Catalina sharing an 8° field with early December. Venus and a thin crescent Moon. E To many people, 4th magni- After the 8th, two weeks of tude hints at naked-eye visibility. Moon-free skies will let observ- 21 Unfortunately, other consider- ers get detailed views of the 109 VIRGO ations come into play. First, cit- comet’s dust and gas tails. From o ies throw up a veil of light that our perspective, the two tails 16 c will hide all but the central area. appear distinct. Although the Second, magnitude predictions comet’s surface remains hidden 11 assume all the light is com- by a veil of dust, the surrounding LIBRA + f pressed into a star-like point. In coma may show a greenish hue. 6 reality, a comet’s glow spreads The Moon returns to the g out into a fuzzy ball and the morning sky in late December ` Dec 1 Spica average surface brightness is as Catalina sets its sights on lower. Finally, the Moon spills its brilliant Arcturus. By then, the 5° own light into the sky during comet likely will have faded to If this visitor from the Oort Cloud matches expectations, it could hit 4th early December. 5th magnitude. magnitude this month as it streaks north toward brilliant Arcturus.

42 ASTRONOMY • DECEMBER 2015 Daytime drama for the Moon and Venus LOCATINGASTEROIDS An asteroid poses as a distant supernova

A small space rock could trigger from a dark site or an 8-inch some excitement among astro- instrument from the suburbs. imagers when it passes close The pair lies less than 1° east- to the popular galaxy M77. On southeast of 4th-magnitude December 9 and 10, asteroid Delta (δ) Ceti. Bump up the 39 Laetitia poses as a 10th- power past 100x, let yourself magnitude star on the fringe of dark adapt a bit, and you this bright spiral, mimicking the should see three other faint light from a supernova nearly fuzzies nearby, all part of the 50 million light-years away. M77 group of galaxies. M77 is the founding member To the eye, Laetitia will not of Carl Seyfert’s class of active move enough to notice during

ANTHONY AYIOMAMITIS ANTHONY galaxies. Hidden from view by a single viewing session. When On June 18, 2007, Earth’s satellite passed in front of Venus during daylight gas, dust, and dense swarms of it’s near M77 or between Delta hours. North American observers can witness a similar event December 7. stars, the galaxy’s core harbors a and the spiral galaxy NGC 1055, supermassive black hole busily make a quick sketch of the field and then its brighter disk cross Mars tracks eastward during devouring its surroundings. and return to it the next clear the jovian cloud tops. Decem ber and passes 4° north You can pick up Laetitia and night. The object that has The angle of the Sun shin- of Spica on the 21st. M77 with a 4-inch telescope shifted position is the asteroid. ing on Jupiter diverges notice- The planet remains on the ably from our line of sight far side of the solar system Cruising by a whale of a galaxy during December. You can get from Earth and thus still N a good idea of the difference looks tiny: a mere 5" across. December 8. Callisto’s shadow At this size, only observers falls on Jupiter’s northern with large scopes can hope to hemisphere starting at 4:17 a.m. pick up subtle surface details. 31 EST when the moon itself is Mars will grow much bigger, CETUS still 1.6' (nearly three Jupiter- brighter, and more detail-rich 26 widths) east of the planet. next spring as it approaches E Path of Laetitia Europa, the target of opposition in late May. 21 NASA’s next major planetary Venus rises about an hour NGC 1055 b mission, casts its shadow on after Mars on December 1 16 Jupiter’s cloud tops early on and appears conspicuous in 11 the mornings of Christmas the eastern sky by 4 a.m. local 6 M77 Eve and New Year’s Eve for time. At magnitude –4.2, it is Dec 1 observers in the Americas. On five full magnitudes (a factor both days, Europa’s shadow of 100) brighter than its com- starts crossing the jovian disk panion Spica, which stands 0.5° 84 about 2.5 hours before the just 4° away. Venus moves The 10th-magnitude asteroid Laetitia skirts north of the bright galaxy moon itself. quickly this month, crossing M77 in Cetus the Whale during the second week of December. The two planets that hud- into Libra on the 11th. Look dled with Jupiter in eastern for the magnitude 2.8 double Leo early last month string star Zubenelgenubi (Alpha [α] no doubt notice the waning for several cities across the out across neighboring Virgo Librae) 2° south of Venus on crescent Moon closing in on country: Seattle at 7:54 a.m. in early December. First to the 17th. Venus. After sunrise, Luna PST; Los Angeles at 8:04 a.m. rise is Mars, which comes up The rapid changes to passes directly in front of PST; Denver at 9:36 a.m. MST; nearly two hours after Jupiter. Venus’ telescopic appearance (occults) the planet for viewers Houston at 11:12 a.m. CST; On the 1st, the Red Planet this autumn slow considerably across North America. You can Chicago at 11:18 a.m. CST; shines at magnitude 1.5 and in December. The apparent follow this occultation through New York at 12:42 p.m. EST; stands 1.5° due south of mag- size of its disk shrinks from binoculars or a telescope. Boston at 12:43 p.m. EST; and nitude 2.8 Gamma Virginis, 17" to 14" while its gibbous The time when Venus dis- Miami at 12:52 p.m. EST. If the constellation’s second- phase fattens from 67 to 77 appears behind the Moon’s you want to view the event brightest star. Mars’ orange percent lit. sunlit limb depends on your through a telescope, be sure hue contrasts nicely with If you’re observing the location. The following are to set up at least 30 minutes blue-white Spica, Virgo’s morning of December 7, you’ll accurate to the nearest minute before these times. 1st-magnitude luminary, which lies some 14° southeast. GET DAILY UPDATES ON YOUR NIGHT SKY AT www.Astronomy.com/skythisweek.

WWW.ASTRONOMY.COM 43 THE LOCAL GROUP Our galactic neighborhood The Milky Way’s family of galaxies is locked in a multi-billion-year battle for gravitational supremacy. by Katherine Kornei

The Milky Way’s nearest cosmic neigh- bors, the (right), reign in the southern sky, but our Local Group is home to many other small satellite galaxies. ESO/BABAK TAFRESHI

— The Cosmic Distance Scale continued from page 35 billion light-years 5.5 5.75 THE MILKY WAY GALAXY is like a hilltop village, according to astronomer Andrew Fox. “At nighttime you can see torches shining in two nearby villages, the Magellanic Clouds, and a more distant one, Andromeda,” he says. “For a long time, those were the only other villages known. Then, one day, someone put two lenses together to make a telescope, looked round, and saw many tiny villages scattered around the surrounding hills and realized that the maps had to be redrawn. That’s the Local Group — it’s where we live.” For most of human history, it was incon- suggests, are low in mass and lack geomet- ceivable that anything existed beyond the rical structure. They’re visually unimpres- Milky Way. After all, the Milky Way is our sive and resemble jumbles of stars — blink home galaxy, and its billions of stars pro- and you’ll miss them — with not a spiral vided enough of a nighttime show to fasci- arm in sight. Dwarf irregular galaxies in nate stargazers for millennia. Even now, in the Local Group can have just a few thou- the era of space-based telescopes and sensi- sand stars, which makes them downright tive cameras, backyard and professional pint-sized compared with the Milky Way astronomers alike can easily devote life- and its hundreds of billions of stars. Even times to studying the Milky Way and its so, astronomers are keen to better under- planets, stars, and nebulae. stand these featherweights of the cosmos. But a peek over the celestial fence beyond “By studying the extremes of any popu- the Milky Way’s tenuous halo yields a rich lation, we learn more about the population view of the nearby universe and galaxy evo- as a whole,” explains Marla Geha, an lution in progress. astronomer at Yale University whose The approximately 85 gravitationally research focuses on the origin and evolu- bound galaxies near the Milky Way are tion of dwarf galaxies. collectively known as the Local Group. This The largest and best-known dwarf population of galaxies, spread over roughly irregular galaxies in the Local Group are 10 million light-years, encompasses not only the Large and Small Magellanic Clouds, the Milky Way and several bright galaxies which look like fuzzy patches in the visible to the naked eye, but also many Southern Hemisphere night sky. The much smaller galaxies that dominate the Magellanic Clouds feature prominently in Local Group by number. By studying the myths: Aboriginal people in Australia tell Local Group, astronomers can observe gal- stories of the Magellanic Clouds alterna- axies in their entirety, no longer confined to tively as a man and a woman, hunters, or understanding a galaxy from the inside out. the ashes of rainbow lorikeets (birds). The Large and Small Magellanic Clouds lie Follow the stream some 160,000 and 200,000 light-years away, Astronomers on Earth have front-row seats respectively, distances slightly greater than to the show of galaxy formation in the Local the diameter of the Milky Group. Comparatively puny galaxies often Way. Given their rela- collide with more massive galaxies, their tive proximity, each meager stockpiles of gas and dust being Magellanic Cloud absorbed into the larger system of stars. holds the dis- Dwarf irregular galaxies, as their name tinction of

Katherine Kornei has a Ph.D. in astronomy and works as a science writer and educator in Portland, Oregon.

66.25 WWW.ASTRONOMY.COM 45 NASA/ESA/J. DALCANTON, B.F. WILLIAMS, AND L.C. JOHNSON L.C. AND WILLIAMS, B.F. DALCANTON, NASA/ESA/J. GENDLER TEAM/R. PHAT WASHINGTON)/THE OF (UNIV. To celebrate the ’s 25th birthday, astronomers created the instrument’s largest image yet thanks to more than 7,000 exposures. This panorama of the Andromeda Galaxy 2.5 million light-years away stretches from our neighbor’s dense central bulge to its sparse outer disk.

being one of a smattering of galaxies in but astronomers have yet to agree whether This cloud stretches across one quarter of which scientists can resolve individual stars. the galaxies have completed several Milky Earth’s sky. Because gas is necessary to “Studying the motion and composition Way orbits or if they’re plunging into our form stars, the Milky Way is collecting star- of individual stars allows us to build a much galaxy’s halo for the first time. forming material and depriving the more accurate and detailed picture of how a “I’d say the jury is still out, chiefly Magellanic Clouds of their stellar fuel. galaxy formed as opposed to studying the because we don’t know the total mass of “There has been a lot of attention in combined light from many stars,” Geha says. the Milky Way very well,” says Fox, the recent years about how much gas there is Hubble Space Telescope images have Space Telescope Science Institute astrono- around galaxies and how that gas enters revealed that the Large Magellanic Cloud is mer who likened the Milky Way to a hill- those galaxies to feed star formation,” forming stars in dense groups that may be top village. “This uncertainty means the says Fox. Observations of the Magellanic the precursors to globular clusters. Some Milky Way’s gravity is also not precisely Stream afford astronomers a close-up view stars in these groups are thought to weigh known, which directly affects the orbits of of gas cycling between galaxies, demon- as much as 100 Suns; they’re prodigiously the Magellanic Clouds.” strating that larger galaxies can act like bright and emit winds that sculpt nearby Regardless of how many times the cannibals and literally consume their gas into bubbles and streamers. These Magellanic Clouds have orbited the Milky smaller companions. intense star formation sites allow a better Way, one fact remains certain: The Milky understanding of high-mass-star formation, Way’s gravity is ripping both apart. Fox and Missing metals and mass a process common in galaxies that are much his colleagues have shown that the Milky Stars are factories for making metals, which farther away than the Local Group. Way is stripping away the gas of the two astronomers consider to be any element The Large and Small Magellanic Clouds dwarf galaxies and collecting it in a massive heavier than helium. Some of the metals are gravitationally bound to the Milky Way, cloud known as the . that stars produce become part of the nebu- lae from which later generations of stars are born. Therefore, the prevalence of metals in a galaxy — its — increases over time, at least for galaxies that are actively forming stars. Larger galaxies contain a higher fraction of metals, on average, than smaller ones. The most massive galaxies in the Local Group — the Milky Way and Andromeda (M31) — contain the highest fractions of metals, which explains the rings on your fingers, your cellphone, and the spoon you used to stir this morning’s coffee. The dwarf irregular galaxies in the Local Group have that are much lower. Astronomers refer to low- metallicity galaxies as being “chemically GALEX/NASA/JPL-CALTECH The Andromeda Galaxy (M31), seen here in ultraviolet light, is the largest galaxy in the Local Group pristine” — they are composed almost with some 1 trillion stars — roughly twice the population of our own Milky Way. entirely of hydrogen and helium, just like

billion light-years 6.5 6.75 46 ASTRONOMY • DECEMBER 2015 B Distance: The Local Group 4.37 million ly Sextans A Edwin Hubble first listed the members of our cosmic Distance: community back in 1936. At the time, our Local Group 4.34 million ly was known to hold the Milky Way, the Large and Small Magellanic Clouds, the Andromeda Galaxy (M31), M32, the Pinwheel Galaxy (M33), NGC 147, NGC 185, NGC 205, NGC 6822, and IC 1613, plus possibly IC 10. The neighborhood has since swelled to more than 85 residents, with new small Distance: Dwarf 2.60 million ly Distance: galaxies being added all the time. Distances are shown in 4.30 million ly light-years (ly). ASTRONOMY: ROEN KELLY Leo II NGC 3109 Distance: Distance: 750,000 ly 4.19 million ly Milky NGC 185 Distance: 838,000 ly Distance: Way IC 10 2.15 million ly Distance: 0° 2.29 million ly 90° And VII Distance: 2.48 million ly

