S&T Test Report: Orion’s New Go To Scope                       p. 52                                  THE ESSENTIAL MAGAZI...
See All You Can See                                                                                                       ...
The Paramount MX goes             wherever astronomy takes you.                           ( $8,500 )            Introducin...
May 2011                                                                VOL. 121, NO. 5                             On the...
Robert NaeyeSpectrum                                                                                                      ...
ONLY MEADE INSTRUMENTS PUTS AN©2011 Meade Instruments Corp. All rights reserved. Specifications subject to change without n...
LettersAstro Outreach in the Canaries                 round-table discussion on our future inWe would like to invite Sky &...
RINGSFor your personal closeencounter with Saturn,take advantage ofCelestron’s Spring SCTSales Event! 10% to 15%off select...
Letters                                                        Alien Misinformation                                  of li...
See us at the:20th AnnualNEAF   NORTHEAST   ASTRONOMY     FORUMSuffern, NY / April 16-17
News NotesKepler’s Latest Planet Bonanza                                                                                  ...
News Notesgravitational wobbles that they induce in                                                                       ...
News Noteswarm (by a few degrees). Leigh Fletcher                             Deep Impact                            Stard...
The Joy of Mathematics                                                                             Taught by Professor Art...
News Notes      2011’s Solar System Bonanza                                  Recovery and Interior Laboratory) will     St...
Check our web site for a full line of:                                                                                    ...
Nature’s Ultimate Whirlpools      Spinning Hearts       of Darkness              Astronomers are measuring the            ...
Illustration by Mark Garlickhigh-energy radiation, orbiting telescopes perform the           GAPING MAW Like almost all ob...
Nature’s Ultimate Whirlpools                                           1.5                                                ...
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  1. 1. S&T Test Report: Orion’s New Go To Scope p. 52 THE ESSENTIAL MAGAZINE OF ASTRONOMY Cosmic Enlightenment: The First Stars & Quasars p. 26MAY 2011 Spinning Hearts of Darkness Astronomers probe whirling black holes. p. 204 Planets Dance at Dawn p. 48Explore the Virgo Cluster p. 66See Superthin Galaxies p. 62 Visit SkyandTelescope.com
  2. 2. See All You Can See TMIn this multifaceted hobby, we share a universal desire to see all we can see. Tele Vue telescopes, eyepieces, and accessories fulfillthat desire. For example, our eyepiece designs provide extraordinary sharpness even to 110° of Apparent Field of View. They aremanufactured with custom coating designs so efficient even our most complex eyepieces are regarded among the best “planetary”performers available. For Newtonians/Dobsonians as fast as f/3, Paracorr flattens the field and sharpens off-axis stars by neutral-izing coma, while Dioptrx lets observers neutralize their eyes’ astigmatism—revealing the inherent performance in their Tele Vueeyepieces. In Chester, NY we design and craft photo/visual APO refractors to deliver startling rich-field vistas yet also resolve thefinest planetary details limited only by aperture. At Tele Vue we are dedicated to the sheer performance of the equipment we createfor you. We are here to speak with you regarding applications. We are here to help you see all you can see. 11 3.7mm Ethos-SX 110° AFoV!APO refractors 60-127mm Dioptrx Naglers—82° AFoV Paracorr Type 2 for f/3 Powermate Celebrate 20-Years of the Northeast Astronomy Forum with 20% Off Tele Vue Eyepieces, Barlows, Powermates & Paracorr We consider NEAF our “home” show. Not because it’s local, or we’re Rockland Astronomy Club members, but because the idea was first discussed between Allan Green (NEAF founder) and Al Nagler at Al’s home. NEAF has grown into an international happening, bringing together “astronuts,” astronauts, and scientists for learning, discovery, and camaraderie. We all owe a hearty “thank you” to theStarbeam supremely dedicated RAC volunteers. To celebrate 20 years, save 20% off every Tele Vue eyepiece, Radian—60° AFoV Barlow, Powermate and Paracorr from now until close of NEAF, April 17th, 2011. And, for Tele Vue eyepieces purchased at NEAF, a donation will be made to the International Dark Sky Association! Image Courtesy R.A.C. Ethos 100° AFoV 21,Nagler Zooms 17, 13, 10, 8, & 6mmCCD Imaging System Filters Dielectric Diagonals Plössl—50° AFoV Alt-Az Mounts Sol Searcher See us at the: 20th Annual NEAF NORTHEAST ASTRONOMY FORUMCamera Adapters Panoptic—68° AFoV Air-Chair Bino Vue Barlows Suffern, NY / April 16-17 T V i s i o Vue ele n a r y ® Chester, York 32 Elkay Drive, Chester, New York 10918 www. eleV 845.469.4551 www. TeleVue.com
  3. 3. The Paramount MX goes wherever astronomy takes you. ( $8,500 ) Introducing the Paramount MX – Space to go. MXPARAMOUNT The Robotic Telescope Mount Engineered To Go Where You Go. FIND YOUR SPACE BISQUE.COM