270° NGC 147 1.5 million Distance: light-years 2.29 million ly 2.5 million 180° light-years

And II Distance: Dwarf 2.07 million ly Distance: 3.26 million ly

And VI Distance: Distance: 1.30 million ly 2.62 million ly Tucana Dwarf Pinwheel Galaxy (M33) Pegasus Dwarf Distance: Distance: Distance: 2.86 million ly 2.76 million ly 3.00 million ly IC 1613 Distance: 2.52 million ly Distance: 2.36 million ly WLM (Wolf-Lundmark-Melotte) Distance: 3.03 million ly

Ursa Major I Dwarf Distance: 330,000 ly Ursa Boötes Dwarf Major II Distance: Dwarf 200,000 ly Distance: 293,000 ly Distance: Sextans Dwarf And X 100,000 ly Distance: 297,000 ly And V Distance: Distance: 240,000 ly Distance: 2.50 million ly 2.59 million ly Sagittarius Dwarf Spheroidal Canis Major Dwarf Distance: 82,000 ly And IX Andromeda Distance: 42,000 ly Distance: Milky 2.54 million ly Galaxy (M31) Distance: 500,000 Way Sagittarius Dwarf NGC 205 2.50 million ly light-years Irregular Distance: Barnard’s Galaxy (NGC 6822) Distance: 3.42 million ly 2.65 million ly And III Distance: 1.61 million ly Carina Dwarf Distance: Distance: 323,000 ly 2.45 million Sculptor Dwarf M32 (NGC 221) ly Distance: And I Distance: 300,000 ly Small Magellanic Cloud And VIII Distance: 200,000 ly 2.54 million ly Distance: Distance: 2.55 million ly 2.50 million ly Large Magellanic Cloud Distance: 440,000 ly Distance: 160,000 ly

KEY Irregular Dwarf elliptical Dwarf irregular galaxies galaxies galaxies Spiral galaxies Elliptical Dwarf spheroidal galaxies galaxies

ESO/SOAR (EL GORDO CLUSTER) El Gordo Cluster 7.25 billion light-years

77.25 WWW.ASTRONOMY.COM 47 The Large Magellanic Cloud is close enough for astronomers to resolve individual stars within it, making this dwarf a rare labora- tory to study how galaxies form. NASA/JPL-CALTECH/M. MEIXNER (STSCI) & THE SAGE LEGACY TEAM

Andromeda and the Milky Way dominate the Local Group of galaxies, but the Pinwheel (M33) packs a still respectable 40 billion stars, making it the third largest in the neighborhood. It’s also often considered the most distant object visible with the naked eye. ESO may sound like a lot of mass, but it’s actu- ally 10 million times less massive than the Milky Way. the universe shortly after the Big Bang. that of the Sun. These measurements is a ladybug to the Milky Way’s These systems are like time capsules: They suggest that Segue 2 has not experienced Volkswagen Beetle. let astronomers study what the early uni- much star formation, which other observa- “These systems have all the properties of verse looked like before stars “polluted” tions confirm. galaxies but ridiculously few stars. That a space with metals. “Right now, Segue 2 has a few thousand galaxy this puny can form at all is a sur- Researchers discovered one such stars, and it took a few hundred million prise to me,” Geha says. chemically pristine galaxy in the Local years to form them,” says Evan Kirby, an And it might not be alone. Astronomers Group using data collected by the Sloan astronomer at the California Institute of suspect that the Local Group could harbor Extension for Galactic Understanding and Technology who studies Segue 2. “So that’s far more galaxies than the roughly 85 that Exploration (SEGUE), a comprehensive an average star formation rate of 0.00001 are currently known. “For every galaxy that survey of the sky conducted between 2004 stars per year, which is pathetic.” we see, there could be a couple hundred and 2008. Astronomers combed through Segue 2 is peculiar in another respect: its that we don’t see,” Kirby says. “To be fair, SEGUE data to search for stars that had mass (or lack thereof). Most galaxies con- most of these unseen ‘galaxies’ may not both similar positions and velocities, hop- tain copious amounts of dark matter, a have any stars — just dark matter — which ing to find new galaxies that were much mysterious substance that interacts gravita- makes it questionable to call them galaxies.” too small and faint to see with current tionally with other matter but does not Searches are underway to find such dark- telescopes. In 2009, scientists in the United emit, reflect, or scatter light. Typical galax- matter-dominated galaxies, and earlier this States and Europe discovered a tiny galaxy ies like the Milky Way have 10 times as year a team of astronomers led by Sukanya in the SEGUE data. The galaxy, which they much dark matter as normal matter. Less Chakrabarti at the Rochester Institute of named Segue 2, orbits the Milky Way but is massive dwarf galaxies have relatively more Technology announced four stars possibly only 1⁄500 its size. dark matter, up to 1,000 times as much as belonging to a Local Group galaxy consist- Since Segue 2 is relatively nearby — normal matter. When Kirby and his team ing mostly of dark matter. 114,000 light-years away, less than the first looked at Segue 2, they expected it to diameter of the Milky Way — astronomers also have lots of dark matter. They studied Toward the Princess are able to study the prevalence of metals in how its stars moved using the Keck II The Milky Way’s gravitational entou- its resolved stars. In astronomy, as in sports Telescope in Hawaii and inferred that the rage — the Magellanic Clouds, Segue 2, and politics, studying individuals as op- ratio of dark matter to normal matter in and other nearby dwarf irregular galax- posed to relying on population averages Segue 2 was at most 300 to 1 — far less ies — isn’t unique in the Local Group. The often leads to new insights. Researchers at than predicted. Andromeda Galaxy, some 2.5 million light- the University of Michigan and the Uni- “The small ratio potentially indicates years away, has several orbiting satellite gal- versity of California, Irvine, used the 6.5- that the Milky Way has stripped off some axies as well. This fact shouldn’t come as a meter Magellan Telescope in Chile to study of Segue 2’s dark matter,” Kirby says. The surprise since the Milky Way and M31 are the metallicity of Segue 2’s brightest star, a observations revealed that the total mass of the largest galaxies in the Local Group. red giant. They found that the star had a Segue 2 — including dark matter — was a “As one of the two most massive mem- surprisingly low metallicity: only 0.1 percent puny 200,000 times that of the Sun. That bers of the Local Group, Andromeda drives

billion light-years 7.5 7.75 48 ASTRONOMY • DECEMBER 2015 much of the evolution of the group,” explains Benjamin Williams, an astrono- mer at the University of Washington who studies stellar populations within nearby galaxies. In fact, M31 shows evidence for even more interactions with its cadre of gravitational groupies than the Milky Way. “It has a more structured stellar halo that is full of streams, indicative of a more violent past,” Williams says. “Its satellite galaxies have much less current star formation, also suggestive of more interactions.” Earlier this year, astronomers at the University of Notre Dame and the Uni- versity of Wisconsin-Madison published the discovery of a massive halo of gas around M31. The halo, which they discov- ered using Hubble, extends halfway to the Milky Way and contains metals such as carbon, oxygen, and silicon, some of which may have been stripped from satellite gal- axies that merged with Andromeda. Demonstrating that large galaxies con- sume smaller galaxies is more than just a morbid pursuit, however. “Observing a dwarf galaxy getting stretched out until it disrupts into the smooth outskirts of the Tiny galaxy Segue 2 is one of the strangest neighbors on our cosmic block. Dwarf galaxies typically Milky Way or Andromeda is to see galaxy have many times more dark matter than normal matter, but this one has relatively little — an indica- growth in action,” Kirby says. Astronomers tion the Milky Way has stripped much of the dark matter away. GARRISON-KIMMEL, BULLOCK (UCI) like Kirby want to understand how galaxies assemble over time into the ones we observe today without having to wait the Andromeda known as M32 — show evi- mergers. Furthermore, its 40 billion stars roughly 5 billion years before the Milky dence for such a monster. It’s no coinci- are arranged in a spiral, a fragile geometry Way and Andromeda collide. dence that these three galaxies are also that would have been destroyed by any sig- “That collision will erase the spiral arms among the most massive systems in the nificant gravitational interactions. of the Milky Way and Andromeda, and we Local Group. “The smallest galaxies with NGC 604, an enormous star-forming will be left with one mostly featureless known central black holes typically have region in M33, is evidence that the galaxy ,” Kirby says. But the dwarf stellar masses of about 1 billion solar contains copious amounts of gas. If the irregular galaxies in the Local Group aren’t masses,” explains Kirby. “Most of the dwarf Pinwheel’s gas reservoirs were somehow destined for such drastic collisions. “Most irregulars in the Local Group top out at 100 stripped away by Andromeda at some point of the dwarf irregulars will continue orbit- million solar masses, so there is not yet any in the past, the gas must have been replen- ing their larger hosts — even after the big evidence for supermassive black holes in ished before NGC 604’s first stars began to collision — and their stellar populations these galaxies.” form several million years ago. will remain unchanged.” Cosmic cannibalism appears to be ubiq- “M33 gives us a nearby subject for learn- When the Milky Way and Andromeda uitous around large galaxies in the Local ing about disk formation and evolution in a do finally collide, the supermassive Group. Even so, some galaxies seem galaxy that experiences little harassment black hole at the center of each to be remarkably immune to it. from satellites,” Williams says. galaxy will orbit around each The spiral galaxy M33, also Far away from the Milky Way and other, releasing copious FAST FACT known as the Pinwheel Andromeda, the Local Group’s remaining amounts of energy. Galaxy, is the third- galaxies exist in relative solitude. They’re Supermassive black holes largest galaxy in the the last torches before reaching other dis- are relatively rare in the 1,000,000 Local Group after the tant galaxy clusters, which themselves are Local Group, however: LIGHT-YEARS Milky Way and M31. part of the larger Virgo Supercluster. Out of the roughly 85 Reach of the Andromeda It orbits Andromeda Astronomers can now study galaxies in our galaxies, only three — Galaxy’s dark matter halo, yet possesses no cen- home supercluster in intricate detail using the Milky Way, which extends halfway tral bulge of stars, advanced telescopes and cameras, but the Andromeda, and a to the Milky Way. which is often a telltale hilltop villages of the Local Group still dwarf elliptical satellite of sign of previous galactic beckon like beacons.

8 WWW.ASTRONOMY.COM 49 THE VIRGO SUPERCLUSTER Our 100,000 closest

he Milky Way sits near the middle near the equator, around which the remain- of an assembly of galaxies called the ing third spread like the globular clusters Local Group. This gathering, 10 mil- galaxies around our galaxy. In total, a mass equiva- lion light-years wide, lives on the edge lent to 10 quintillion Suns fills our cosmic of a collection of galaxy clumps called supercity. Much of that mass, however, isn’t Tthe Local, or Virgo, Supercluster. Small groups luminous. Just as dark matter dominates British astronomers and father-son duo the universe, it also fills the supercluster. William and John Herschel pointed their combine with larger Still, lots of pretty light-emitting objects telescopes all over the sky in the 18th and live next door, cosmically speaking. 19th centuries, taking samples of objects clusters to form they called “nebulae.” In using the word Meet the neighbors nebula, they didn’t always mean what we a vast network of Closest to us, perhaps 11 million light- do today. They merely meant a celestial years away, live the Maffei I and II groups. object that wasn’t a comet but nonetheless interconnected Although nearby, they went unnoticed appeared fuzzy. And they believed all these until 1968 because they lie right behind nebulae lay within the Milky Way. galaxies that the Milky Way’s plane, so the material that In their 1864 catalog, John noted that makes up our home drowns out the light more nebulae populated the area around spans 110 million from Maffei’s constituents. But Italian the constellation Virgo. (French astrono- astronomer Paolo Maffei noticed some- mer Charles Messier also had noted such light-years. thing strange when he looked at IC 1805 structure a century earlier.) The reason, (a true nebula). A nearby object shone in though, remained nebulous. by Sarah Scoles infrared light. He guessed it was a galaxy, In 1920, the “Great Debate” attempted obscured by the Milky Way. In the past to resolve the nature of nebulae. American 20 years, astronomers have found 17 more astronomers Harlow Shapley and Heber associated galaxies. Curtis argued about whether these objects Supergalaxy. Five years later, he changed it A million light-years farther away, the represented nearby cloudiness or whether to the Local Supercluster. Sculptor is easier to pick out. they were galaxies in their own right, ren- As telescopes became more powerful, From 2011 to 2013, a group of astronomers dered fuzzy by distance. The latter inter- astronomers could take larger surveys, cat- led by Tobias Westmeier of the International pretation won out. But why did these aloging more galaxies and their motions. Centre for Radio Astronomy Research in galaxies cluster around Virgo? Was it These pixeled together a bigger picture: a Australia studied how high-velocity clouds chance or something more? concentration of galaxies along a plane in (HVCs) of hydrogen enrich member galax- In the 1950s, French astronomer Gérard space. Just as most of the Milky Way’s stars ies NGC 55 and NGC 300. HVCs don’t de Vaucouleurs observed how these Virgo reside in a thin equatorial disk, so most of rotate with galaxies; instead, they come galaxies traveled through space. Curiously, the galaxies in the Local Supercluster lie from outside and stream in like missiles. they seemed to be moving away from us at along its equator. Science had spoken — Astronomers believe they may provide fuel the same speed and lay fairly close to one stars cluster in galaxies, galaxies cluster to produce more stars as galaxies run out another and to us. They were, in astron- in clusters, and clusters cluster in super- of their own gas. But HVCs have to come omy parlance, “dynamically related.” In clusters. And the center of our supercluster from somewhere. 1953, he christened this crowd the Local shared a spot in the sky with Virgo. At the time of Westmeier’s study, two If you placed 1,000 Milky Ways end to theories dominated, and he wanted to Former Astronomy Associate Editor Sarah end, they would span 100 million light- find which was true. In one, HVCs are Scoles is a freelance writer currently living in years — the size of our supercluster. Two- clumps of gas shot out of a galaxy by super- the San Francisco area. thirds of the bright galaxies live in a disk novae; in the other, they are dwarf galaxies

billion light-years 8.25 8.5 The Southern Pinwheel Galaxy (M83) is the second-brightest member of the Group. At a distance of some 15 million light-years, it lies some 3 million light-years beyond the group’s brightest member, Centaurus A. NASA/ESA/THE HUBBLE HERITAGE TEAM (STSCI/AURA)

8.75 9 WWW.ASTRONOMY.COM 51 A more visually recognizable crowd Maffei II — the — resides at the same distance. Located inside the boundaries of and Camelopardalis, it con- tains 1 trillion solar masses and 34 known galaxies, the most famous of which are

Maffei I M81 and M82. The two tug on each other, which has turned M82 into a star factory. The galaxy’s center glows 100 times brighter than the Milky Way’s as hydrogen gas falls from intergalactic space toward the gravitational hub. Galaxies Maffei I and Maffei II are the centers of the two closest galaxy groups. This infrared The galaxy causing some of this ruckus, image penetrates material in our galaxy that M81, holds the title of “Biggest Galaxy in hides the two in visible light. NASA/JPL-CALTECH/WISE TEAM its Group.” At the center of its perfect spiral arms lurks a supermassive black hole weighing 70 million solar masses. only if the galaxy slammed into a dense intergalactic medium. While astronomers Chaotic concentrations know that large clusters bathe in dense In contrast to the order in the M81 Group, soups of material, they thought smaller the Canes Venatici I Group seems chaotic. groups sat in thin broths. Located 15 million light-years away in the The pressure blows gas away, meaning Canes Venatici and Coma stars can’t form as well — or at all in the Berenices, this group’s members are bound case of small satellites. “As gas constitutes only loosely to one another. Not all of its the fuel for future star formation, its approximately 20 galaxies have settled NGC 300 resides on the near side of the Sculptor removal could potentially shut down star- into stable orbits, and they seem to be just Galaxy Group, whose bulk lies 12 million light- years from Earth. The galaxy appears nearly as formation activity in dwarf galaxies living weakly connected gravitationally. big on the sky as the Full Moon. ESO in group environments,” says Westmeier. Several million light-years farther out This discovery helps solve a mystery and back in the direction of Ursa Major, called the “missing satellites problem,” the has the same loose, dis- dominated by dark matter and gas that which arose about 15 years ago when com- connected air. This group’s namesake never formed stars. “Due to its proximity, puter simulations showed that galaxies like keeps the family together. Most of the the was the most obvious the Milky Way should have more dark other galaxies in the group are merely choice for this study,” he says, “enabling dwarf-sized companions than they do. companions of this Milky Way twin. observations at sufficient resolution and “This discrepancy suggests that either there M101 spans 170,000 light-years, has tightly sensitivity to detect potential HVCs.” is something wrong with the simulations wound spiral arms, and holds 100 billion They found the population they or some mechanism has prevented the solar masses’ worth of material. expected around NGC 55 but none around majority of dark matter halos from turning Less than 20° to the southwest, we NGC 300. This discovery automatically into proper galaxies,” says Westmeier. If find the Canes Venatici II Group. Thirty knocked out the idea of dark satellite gal- pressure strips gas away, as it seems to axies because both objects, being around in the Sculptor Group, small galaxies the same size, should have formed a simi- wouldn’t be able to form stars, meaning the lar set of satellites. The lack of clouds, then, satellites aren’t missing — they’re invisible. suggested that HVCs come from gas flow- ing into or out of the main galaxy, which The unusual suspects happens when bigger galaxies eat smaller The Centaurus A Group lies at a similar ones or when stars form. distance as Sculptor, some 12 million light- The big surprise from the study, how- years from Earth. The group’s dominant ever, was about wonkiness in the big galax- member, Centaurus A, is the closest radio ies’ gaseous disks. They appear galaxy. This massive object swallowed a to wobble as they travel spiral galaxy about 500 million years ago. through space under The digestion is ongoing, though, and the influence of a force it causes radio waves to blast into space. called ram pressure. Centaurus A serves as an example of what Centaurus A dominates its group from a distance This could happen could ensue when the Milky Way and of 12 million light-years. The massive galaxy Andromeda galaxies collide 4 billion shines brightly in both visible light and radio waves, the latter a result if its having consumed Twin Quasar (Q 0957+561) years from now. a companion galaxy 500 million years ago. ESO

9.18 billion light-years CROSS) EINSTEIN’S (TWIN QUASAR, NASA/ESA/HUBBLE

billion light-years 9.25 9.5 52 ASTRONOMY • DECEMBER 2015 Local Supercluster NGC 5005 Group Distance: 60 million ly NGC 4565 Virgo Cluster Group Distance: 55 million ly Distance: Ursa Major South Group 57 million ly Distance: 56 million ly NGC 5746 Group NGC 4274 Distance: 64 million ly Group Distance: M61 Group Ursa Major North Group 53 million ly Distance: 60 million ly Distance: 57 million ly NGC 4179 Group NGC 3665 Distance: 52 million ly Group NGC 5364 Distance: Group NGC 4666 Group 50 million ly Distance: Distance: 51 million ly NGC 5866 Group 50 million ly NGC 5084 Group Distance: 50 million ly NGC 5775 Distance: 60 million ly Canes Group Venatici II Distance: Group 52 million ly NGC 4697 Group NGC 3585 Group Distance: Distance: 40 million ly 30 million ly Distance: Distance: 64 million ly 36 million ly M104 Group Distance: 36 million ly M66 Group Canes NGC 5121 Group Venatici I Distance: Distance: 52 million ly Distance: 33 million 26 million ly Group Distance: ly NGC 4976 M101 Group 15 million ly NGC 2273 Group NGC 3175 Group Group Distance: Distance: 48 million ly Distance: Distance: 58 million ly 18 million ly 0° 47 million ly Centaurus A M81 Group Group NGC 2835 Group Distance: Distance: Distance: 33 million ly 90° 12 million ly 12 million ly Maffei I and II Groups NGC 2997 Distance: Group 11 million ly Distance: Local 37 million ly Group NGC 7331 Group 20 million 270° Distance: light-years 40 million 42 million ly light-years Sculptor Group Distance: 12 million ly 180° NGC 1023 Group Distance: 32 million ly

NGC 1672 Group IC 5181 Distance: 39 million ly IC 5332 Group Group The Virgo Supercluster Distance: Distance: 47 million ly NGC 1808 Group is a vast assemblage of 32 million ly Distance: 42 million ly relatively small groups and M77 Group large clusters extending Distance: 40 million ly more than 50 million light- years from its center in the massive Virgo Cluster. This illustration is centered on Dorado Cluster the Local Group and shows NGC 7582 Distance: 59 million ly all groupings that contain NGC 1097 Group Group Distance: at least three relatively Distance: 61 million ly NGC 1433 Group large galaxies. The size of NGC 1084 46 million ly Distance: 55 million ly each sphere reflects the Group number of galaxies in the Distance: 56 million ly group or cluster. Distances Fornax I Cluster are shown in light-years (ly). NGC 681 Group Distance: 62 million ly ASTRONOMY: ROEN KELLY Distance: 63 million ly NGC 908 Group Distance: 62 million ly NGC 1255 Group Distance: 65 million ly

Einstein’s Cross 9.85 billion light-years

9.75 10 WWW.ASTRONOMY.COM 53 Spiral galaxy M101 is the dominant member of its galaxy group, which lies 18 million light-years from Earth. The 170,000-light-year-wide island Galaxy M82 resides in a large group named for its neighbor, M81. An ongoing interaction between universe appears face-on from our perspective. these two has sparked M82 to produce vast numbers of new stars. NASA/ESA/THE HUBBLE HERITAGE TEAM (STSCI/AURA) NASA/ESA/K. KUNTZ (JHU), ET AL./STSCI million light-years from home, this group’s into stars. For galaxies to form, astrono- “Local Supercluster,” has a center some 55 largest and most famous member is M106. mers think cold primordial gas has to fall million light-years away. As its name sug- Water vapor in this galaxy pulses out into the system, like high-nutrition food to gests, you can find it in the direction of microwave radiation, creating a giant help fuel the growth of early years. But no the constellation Virgo. Compared to the microwave equivalent of a laser called a telescope had seen such old-school atoms meager dozens of galaxies in the previ- . Astronomers used the water around a growing galaxy. The Leo Ring, ous groups and clusters, Virgo has 1,300 orbiting M106’s supermassive these scientists thought, might be just that. — and maybe even 2,000 — constituents. black hole to directly measure its distance. But when they turned their telescope on They add up to 1.2 quadrillion - The galaxy also contains visible it, they found bright optical light, the kind es spread across 7.2 million light-years. Cepheid variable stars, “standard candles” that massive young stars emit. Primordial In between these galaxies roams a whose predictable brightness changes gas, by definition, cannot create such stars. rogue set of intergalactic stars — up to astronomers also can use to tabulate dis- They had a new mystery. 10 percent of the cluster’s total. Globular tance. These stars dim and brighten peri- Using computer simulations, the team clusters, dwarf galaxies ripped from par- odically like seizure-inducing Christmas discovered that the ring represents a scar ents, and at least one star-forming region lights, and the period gives away the star’s left over from a colossal collision: NGC also populate the intergalactic region. Hot intrinsic luminosity. By comparing this to 3384, a central elliptical galaxy, and M96, a X-ray-emitting gas permeates the space the star’s observed brightness, astronomers peripheral spiral, smashed into each other where these orphans live. can calculate how far away it is. Thus, the more than a billion years ago. The gas from The Virgo Cluster is so big that it actu- water megamasers and Cepheid variables one blew away, eventually forming the ring. ally has subclumps: Virgo A, Virgo B, and in M106 help scientists calibrate the cos- Virgo C. Virgo A dominates, encompass- mic distance scale. The biggest and baddest ing 10 times as much mass as the other The Virgo Cluster, the most significant col- two. The three eventually will merge into One ring rules them all lection of galaxies with the home address a single giant cluster. Because these sub- The M96 Group in Leo lies some 36 mil- groups have yet to coalesce, astronomers lion light-years away and has a number of suspect Virgo is young and still figuring big, bright galaxies — 12 with diameters out its identity. above 30,000 light-years. In 2010, it also At the cluster’s center — and the center helped astronomers investigate how gal- of subgroup Virgo A — lies the giant ellip- axies form thanks to a ring of chilly gas tical galaxy M87. Bright galaxies like M87 650,000 light-years wide that encircles the fill the Virgo Cluster. They are, in fact, group. For 30 years, no one understood the concentration that Messier where it came from or what exactly it was. and the Herschels noticed Then a team led by astronomers at Lyon — the first evidence that Observatory decided to tease out the ring’s superclusters exist. nature. The researchers believed that it Fifteen of Messier’s might reveal itself to be “primordial gas,” Microwaves from water vapor in M106 helped pin objects are found in DSS2 (3C9); NASA/ESA/R. GOBAT, ET AL. (CL J1449+0856) gas that has never lived inside another gal- down its 30-million-light-year distance. NASA/ESA/THE this congregation. axy and can’t, in its current form, transform HUBBLE HERITAGE TEAM (STSCI/AURA)/R. GENDLER (FOR HUBBLE HERITAGE TEAM) And you can see 3C 9 (quasar) 10.44 billion light-years

billion light-years 10.25 10.5 54 ASTRONOMY • DECEMBER 2015 many of its members — including M84, M86, M87, and the Sombrero (M104) and Black Eye (M64) galaxies — through mod- est backyard telescopes. Because Virgo has so many galaxies at approximately the same distance from Earth, astronomers use the cluster as a lab- oratory to study galaxy evolution. Just as an alien visitor would learn more about human development by looking at a sta- dium full of different people than at a sin- gle family, so too can astronomers learn more about galaxies from a huge batch like the one in Virgo. The giant Virgo Cluster contains more than a One thing they learned recently deals thousand galaxies. The central region seen here with star formation — more specifically, includes the cluster’s most massive member, stars that are not forming as fast as many M87, below center. TERRY HANCOCK expected. A 2014 study found that turbu- lence — the kind that shakes airplanes — in the center of Virgo shakes up star- that the Ursa Major groups contain just 5 I/AURA) making gas, keeping it hot for billions of percent of Virgo’s mass. C years. That turbulence, which comes from Thirty-two galaxies call the Ursa Major active black holes in galaxy centers shoot- North Group home, including the bright ing out powerful jets, prevents the gas from spirals NGC 3631, NGC 4088, NGC 3953, becoming stable enough to form stars. and M109. The last of these looks most “The energy in such slow motions is similar to the Milky Way. Of all 109 objects more than enough to stop gas from cata- in the Messier catalog, it lies farthest away, strophic cooling and star formation,” says meaning that 18th-century telescopes NASA/ESA/THE TEAM (STS HUBBLE HERITAGE lead researcher Irina Zhuravleva of Stan- couldn’t decipher things beyond this point. At 62 million light-years, the Fornax I Galaxy ford University in Palo Alto, California. Now, however, large scopes on Earth and Group (and NGC 1427A, seen here) is one of the Understanding why stars stop forming at in space give us glimpses of galaxies bil- Local Supercluster’s most distant members. the centers of galaxies and clusters helps lions of light-years more distant. astronomers understand how their lives progress and how the life of our own gal- Moving targets spheroidals tended to hover near a cluster’s axy and group might unfold. But classifying those galaxies into catego- center — either because they began there ries has always been a problem. They orient or migrated there. Fish and bears themselves differently relative to us, and “One of the long-debated ideas is the Even farther out, a 59-million-light-year a face-on spiral looks quite different from question of nature versus nurture in galaxy jaunt, lies the . This large one seen edge-on. We only have a 2-D evolution,” says Scott. Is how a galaxy looks collection comprises 70 galaxies in the sense of galaxies, as if they’re projected on today totally determined by its properties direction of the southern constellation a screen, rather than a true 3-D picture. in the early universe, or do its surround- Dorado the Dolphinfish (a species that The problem is particularly acute for disk- ings play a role too? “This study shows that, appears on menus as “mahi-mahi”). One shaped galaxies, such as spirals, and sphe- for at least some galaxies, the environment of its most prominent members, NGC roidals. You can tell the difference, though, does play a major role.” He adds that many 1483, shows a luminous central bulge and by watching how their stars move. If they questions still exist, which further cluster jumbled spiral arms that host bright star- rotate slowly and disorderly, they’re sphe- studies can answer. forming regions and young star clusters. roidal. If they rotate fast and in an orderly More questions always exist about At approximately the same distance but manner, they are disk galaxies. the cosmos — its stars, galaxies, groups, in the direction of Ursa Major, two groups A team of astronomers led by Nicholas clusters, superclusters, and beyond. And form a defined band. Most of its major Scott of the University of Sydney in Aus- because the universe is organized into galaxies are spirals, and these com- tralia set about to study the stellar motions superclusters, their constituents stay “close” bine with the minor ones to pro- in the Fornax I Cluster, 62 million light- together. Astronomers can swing their duce 30 percent as much light years away. They found that 93 percent of telescopes and find a stadium alive with as the Virgo Cluster. Thirty the cluster’s galaxies were fast rotators (spi- variety. They learn not just what the more percent may not sound rals) and just 7 percent were slow rotators distant universe is like, but also what it impressive, but consider (spheroidals). Combining this study with used to be like, what it will be like, and those of other clusters, including the larger what has happened and will happen even CL J1449+0856 (farthest galaxy cluster) one in Virgo, Scott’s team found that the closer to home. 10.51 billion light-years

10.75 WWW.ASTRONOMY.COM 55 LIMITS OF THE COSMOS The far reaches of space

Using the relative velocities of galaxies, astronomers in September 2014 defined the (the region within the outline) as the one that contains the Milky Way, the Virgo Supercluster, and several others. Laniakea contains more than 100,000 galaxies with a combined mass of 1017 Suns in a region of space 520 million light-years across. R. BRENT TULLY, HELENE COURTOIS, YEHUDA HOFFMAN, AND

billion light-years 11 11.25 Everything astronomers have learned points to a universe populated by superclusters of galaxies, incredibly long filaments, and colossal voids. by Liz Kruesi

efore extending to what lies in the distant cosmos, scientists must under- stand our neighborhood. As we’ve seen, the force of gravity binds the Milky Way and its Local Group neighbors Bto thousands of other galaxies in the Virgo Supercluster. For decades, astronomers believed that this was the largest gravitation- ally bound object in which we live. But a study published in 2014 showed that our home supercluster is just one lobe of a larger grouping called Laniakea, the Hawaiian word for “immense heaven.” And although the Virgo Supercluster spans a worthy 100 million light-years, Laniakea has a diameter of more than 500 million light-years. While distance is an easy value to measure on Earth and even within our solar system, it’s much more difficult for locations where no human-made objects have yet explored. Astronomers can’t run a tape measure between another galaxy and Earth. Instead, they study a celestial object’s light.

11.5 11.75 WWW.ASTRONOMY.COM 57 In 2009, a group of astrophys- icists from five countries ran AB the Millennium-II Simulation, which created a virtual cube of space 400 million light- years on a side. The research- ers then traced the evolution of more than 10 billion “par- ticles,” each made up of 6.9 million solar masses of dark matter. These four images, which show a field of view 50 million light-years on a side, zoom forward in time (from A to D) 11.4 billion years. BOYLAN-KOLCHIN, ET AL. (2009)

Measuring the expanse The other method of measurement analyzes the Scientists have two different methods to compute light from the brightest galaxies in a cluster to the distance to a faraway galaxy. The simplest is to determine how the members it contains are mov- use the brightness of the light they collect from it. ing. This is a harder process, but it relies on a sim- The University of Lyon’s Helene Courtois, one of the ple fact: Light can tell scientists about an object’s researchers who defined the Laniakea Supercluster, movement toward or away from us. The light’s color likens this method to determining how far away a shifts bluer if the galaxy is moving toward Earth 60-watt light bulb is. You already know how lumi- and redder if it’s moving away. That movement can nous the bulb is, so once you measure how bright it arise from the gravitational push and pull of other appears, you can calculate its distance. galaxies or from the expansion of the universe. Because intervening dust blocks visible light We now know that the cosmic fabric is expand- from these galaxies, astronomers look at the ing, and it’s bringing galaxies along for the ride. strength of radio waves from them to estimate how Once scientists compare the movements of many much energy the galaxies release in visible light. objects in the same area of sky, they can break apart They then compare the intensity of light collected the different motions. When mapping galaxies in a to their calculations of actual energy released to supercluster, astronomers focus on the gravitational determine the distances to the galaxies. pull of both visible matter and unseen dark matter. In the Laniakea discovery, Courtois and her col- leagues used both methods to study the distances to and the movements of more than 8,100 relatively nearby galaxies. They found the boundaries where some galaxies were moving toward one region and others were moving in another direction. That tip- ping point, like the top of a hill where a stone can roll toward one valley or another, marks the edge of the Laniakea Supercluster. But what lies farther out? Filaments and voids Astronomers began mapping cosmic structure decades ago by analyzing the light from each glow- This map of 125,071 gal- ing galaxy using the two previously mentioned axies in the techniques. In the late 1970s and early 1980s, a pat- nearby uni- tern began to emerge: galaxies huddling together verse shows with voids in between. how they clus- ter together. By the late 1980s, astronomers also saw that gal- Astronomers pro- axies near ours are moving toward a specific region duced this image as part of the 6 Degree Survey. Astronomy Contributing Editor Liz Kruesi writes about C. FLUKE/6DF SURVEY pieces of the cosmic puzzle from her home in Austin, Texas.

billion light-years 12 12.25 58 ASTRONOMY • DECEMBER 2015 CD

of space in the Southern Hemisphere. The force of “At the end of the simulation, when the virtual gravity encourages this movement, which means universe is mature, we compare it to the observed that this region must contain a huge amount of universe,” says Courtois. The computer model THE mass; scientists called it the Great Attractor. All the reveals more structure than observers detect, a cos- DIFFICULTY galaxies in the Laniakea Supercluster are falling mic web stretching back billions of years into cos- toward this spot. mic history. OF DEFINING More discoveries followed as astronomers con- DISTANCES ducted bigger surveys to cover larger areas on the Brilliant signposts sky. Telescopes and cameras continued to evolve Even the brightest galaxies can’t compete with the The fabric of space-time rapidly, and their larger light-grasp and more sensi- luminosity of actively dining supermassive black has been expanding since tive detectors allowed scientists to see farther out holes. While every large galaxy harbors one such the universe came into existence 13.82 billion and further back in time. behemoth, only a fraction of them are feeding, a years ago. That expansion Researchers looked at a large-scale view reaching classification that astronomers call “active.” A qua- ignores the speed limit halfway across the universe to determine the distri- sar is one type of . that governs moving bution of galaxies. They saw that galaxies and their The black hole feeds on material like stars or gas objects — the speed of home clusters congregate in clumps connected to clouds that pass too close. As gravity funnels that light. Whereas on Earth other clumps by strings of galaxies, which create material toward the high-mass black hole — like (but not just on Earth), nothing can travel faster seemingly empty voids. This pattern forms the water spiraling toward a drain — it forms a disk. than light, the cosmic fab- “cosmic web.” And it appears to repeat, showing The material rubs against itself and glows due to ric can. That means two that no location in the universe is more important friction. This light is visible across vast distances spots in our universe that than any other at the largest scales. — so far, in fact, that the light from the nearest were near each other and But there’s even more material that no telescope quasar has taken 600 million years to reach us. could send signals to can see. The strings of galaxies lie upon a thicker Astronomers have found scattered across each other right after the scaffolding of dark matter, a mysterious material the universe. The most distant one existed just 750 Big Bang have been pulled apart by cosmic that only shows itself through its gravitational million years after the Big Bang. Its light has trav- expansion to much far- interactions with stars and galaxies that we can eled for more than 13 billion years to reach us. ther than 13.82 billion detect. Dark matter oozes between galaxies in clus- Quasar light is a multiuse tool for studying the light-years distant. In fact, ters and holds together the strings, or filaments, distant universe. Scientists can map these objects in they now lie some 95 bil- between superclusters. This universe holds five to the same way they map other galaxies to find struc- lion light-years apart. This six times as much dark matter as normal matter. tural filaments and voids. Quasars also can act as topic commonly causes Because astronomers can’t see dark matter, the flashlights to illuminate the gas that lies around confusion, and it is one reason why when talking bulk of cosmic structure is invisible. To understand their home galaxies, the gas between galaxies, and about distant galaxies, what it looks like, scientists build universes in com- even the gas falling along nearby filaments. astronomers state how plex computer simulations that compress all of cos- University of California, Santa Cruz, astronomer long the light has been mic evolution into mere weeks. They know roughly J. Xavier Prochaska and colleagues have studied traveling through the cos- how much normal matter and how much dark mat- some 20 distant quasars, and they’ve made two big mos instead of a distance ter the universe has now. They load this informa- discoveries. First, they saw a huge clump of hydro- value. — L. K. tion, along with the laws of physics, into the gen surrounding a quasar that was much larger computer model. than the galaxy should be. They reason this gas lies

12.5 12.75 WWW.ASTRONOMY.COM 59 How it all formed

Cosmic microwave background 380,000 years

Clouds First stars of neutral and hydrogen galaxies form OF IONIZATION DARK AGES Logarithmic scale

Three 10,000 100,000 1 million 10 million 100 million minutes years years years years years

BIG BANG

Astronomers think a span of time that lasted hundreds of millions of years called the reionization era began when the first stars and galaxies emitted radiation that turned hydrogen VOIDS AREN’T atoms (one proton, one electron) into hydrogen ions (a proton without an electron). ASTRONOMY: ROEN KELLY EMPTY

The spaces between galax- along a filament that contains the quasar. Its light pieces. The cosmos has been expanding since that ies are not empty. Huge has been traveling for nearly 11 billion years. moment 13.82 billion years ago. reservoirs of hot gas — Second, the researchers recently found four qua- Once it had expanded enough for the tempera- invisible to human eyes but sars close to one another and embedded in an enor- ture throughout the universe to cool to about glowing in X-rays — fill the mous cloud of hydrogen. “It’s a structure that will 4,900° F (3,000 ), each proton grabbed a gaps. And even the large- scale voids in the cosmic evolve into something like [the] Virgo [Cluster] nearby electron to form a neutral hydrogen atom. web structure aren’t today,” says Prochaska. With fewer particles floating around, the pinball vacant. The Cosmic In both discoveries, the light he and his col- game was over. Infrared Background leagues see comes from fluorescence, he says, At that point, light was free to stream about the Experiment (CIBER), with its “where the quasar shines at a range of energies onto cosmos. That light has been traveling along the fab- onboard Wide Field Imager a galaxy and the galaxy actually shines back.” ric of space-time ever since. Today, it bathes the sky (WFI) took four 7-minute Prochaska thinks that with the observing tools set in a cool microwave glow, its wavelength stretched rides on sounding rockets between 2009 and 2013. to come online in the next five to 10 years, astrono- by cosmic expansion. WFI collects near-infrared mers will find hundreds of quasars embedded in This cosmic microwave background (CMB) radiation with two 4.4-inch filaments and forming galaxy clusters. looks nearly the same in every direction. It reveals telescopes. During two of to astronomers what the universe looked like just those rides, WFI found a To the early cosmos 380,000 years after the Big Bang: an almost feature- brighter glow in the uni- The farther astronomers look from Earth, the less less soup of hydrogen and helium. verse than expected. The organization they see. That’s because they’re see- The tiny differences in temperature it contains scientists involved with CIBER think there are far ing the cosmos at an earlier stage, and modern-day reflect tiny differences in density. Eventually, those more stars wandering structures — like spiral galaxies and dense clusters denser areas grew into galaxies and galaxy clusters, through the cosmic voids, of thousands of galaxies — didn’t exist. The uni- while the least dense regions emptied. likely flung from their verse was not born complex. Instead, after the Big “Understanding that transition, from a simple home galaxies by gravita- Bang, it was dense and hot, filled with electrons, universe to something with interesting structure in tional interactions. — L. K. protons, and light bouncing between those atomic it, is a crucial missing piece in astronomy,” says Steve Furlanetto of the GRB 090423 University of California, Los (farthest EGSY8p7 Angeles. gamma-ray (farthest This transformation hap- burst) galaxy) ULAS J1120+0641 (farthest quasar) 13.10 billion 13.14 billion pened in the astronomical Dark ESO/UKIDSS/SDSS I. LABBÉ (ULAS (LEIDEN J1120+0641); UNIVERSITY)/NASA/ESA/JPL-CALTECH (EGSY8P7); ESA/PLANCK COLLABORATION (CMB) 12.96 billion light-years light-years light-years

billion light-years 13 13.25 60 ASTRONOMY • DECEMBER 2015 Present day 13.8 billion years

1 billion 10 billion years years

This image of 3C 348 combines visual data from the Hubble Space Telescope with radio data from the Karl G. Jansky (VLA). Hubble captured the yellowish elliptical galaxy at center. The VLA revealed an active galactic nucleus (AGN) with 1.5-million-light-year-wide jets of high-energy plasma coming from the region of a 2.5-billion-solar-mass black hole. Astronomers use quasars, another type of AGN, to study the distant universe. NASA/ESA/S. BAUM AND C. O’DEA (RIT)/R. PERLEY AND W. COTTON (NRAO/AUI/NSF)/THE HUBBLE HERITAGE TEAM (STSCI/AURA)

Ages when material wasn’t yet dense enough to cosmos, researchers expect there was enough of it form stars, which could light the way. When the to faintly glow as radio waves. This makes for a first stars and the galaxies they congregated in region nearer to us with no 21-centimeter signal formed, the overall mix developed into today’s cos- and a stronger radiance farther away. mos. But the first galaxies, says Furlanetto, were up The search for this radiation — evidence of an to a million times smaller than the Milky Way and epoch astronomers call reionization — has only just lie so far away from us that telescopes cannot see begun, and no instrument has picked up this faint them. Instead, astronomers search for these first glow yet. Radio telescopes with the ability to detect objects by how they affected material around them. this emission started operating recently, and Finding this intermediate range, which lies another will come online in a few years. between the CMB (380,000 years into the universe’s Scientists who are on the hunt to map reioniza- history) and the quasars and galaxies that lived 1 tion point out that the transition between the neu- billion years after the Big Bang, revolves around the tral, bland universe and the ionized, lumpy universe most prevalent element in the cosmos: hydrogen. As is a long process — hundreds of millions of years. stars and galaxies lit up, they spewed high-energy That is a major missing section along the cosmic light. This radiation is powerful enough to knock distance scale. Zaroubi agrees: “It’s an important away the one electron a hydrogen atom contains, step in this scientific narrative of the formation creating a hydrogen ion. of the universe from the beginning until now.” Those light sources continued to emit energy, Astronomers have learned an incredible “and then bit by bit, the first sources carved cavities amount about how our universe looks at the of ionized material,” says Saleem Zaroubi of the largest scales and also what it was like in its Kapteyn Astronomical Institute in the Netherlands. infancy. Unfortunately, they still are missing These cavities grew while more stars lit up, eventu- a major chapter in the cosmic story. Cosmic ally ionizing all of the neutral hydrogen in their Yet the scientists who study reionization, like microwave regions of space. Zaroubi and Furlanetto, are confident that in the background radiation The key to spotting the transition is the predic- next couple decades observations will uncover the 13.82 billion tion, from 1944, that neutral hydrogen can emit distant radio light that tells the history of how our light-years radio energy with a wavelength of 21 centimeters. universe evolved from a neutral bath of hydrogen Ionized hydrogen, however, doesn’t emit this radia- into the complex web of galaxies, stars, and planets Big Bang tion. Because neutral hydrogen filled the early we now call home. 13.82 billion light-years

13.5 13.75 WWW.ASTRONOMY.COM 61 ASKASTR0 Astronomy’s experts from around the globe answer your cosmic questions. GATHERING THE DARK Q: DOES DARK MATTER COLLECT INTO DENSE CONCENTRATIONS LIKE STARS? AND, IF NOT, WHY? David White, Arlington, Massachusetts

A: This is an excellent question Whether dark matter can and one that is at the forefront collapse into these MAssive of cosmology research. The Compact Halo ObjectS (or Astronomers have strong evidence that halos of dark matter surround short answer is that we do not MACHOS, as they have been most galaxies, like the one shown in this simulation. KAVLI INSTITUTE/SIMULATION: yet know. popularly known since the HEIDI WU, OLIVER HAHN, RISA WECHSLER; VISUALIZATION: RALF KAEHLER Dark matter particles, being 1980s) is uncertain. responsive to Newton’s laws of From the theory side, the Q: WOULD THE THIN ATMO- sunset or sunrise. But the gravity, are certainly allowed to clumping depends on the exact SPHERE AND ABSENCE OF brightest lights on Mars are clump just like the ordinary particle physics properties of LIGHT POLLUTION ON MARS Curiosity’s Mars Hand Lens matter that is made of protons, dark matter particles, for exam- MAKE FOR A FANTASTIC Imager’s LED lights and Chem- neutrons, and other familiar ple, the presence of any non- NIGHT SKY, OR WOULD Cam laser. And in the skies, subatomic particles. Dark mat- gravitational interactions MARTIAN DUST/TWILIGHT the moons are much smaller ter could, therefore, collapse between them. In any case, SPOIL THE VIEW? WHY and fainter than ours. into objects of any size. In fact, such clumping is certainly DON’T WE HAVE BROAD But the dust is a problem, there is strong evidence today allowed in principle. NIGHT-SKY PICTURES? even in the darkest skies. Typi- for the existence of dark matter On the observational side, Dustin Cable cally, stars near the zenith lose “halos” — spherical clumps of there exist upper limits on the Menlo Park, California nearly 1 magnitude due to dark matter centered around abundance of MACHOS — made dust; that increases to 3 to 4 every galaxy that extend 10 up of either ordinary or dark A: Martian astronomy has magnitudes at 15° altitude. times larger than the radius matter — in our galaxy and the some advantages. The surface The rover cameras have sen- spanned by the galaxy’s stars. neighboring Andromeda Galaxy pressure is near 8 millibars sitivities similar to the human However, the existence of (M31). Convincing evidence for — less than 1 percent of eye (Opportunity’s Pancam “compact” dark matter objects, the existence of MACHOS made Earth’s surface pressure — can just pick out magnitude 6 such as stars or planets, is more up of dark matter would be meaning that atmospheric or 7 red stars on a clear night, difficult to ascertain. Ordinary incredibly exciting and would refraction is insignificant. One and Curiosity’s Mastcam can matter has an advantage over provide significant new informa- effect of that is the stars would see magnitude 7 blue stars). dark matter in forming stars or tion in our understanding of not twinkle. Another is truly That means only the brightest planets because of its electro- dark matter properties. dark night skies. Twilight is stars are visible at lower alti- magnetic interactions, which Dragan Huterer sometimes very long, such that tudes. The cameras’ low sensi- facilitate collapse into small, University of Michigan astronomical imaging needs to tivity — and the fact that they compact structures. Ann Arbor be at least two hours away from cannot move while exposing

Rigel

Saiph

Orion Nebula

Orion’s Belt Orion (Simulation using Starry Night software) 10-second exposure 30-second exposure 60-second exposure Bellatrix NASA’s Spirit rover turned its panoramic camera skyward above Gusev Crater in 2005 to capture these views of Orion. The famous constellation appears upside down to Earth’s Northern Hemisphere viewers because of the rover’s southerly path on the Red Planet. NASA/JPL/TEXAS A&M/CORNELL/SSI

62 ASTRONOMY • DECEMBER 2015 — means they can see only the 2014 brightest deep-sky objects. The Andromeda Galaxy (M31) is the farthest object to 1995 be (barely) imaged; the Orion Nebula (M42) and the Magel- lanic Clouds are among the few visible deep-sky-objects. A pan-sky image would require about 150 images — over an hour — and one or two Mars days’ worth of the rover’s avail- able bandwidth. So, targeted By comparing a new image imaging always wins. In the of the Eagle Nebula’s “Pillars of future, perhaps a wide-angle or Creation” to the original, astrono- mers are beginning to watch the zoom camera will be able to famous stellar nursery evaporate accomplish a pan-sky image. before their eyes. NASA/ESA/HUBBLE HERI- Mark Lemmon TAGE TEAM (STSCI/AURA)/J. HESTER, P. SCOWEN (ASU) Texas A&M University College Station A: When astronomers talk about retrograde motion, gen- erally they refer to the apparent Q: SINCE THE DISCOVERY motion outer planets make as OF THE “PILLARS OF CRE- Earth passes them while orbit- ATION,” WHAT CHANGES ing the Sun. Most of the time, HAVE OCCURRED WITHIN all the outer planets appear to THEM AND THE SURROUND- move eastward through our ING AREA? Editor Jeff Hester — who actu- The view pierced the dust and sky. During retrograde motion, Bobbie Hibbert ally took the photo — said in revealed the pillars’ tenuous however, each appears to Ashton, Idaho April’s Hubble commemorative true nature as well as infant reverse direction and head issue. The image has graced stars hidden among the gas. westward. Note that this is an A: The Hubble Space Tele- everything from U.S. postage One prominent jet, perhaps apparent motion. The same scope’s iconic image of the stamps to classroom posters. from another forming star, has thing happens when you, in a Eagle Nebula’s “Pillars of And yet the Pillars of Cre- already increased its reach some car, pass another car going in Creation” heralded the instru- ation might not even exist any- 60 billion miles (97 billion kilo- the same direction. While you ment’s rebirth in 1995 and more. NASA captured this meters) farther into space. And are passing that car (and showed the public just how stellar nursery at a fleeting scientists suspect that a nearby because you are traveling incredible astrophotography moment. The region is packed supernova, like the one thought faster), it appears to move in could be above Earth’s atmo- with gas and young stars. And to have brought our solar sys- the opposite direction. sphere. “No one imagined a as those stars ignite, the gases tem radioactive elements, might The two inner planets, Mer- reaction that would turn the evaporate into space, as seen in have already ablated what was cury and Venus, don’t exhibit image into a cultural icon,” the green streaks that surround left of the gas. Spitzer Space retrograde motion for the same Astronomy Contributing the columns. But because the Telescope images show the reason because they move nebula sits some 7,000 light- supernova’s shock wave was faster than Earth. So, our years from Earth, the light we racing at the pillars 6,000 years planet never passes either of Send us your see left the nebula 7,000 years ago. We should know for sure in them. Some astronomers, how- questions ago, as agriculture spread across a couple thousand years. ever, define retrograde motion Send your astronomy Europe. Intense ultraviolet Eric Betz as any westward motion by a questions via email to radiation from young stars has Associate Editor planet. For those that agree [email protected], stripped much of the gas since with that definition, even the or write to Ask Astro, that time. inner planets retrograde as P. O. Box 1612, Waukesha, We’re already starting to see Q: CAN YOU EXPLAIN they move farther from the WI 53187. Be sure to tell us the signs of that destruction. MERCURY’S RETROGRADE Sun (or the horizon) in the your full name and where NASA revisited the Eagle Neb- MOTION? DO OTHER PLAN- eastern morning sky or you live. Unfortunately, we ula last year and this time took ETS APPEAR TO DO THIS approach the Sun (horizon) cannot answer all questions images in the near-infrared ALSO? in the western evening sky. submitted. part of the light spectrum as Robert Schneider Michael E. Bakich well as the visible. Merida, Mexico Senior Editor

WWW.ASTRONOMY.COM 63 Theophilus Cyrillus OBSERVINGBASICS Catharina BY GLENN CHAPLE Fracastorius Piccolomini

The Mutus X Janssen Seek out this interesting feature of light and shadow Hommel a few days after Full Moon. Mutus

n response to my August Schmidt-Cassegrain telescope 2013 column, “Elvis and and imaging equipment. the alphabet,” in which I Mutus is in an area littered made reference to the Lunar by craters like dimples on a golf X, Richard Edmonds of ball. To find the Mutus X, I’d IFlagstaff, Arizona, emailed: “You need to crater-hop much the way didn’t include an image of the I star-hop when seeking a deep- X and I didn’t retain my copy of sky object in a star-rich region. the September 2012 issue where Fortunately, Edmonds had “The Face” Mutus X you featured it and may have included a whole Moon image While searching his images of the Moon for the Lunar X, the photographer came across included an image. I searched that pinpointed its position. two interesting features near the crater Mutus. RICHARD EDMONDS my recent lunar photographs On the evening of Sept- and found this image. Is this the ember 22, 2013, 4½ days after month but that lunar libration Simpelius and Simpelius A (the Lunar X to which you refer?” Full Moon, I tried my luck. [the Moon’s side-to-side and eyes) and Schomberger (the I looked at the image. It Starting with the prominent up-and-down wobble] has a mouth). Between the eyes and wasn’t. The X that I described three-crater chain formed significant effect on the shad- mouth, shadows in a small cra- is the one associated with the by Theophilus, Cyrillus, and owing that can bring out the X ter create nostrils. crater Werner, located near the Catharina, I jumped to the cra- in sharp contrast. Several times, Last July, three days after lunar meridian about a third ter Fracastorius on the southern even with an active imagina- Full Moon, I checked out the of the way up from the lunar shore of the Sea of Nectar and tion, the X was not recognizable Mutus X and then looked for south pole and visible for only a then to Piccolomini farther due to libration.” the face. Even with low mag- few hours around First Quarter south. A southeastward jaunt Now it’s your turn to dis- nification, it was easily visible. Moon. The X that Edmonds from Piccolomini brought me cover the Mutus X. The labeled The smile was wider than found is close to the crater to the huge crater Janssen and guide on this page, taken from that in the image Edmonds Mutus near the lunar south pole then, with a southward turn, Edmond’s all-Moon photo, sent (shown above), stretching and is observable for several to a crater group surround- retraces the path I took from into adjacent Schomberger C consecutive evenings after Full ing Hommel. Continuing the Theophilus to the X. Conduct and creating a wide troll-like Moon. He had stumbled upon southerly drift finally brought your search a few days after grin. The following night, it while “strolling” around a me to the crater Mutus and, just Full Moon when the terminator the mouth was limited to just 19-day-old Moon with a 10-inch below it, the X. (the dividing line between night Schomberger Crater, becom- Without the chart, I doubt I and day on the lunar surface) ing the silly grin Edmonds had would have found the Mutus X. reaches the eastern edges of imaged — from troll to droll in FROM OUR Barely half the size of Jupiter’s the Sea of Tranquillity and the a single evening! INBOX disk, it was just visible through Sea of Nectar. Make the crater- His experience leads my 4.5-inch reflector at 150x hop with low power, and then Edmonds to surmise, “I’m sure Corrections and still appeared small in a switch to high magnification there are other real and imagi- On p. 47 in the August 10-inch reflector at 208x. Its (100x or more). nary objects to be found on issue, the Big Bear Solar image contorted by atmospheric Once you’ve spotted the the Moon.” There are! As the Observatory was incor- turbulence, it was indeed a Mutus X, be sure to look a terminator drifts east to west rectly cited as closing in challenge to capture! little farther south to another across our satellite’s surface, 2017. Officials say they After spending a year study- interesting feature Edmonds dozens of clair-obscur (liter- expect funding to continue ing the Mutus X, Edmonds encountered. “By the same ally, light and shadow) features for the foreseeable future. concludes: “It takes an 18- to strolling method that netted the come and go. For an extensive — Astronomy Editors 19-day-old Moon to really see X,” Edmonds relates, “this guy’s list, log on to www.the-moon. it. … It seems to result in part face popped out at me. This one wikispaces.com/clair-obscur. On p. 58 in the August from a shadow on the western was quite startling considering I Questions, comments, issue, “8 Metis” should be side of Mutus F [a companion was not expecting it. A bit com- or suggestions? Email me at “9 Metis” under the illus- crater to Mutus] and a smaller ical when you get used to it.” [email protected]. Next tration. — Astronomy Editors crater immediately to the north. This miniature “Man in the month: The LVAS Observer’s I would say the X is visible every Moon” is formed by the craters Challenge! Clear skies!

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64 ASTRONOMY • DECEMBER 2015 What’s new at Astronomy.com. REGISTER TODAY! Go to www.Astronomy.com/register WEBTALK BY KARRI FERRON for access to bonus articles, photos, videos, and more.

50 Weirdest Objects OBSERVING TOOLS The Sky this Week Cosmic oddities with Bob Berman This daily digest of celestial events highlights the brightest objects you can Whether relatively close to home or billions of light-years from us, the cos- observe each night. In 10-day increments, learn when and where to spot each mos is filled with weird and wacky wonders. After all, as Contributing Editor planet, the best meteor showers, bright comets and asteroids, notable con- Bob Berman aptly demonstrates each month in Astronomy, we really do live stellations and asterisms, a few deep-sky objects, and more. Each daily entry in a strange universe. And for the past year, maga- offers essential details of the event or object and how to locate it in your sky. zine subscribers have been able to follow Berman’s See what’s happening tonight at www.Astronomy.com/skythisweek. countdown of the 50 strangest and most fascinating oddities he’s come to know over the years. Each week COMMUNITY at www.Astronomy.com/50weirdest, Berman has covered a new object he believes should be inducted into the “Cosmic Hall of Weird,” starting at No. 50 and Reader Photo Gallery counting down. No. 1 will be revealed by New Year’s Browse thousands of beautiful astroimages Eve. What will it be? like this one of the Owl Cluster (NGC 457) by Magazine subscribers can still look back on the Jaspal Chadha. Submit your own images at countdown at www.Astronomy.com/50weirdest, www.Astronomy.com/readergallery. getting to tour everything from the coldest place in space and some planetary wannabes to invisible particles and dark galax- Dave’s Universe blog ies. Be sure to follow Astronomy on Facebook and Twitter for updates on Astronomy Editor David J. Eicher shares amazing astrophotos, highlights when we publish the top five objects (along with lots of other fun stuff). from trips, insights from popular names in the world of amateur astronomy, But remember, the online version of Bob Berman’s 50 Weirdest Objects in videos about his favorite topics or a recent scientific pet peeve, and much the Cosmos is only available to magazine subscribers. Join the final weeks more at www.Astronomy.com/davesuniverse. of the countdown and receive in-depth articles and tips delivered direct to your door as well as other great online features FOLLOW www.twitter.com/ www.facebook.com/ plus.google.com/ by subscribing to Astronomy. Just visit ASTRONOMY AstronomyMag AstronomyMagazine +astronomymagazine www.Astronomy.com/subscribe.

WWW.ASTRONOMY.COM 65 SECRETSKY BY STEPHEN JAMES O’MEARA Out-of-focus observing Sometimes, a slightly fuzzy view can help you.

btaining a sharp matter of milliseconds because focus is critical to of the refractive effects of our many aspects of ever-moving atmosphere. amateur astron- When it does, it displays an To estimate the brightness of an object like Comet PANSTARRS (C/2012 K1), ignore its tails and memorize the size and brightness of the greenish coma. Then, compare the omy. Ironically, array of prismatic colors as it comet’s in-focus image with a similar size out-of-focus star image. When the star’s fuzzy Ohowever, placing the object of scintillates (twinkles) like a image equals the comet’s sharp one, their magnitudes will be the same. GERALD RHEMANN attention purposefully out of diamond under light. focus can help us make better Viewing the same star at stars (those that change in Cluster patterns sense or more accurate obser- high power and slightly out of brightness). To do this, they Do you enjoy drawing deep- vations of what we see. Let’s focus averages out the scintilla- use comparison stars that lie sky objects, especially fanciful look at some examples. tion, leaving us with a good nearby (or better yet, in the open star clusters? One way to representation of the star’s true same field of view). Sometimes, enhance patterns of stars with- Star color color. This method helps espe- it’s hard to judge the brightness in them is to slightly defocus The surfaces of stars radiate at cially in double star observing; of faint variables. the view. Doing so blends the different temperatures, which just be sure not to rack the One way to improve your light of individual points and to our eyes appear as different stars so far out of focus that the accuracy is to keep racking the reveals the most pronounced colors: Blue stars are hot; red disks overlap. variable out of focus until stellar groupings and interven- stars are cool; yellow-white either it or a similarly bright ing voids more distinctly. stars (like our Sun) are in comparison star disappears. between. If you view a star in brightness The first one to vanish from Dark nebulae focus, its color can be difficult Observers often try to estimate view is the fainter of the two; if, Although they can look inky to pin down. It can change in a the brightnesses of variable however, they disappear simul- black and dramatic in images, taneously, you won’t have to dark nebulae can be difficult repeat the process with another to detect visually if they don’t COSMIC WORLD comparison star. appear against a bright region of Milky Way. So, if the nebula A look at the best and the worst that astronomy and Comet brightness winds through star fields inter- space science have to offer. by Eric Betz Cold as Supernova Amateur astronomers use two laced with regions devoid of space hot different acceptable methods bright stars, you might lose the to accurately determine the dark nebula among the web- What’s in a Alien ant farm Space salad Reboot the suit name? brightness of a comet that like confusion. sports a significant coma. The The remedy is to slightly first is the Vsekhsvyatskij- defocus the Milky Way until it Steavenson-Sidgwick method, appears as a smooth back- where you compare a memo- ground glow. Against it, the The International Stephen Hawking NASA astronauts The Smithsonian rized in-focus image of the size and shape of the dark neb- Astronomical throws support to Scott Kelly and raises $700,000 comet to a defocused image of ula will stand out. Union (IAU) the Breakthrough Kjell Lindgren to reboot Neil a comparison star. It also helps to focus your announces new Initiatives’ E.T. eat the first food Armstrong’s suit The second is the O’Meara- attention on the darkness publicly chosen hunt, despite his ever grown on for display after nicknames for belief advanced the International Apollo 11’s 50th Morris method, where you rather than the light. You may dozens of exo- aliens could crush Space Station — anniversary in defocus the stars more than the think it odd, but “telling” your planets. Of course, us like humans lettuce. Up next: 2019. As a comet. This yields a more cor- eyes what to see can help you by IAU rules none stomping on an potatoes, which stretch goal, rect surface brightness for the focus on your target in the out- of the planets are anthill. Out of a every astronaut they rehab a actually planets billion Earths, knows are vital lunar module images of both the comet and of-focus field. since they don’t surely we’re not for surviving and reboot the star. See http://www.icq.eps. As always, please send your “orbit the Sun.” the only jerks. on Mars. space program. harvard.edu/ICQMM.html for thoughts related to my column

ESO/M. KORNMESSER/N. RISINGER (SKYSURVEY.ORG) (WHAT’S IN A NAME?); NRAO/AUI (ALIEN ANT FARM); NASA/GIOIA MASSA (SPACE SALAD); NASA (REBOOT THE SUIT) more details. to [email protected].

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WWW.ASTRONOMY.COM 67 COSMICIMAGING BY ADAM BLOCK Understanding wavelets

Representing images in ways Wavelets get around this other than pixels allows for restriction because these func- powerful processing. This tions have shapes that limit subject, based on signal pro- their oscillations. You can cre- cessing, requires a mathemati- ate a new image in a wavelet cal fluency to understand the domain instead of the strict concepts fully. Many software frequency domain by using Image #2. The author deconstructed this image of the Lagoon Nebula (M8) using wave- programs include tools such these shapes. lets. It contains layers that correspond to four pixels, 32 pixels, and all residual pixels. as fast-Fourier transform fil- The wavelet domain can You’ll find this image online at http://skycenter.arizona.edu/gallery/Nebulae/M8_32in. ters and wavelet-based filters. isolate structures by their sizes. Luckily we don’t need to derive So when you use a “Wavelet the wavelet function you select an image’s small stars, just the math to understand how Transform,” the image decon- from the pull-down menu in decrease that layer’s weight. to use these utilities. However, structs using the same wavelet the form of a kernel filter (the Another popular program some background informa- function at different scales, discrete representation of a that uses wavelets for planetary tion can make parameters a bit rather than the same sine wave wavelet). You choose how many processing is RegiStax. The more understandable. at different frequencies. layers (scale sizes) to decon- layer adjustments are similar We can deconstruct images Using a particular wavelet struct the image into. with the sliders being like the as the sum of periodic varia- function, I’ve broken an image You can set the layers up as “Bias” setting in PixInsight. In tions of brightness and repre- of the Lagoon Nebula (M8) a doubling scheme (a scale of fact, here’s a secret many of the sent the frequencies we get by into small-scale features one pixel for the first, two for top planetary imagers know: sine and cosine functions. This around four pixels in size, the second, etc.). Turning off They modify the wavelet (filter process is a Fourier transform. large-scale features 32 pixels in (removing) the first layer and kernel) and increase the weight In fact, you can represent size, and a residual image that combining the remaining ones of the center of the matrix images by transforming them is everything else (Image #2). will remove features on the (Image #4). Changing the from brightness at any pixel Adding these “layers” back order of one pixel in size. This shape of the wavelet in this way position to a frequency with a together gives me my original example shows how you might (making it more “peaked”) particular amplitude. image. But now I can modify remove noise in your image. better probes small features Image #1 shows an example any layer to enhance or dimin- Changing the “Bias” of a when you adjust the scale size. for a repeating pattern. In this ish the information there. layer gives it more or less In my next column, I’ll show domain, just a few dots charac- PixInsight software has a weight in the reconstruction, you how I used wavelet layers terize the image. If you modify “Wavelet Transform” tool which makes information at a in combination with high one of the dots by, say, erasing (Image #3) with many param- particular scale size more or dynamic range processing to it (making it black) and trans- eters. The “Scaling Function” is less obvious. To de-emphasize handle the M8 image. form back to the original image, you’ll remove the signal that repeats at that frequency. Note, however, that this modi- fication will affect all struc- tures because functions that model the image are infinite and range the whole image.

ALL IMAGES: ADAM BLOCK ADAM IMAGES: ALL Image #3. This screen shot Image #4. This screen shot Image #1. In this repeating pattern, the shows PixInsight’s “Wavelet from RegiStax shows the dots represent the frequency values of Transform” tool. screen that allows you to the variation in the original image. modify wavelets.

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68 ASTRONOMY • DECEMBER 2015 See the Universe in color live with NEXTCOMING ISSUE IN OUR Astronomical Video CCD Cameras TOP 10 MALLINCAM.COM SPACE STORIES *REWARD* $10,000.00 OF 2015 “The Theories of Relativity, Special Astronomers find signs Relativity, String, Steady State and of dark matter close to home, Nebula Hypothesis’ () have unravel the mystery of a famous all been disproven. You’re invited supernova, and take a to disprove “The AP Theory”. So trip to Pluto confident are we that “The AP Theory” most logically describes the forma- tion of water and our solar system and how our atmosphere is being held down without gravity in the most logical way that we’re offering a $10,000.00 REWARD to the first person to disprove it. Must include The life and an example in nature (on Earth) and a times of description of a successful experiment. Stephen Hawking READ: www.aptheory.info COMMENT: [email protected]

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...... 75 and witness and Astronomy magazine Astronomy EXCLUSIVE • Objects: Weirdest 50 • archive: Day ofthe Picture • archive: Astro Ask • this Sky Month:The • Interactive Star Atlas: • archive: review Equipment ASTRONOMY.COM SUBSCRIBE online today Benefi Astronomy.com to the universe’s oddities. celestial images. reader questions. each month’s events. sky objects. deep-sky 800 cameras,binoculars, and more. Not yet asubscriber? ts for Subscribers for ts staff William (landscape) Cho

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WWW.ASTRONOMY.COM 71 READER GALLERY

1. SWEET SPIRAL NGC 4945, also known as Caldwell 83, is an edge-on spiral galaxy in the constellation Centaurus. It shines at magnitude 8.8 and lies some 12 mil- lion light-years away. (16-inch Dream Telescopes astrograph at f/3.75, Apogee Alta U16M CCD camera, LRGB image with exposures of 70, 18, 18, and 18 minutes, respectively) • Kfir Simon

2. CHANGING FACE From July 25 to August 8, 2015, Venus showed rapid changes. The planet’s phase dropped from 12.9 percent (left) to 2.6 percent, and its apparent size increased from 47.6" (left) to 56.2". (14-inch Celestron Schmidt-Cassegrain telescope, ZW Optical ASI174MM CCD 1 camera) • Pete Lawrence

2

3. LOOP DE LOOP The so-called Cygnus Loop is a . It contains many individual sections designated by names and numbers, including those in the Veil Nebula complex. The entire loop has an apparent diameter of 3°. (Quantum Scientific Instruments 683ws-8 CCD camera, 200mm Canon L II f/2.8 lens set at f/4, Hα/OIII/SII image with 4 hours and 20 minutes of expo- sures) • Richard S. Wright Jr.

4. MOST TRANQUIL The compact grouping of the Moon, Venus (the brighter of the two points below the Moon), and Mars on February 21, 2015, was hidden behind clouds, which dumped about 16 inches (40 centimeters) of snow on the area around Dagyenice Reservoir near Bursa, Turkey, but thankfully for the photographer the next evening was clear. (Canon 6D, 24mm f/1.4 lens set at f/3.2, ISO 400, 2-second exposure, 3 4 taken February 22, 2015) • Tunç Tezel

72 ASTRONOMY • DECEMBER 2015 5

5. GLORIOUS GLOB M30 is a magnitude 6.9 globular cluster that lies in Capricornus the Sea Goat. It orbits our galaxy within a halo region some 27,000 light-years away. NGC 6866 The bright star to its left is magnitude 5.2 SAO 190559. (3.6-inch Astro- Tech AT90EDT refractor at f/6.7, SBIG ST-8300M CCD camera, LRGB image with exposures of 120, 40, 40, and 40 minutes, respectively) • Dan Crowson

IC 1311 6. SPACE GAS LBN 243 The region near the star Sadr (Gamma [γ] Cygni) boasts glowing hydrogen clouds, most notably the Butterfly LBN 239 Nebula (LBN 239 and LBN 251) and the Propeller Nebula (DWB 111). (7.2- inch Takahashi E-180 astrograph at f/2.8, QHY CCD QHY11S CCD camera, α IC 1318a H LRGB image with a total exposure time of 4 hours) • Terry Hancock DWB 111

7. CHASING A WATERFALL You’ll find the asterism observers call Kemble’s Cascade in one of the sky’s 6 faintest constellations, Camelopardalis. It lies between Cassiopeia and Polaris (Alpha [α] Ursae Minoris). The two dozen stars in this string stretch 2.5° and range from 5th to 9th magnitude. (5.5-inch Newtonian reflector at f/5, Canon Rebel XSi, ISO 800, 5-minute exposure) • 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 expo- sures. Submit images by email to 7 [email protected].

WWW.ASTRONOMY.COM 73 BREAK THROUGH The Owl and the Snake

One of the sky’s brightest planetary nebulae — the Owl Nebula (M97) in Ursa Major — lurks just below the bowl of the Big Dipper. But the southern sky harbors a fainter ver- sion of this stealthy preda- tor. The Southern Owl Nebula (ESO 378-1) seen here soars among the stars of Hydra the Water Snake. Like all planetaries, the Southern Owl repre- sents the death throes of a Sun-like star. After the star puffs off its outer layers, its white-hot core energizes the ejected gas and causes it to glow. ESO 378-1 spans nearly four light-years and lies about 3,500 light-years from Earth. ESO

74 ASTRONOMY • DECEMBER 2015

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Adorama – 800.223.2500 – www.adorama.com High Point Scientific – 800.266.9590 – www.highpointscientific.com Astronomics – 800.422.7876 – www.astronomics.com OPT Telescopes – 800.483.6287 – www.opttelescopes.com B&H Photo – 800.947.9970 – www.bhphotovideo.com Focus Camera – 800.221.0828 – www.focuscamera.com Optics Planet – 800.504.5897 – www.opticsplanet.com Hands On Optics – 866.726.7371 – www.handsonoptics.com Woodland Hills – 888.427.8766 – www.telescopes.net SOUTHERN MARTIN GEORGE describes the solar system’s changing landscape SKY as it appears in Earth’s southern sky. February 2016: Bright planets on display

As evening twilight fades in brighter than the star, increas- Brilliant Venus arrives next, Of course, things are not early February, no bright plan- ing from magnitude 0.8 to 0.3 cresting above the eastern hori- that way. From our southern ets grace the sky and observers during February. zon a bit more than two hours perspective, Orion stands on will have to be content with Following a year in which before the Sun. Gleaming at his head and his sword points viewing the glittering summer Mars appeared so small that magnitude –3.9, the planet out- upward. People often remark stars. By late evening, however, it showed little if any detail shines every other celestial that Orion has a rather small brilliant Jupiter rises in the through a telescope, it is now object except for the Sun and head for such a large body, but east. Now just a month away starting to become attractive. Moon. Still, the inner world is it holds a lot of interest for from opposition and peak vis- The planet’s disk grows from moving away from Earth and observers. The stars Lambda ibility, the giant planet gleams 6.8" to 8.6" across in February doesn’t look like much through (λ), Phi1 (ϕ1), and Phi2 (ϕ2) at magnitude –2.4 and domi- and should reveal a few subtle a telescope. All you’ll see is a Orionis form the Hunter’s head. nates the overnight hours. surface markings. Also see if 12"-diameter disk that is nearly They make a small triangle Jupiter resides among the back- you can detect Mars’ phase, 90 percent lit. with Lambda at the bottom. ground stars of southeastern which reaches a minimum of Mercury has crept away Magnitude 3.4 Lambda Ori Leo, on the right end of the 90 percent lit this month. from the Sun and now joins is the brightest of the three and upside-down creature where it About 90 minutes after Venus in February’s early has the proper name Meissa, looks like an extension of the Mars rises, Saturn comes into morning sky. Look for the which comes from an Arabic Lion’s rear leg. view. The ringed planet inhab- innermost planet to the lower term meaning “The Shining It’s best to wait until the its the constellation Ophiuchus right of Venus all month. One.” Meissa is a fine climbs higher in the sky the Serpent-bearer, the little- Mercury reaches greatest elon- star whose components shine at around midnight or later if you known 13th member of the gation February 7, when it lies magnitudes 3.6 and 5.5 and are want to observe it through a zodiac. (A nearly 20°-long slice 26° west of the Sun. It then separated by a reasonably close telescope. The gas giant world is of the ecliptic — the Sun’s stands 11° high in the east- 4.4". Moderately high magnifi- well worth a look because of the apparent path across the sky southeast an hour before sun- cations split them quite easily. intricate detail in its cloud tops. that the planets follow closely rise. Coincidentally, the waning Meissa is also the brightest Even a small instrument reveals — runs through Ophiuchus.) crescent Moon appears just to member of an open star cluster an alternating series of bright At magnitude 0.5, Saturn the lower left of Mercury that known as Collinder 69, from zones and darker belts that run shines significantly brighter same morning. Through a tele- Swedish astronomer Per parallel to the equator. The than anything else in this scope, the planet’s disk then Collinder’s catalog of 471 planet spans 44" at midmonth, constellation. spans 7" and appears slightly open clusters published in almost imperceptibly smaller The planet’s southerly dec- more than half-lit. 1931. It covers about 1° of sky than its opposition diameter. lination makes it an attractive and shows up best through Also watch for night-to-night target for telescope owners, The starry sky binoculars or a telescope with changes in the pattern of its particularly once it climbs high As darkness falls on February a wide field of view. four bright Galilean moons. in the east before morning evenings, Orion the Hunter There is much more to this Nearly three hours after twilight begins. Saturn’s disk stands tall in our northern part of the sky, however. A Jupiter rises, ruddy Mars pokes measures 16" across in mid- sky. In fact, this is one of the huge region of faint nebulosity above the horizon. The Red February while the rings span few constellations that appears that shows up only on photo- Planet travels eastward through 37" and tilt 26° to our line of prominent from both north graphs surrounds Lambda. The the dim constellation Libra the sight. In moments of good see- and south of the equator. If so-called Lambda Orionis Scales this month, moving ing, the dark Cassini Division you ignore the Hunter’s head, Nebula is a massive cloud of toward the head of Scorpius separating the outer A ring sword, and shield, you can star formation that spans about the Scorpion. Mars’ strong from the brighter B ring should envision him as an upright 10°. American astronomer color makes it easy to identify, appear obvious even through figure even from the Southern Stewart Sharpless included it as but take care not to confuse it small scopes. Also look for 8th- Hemisphere. Picture brilliant number 264 in his second cata- with 1st-magnitude Antares, magnitude Titan, Saturn’s big- blue-white Rigel and Saiph log of emission nebulae. If we Scorpius’ brightest star, which gest and brightest moon. as his shoulders or arms with could see this glowing cloud rises somewhat later. The The parade of planets con- Bellatrix and ruddy Betelgeuse with naked eyes, Orion’s head planet shines noticeably tinues as the night wears on. as his legs. would look a lot bigger. STAR S

DOME AUSTRALE

THE ALL-SKY MAP TRIANGULUM

SHOWS HOW THE

SKY LOOKS AT:

P.M. α

10 February 1

9 P.M. February 15 TUCANA

OCTANS

8 P.M. February 29 APUS AURUS

NT

β Planets are shown 104 NGC

at midmonth

SCP

NGC 4755 NGC

SMC PHOENIX

α

β HYDRUS CHAMAELEON

γ

Achernar δ

MENSA CRUX

CARINA

α

HOROLOGIUM NGC 3372 NGC

NGC 2070 NGC

LMC

SCULPTOR

NGC 253 NGC

VOLANS

DORADO

SGP

PICTOR

NGC 2561 NGC

β

FORNAX

α

VELA

Canopus

γ

COLUMBA

CAELUM

CETUS

ζ

ANTLIA

PUPPIS

ERIDANUS

LEPUS ε

W 2477 NGC

M41 PISCES Mira β CANIS MAJOR ο M47 Rigel Sirius α α α β Alphard M42 MONOCEROS

α

Pa th of th e S α un Procyon (e α cl Betelgeuse β ipt ic) ORION TAURUS CANIS MINOR

` α ARIES γ Aldebaran α ζ M44 M1 CANCER Pleiades M35 GEMINI β β Pollux α MAGNITUDES M36 M37 Algol Castor β Sirius Open cluster PERSEUS M38 0.0 Globular cluster AURIGA 1.0 Diffuse nebula 2.0 α β 3.0 Planetary nebula Capella 4.0 5.0 Galaxy

N HOW TO USE THIS MAP: This map portrays the sky as seen near 30° south latitude. FEBRUARY 2016 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

NT direction label matches the 1 Last Quarter Moon occurs at 15 First Quarter Moon occurs at CE direction you’re facing. 3h28m UT 7h46m UT The stars above the map’s horizon now The Moon passes 3° north of Asteroid Astraea is at opposition, match what’s Mars, 9h UT 12h UT

NGC 5139 NGC in the sky.

3 The Moon passes 3° north of 16 The Moon passes 0.3° north of NGC 5128 NGC Saturn, 19h UT Aldebaran, 8h UT

6 The Moon passes 4° north of 22 Full Moon occurs at 18h20m UT Venus, 8h UT 24 The Moon passes 1.7° south of The Moon passes 4° north of Jupiter, 4h UT Mercury, 17h UT 27 The Moon is at apogee 7 Mercury is at greatest western (405,383 kilometers from Earth), elongation (26°), 1h UT 3h28m UT

CORVUS 8 New Moon occurs at 14h39m UT 28 Neptune is in conjunction with the Sun, 16h UT

10 The Moon passes 2° north of M104 Neptune, 0h UT 29 The Moon passes 4° north of Mars, 18h UT 11 The Moon is at perigee (364,360

kilometers from Earth), 2h41m UT HYDRA

CRATER 12 The Moon passes 1.7° south of E Uranus, 14h UT

SEXTANS

Jupiter

M65 Regulus M66

α

γ

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

Illustrations by Astronomy: Roen Kelly

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