  4. 4. May 2011 VOL. 121, NO. 5 On the cover: THI S M O N TH ’ S S K Y AL S O IN THI S I S S U E This artwork portrays a black 40 Northern Hemisphere’s Sky 6 Spectrum hole that must By Fred Schaaf By Robert Naeye be spinning fast, because mate- 43 May’s Sky at a Glance 8 Letters rial in the disk is orbiting very close. 45 Binocular Highlight 10 50 & 25 Years Ago MARK GARLICK By Gary Seronik By Leif J. Robinson FE ATURE S 46 Planetary Almanac 12 News Notes 20 Spinning Hearts 48 Sun, Moon, and Planets 38 New Product ShowcaseCOVER o Darkness of By Fred SchaafSTORY A Astronomers are measuring 62 Going Deep th rotation rates of black holes the 50 Exploring the Solar System By Steve Gottlieb to determine their formation By Sean Walker and history. 64 Telescope Workshop By Laura Brenneman 56 Celestial Calendar By Gary Seronik By Alan MacRobert 76 Gallery 26 End of the 58 Deep-Sky Wonders Cosmic Dark Ages By Sue French 86 Focal Point A novel radio array may be first By Al Schlafli instrument to detect the elusive S &T TE S T R E P O R T signal from the Epoch of Reionization. 52 Orion’s Robo Dobsonian By Govert Schilling Good things happen when Go To pointing meets a 32 Building My mass-market Dobsonian. Find us on Dream Observatory By Gary Seronik Facebook & Twitter A veteran telescope maker spent four years building this XX novel 32-inch reflector and home observatory. 26 By Mario Motta 66 Galaxy-Hopping the Virgo Cluster Explore the wonderland of galaxies on the border between Virgo and Coma Berenices. By Ted Forte B. PREMKUMAR / NCRA 72 Residual Bulk Image: Cause and Cure Understanding the subtleties of your CCD camera will help SKY & TELESCOPE (ISSN 0037-6604) is published monthly by Sky & Telescope Media, LLC, 90 Sherman St., Cambridge, MA 02140- you create better images. 3264, USA. Phone: 800-253-0245 (customer service/subscriptions), 888-253-0230 (product orders), 617-864-7360 (all other calls). Fax: 617-864-6117. Website: SkyandTelescope.com. © 2011 Sky & Telescope Media, LLC. All rights reserved. Periodicals postage paid By Richard Crisp at Boston, Massachusetts, and at additional mailing offices. Canada Post Publications Mail sales agreement #40029823. Canadian return address: 2744 Edna St., Windsor, ON, Canada N8Y 1V2. Canadian GST Reg. #R128921855. POSTMASTER: Send address changes to Sky & Telescope, PO Box 171, Winterset, IA 50273. Printed in the USA. 4 May 2011 sky & telescope
  5. 5. Robert NaeyeSpectrum Founded in 1941 by Charles A. Federer, Jr. and Helen Spence Federer The Essential Magazine of Astronomy EDITORIAL The Exoplanet Editor in Chief Robert Naeye Senior Editors Dennis di Cicco, Alan M. MacRobert Associate Editor Tony Flanders Imaging Editor Sean Walker Editorial Assistant Katherine L. Curtis Explosion Editors Emeritus Richard T. Fienberg, Leif J. Robinson Senior Contributing Editors J. Kelly Beatty, Roger W. Sinnott Contributing Editors Greg Bryant, Paul Deans, Thomas A. Dobbins, David W. Dunham, Alan Dyer, Sue French, Paul J. Heafner, Ken Hewitt-White, Johnny Horne, E. C. Krupp, Emily Lakdawalla, Jonathan McDowell, Donald W. I have covered developments in the field of extrasolar planets since Olson, Fred Schaaf, Govert Schilling, Ivan Semeniuk, Gary Seronik, William Sheehan, Mike Simmons, Charles A. Wood, Robert Zimmerman 1995, when the first exoplanet orbiting a Sun-like star was announced around Contributing Photographers P. K. Chen, Akira Fujii, Robert Gendler, 51 Pegasi. With the pace of discovery advancing at what seems like an expo- Babak Tafreshi nential rate, I feel a bit jaded whenever a new exoplanet result is announced. It ART & DESIGN Design Director Patricia Gillis-Coppola takes a lot to impress me. Illustration Director Gregg Dinderman But suffice it to say, my jaw practically dropped to the floor when I heard Illustrators Casey Reed, Leah Tiscione the latest Kepler results, which we cover in our News Note that begins on page PUBLISHING 12. Not only have Kepler scientists identified a bizarre new system with six VP / Publishing Director Joel Toner Advertising Sales Director Peter D. Hardy, Jr. transiting planets, they have announced more than 1,200 planet candidates, Advertising Services Manager Lester J. Stockman 54 of which orbit within the habitable zones of their host stars, with 5 of those VP, Production & Technology Derek W. Corson Production Manager Michael J. Rueckwald being roughly the diameter of Earth. Most of these candidates are real planets. Production Coordinator Kristin N. Burke The pace of exoplanet research is dizzying. I IT Manager Denise Donnarumma remember the time in the 1990s when the discovery VP / eCommerce Dennis O’Brien VP / Circulation Nicole McGuire of every new hot Jupiter would generate headline Consumer Marketing Nekeya Dancy, Hannah di Cicco, MaKenzie Dykstra, news around the world. Then came planets on more Bryan Griffith, Joseph Izzo, Jodi Lee, Adriana Maldonado, T.J. Montilli distant orbits, planets on eccentric orbits, transiting NEW TRACK MEDIA LLC Chief Executive Officer Stephen J. Kent planets, puffed-up planets, multi-planet systems, Executive Vice President / CFO Mark F. Arnett resonant planets, detections of exoplanet atmospheric Corporate Controller Jordan Bohrer Office Administrator Laura Riggs gases, direct images of exoplanets, mini-Neptunes, Editorial Correspondence: Sky & Telescope, 90 Sherman St., Cambridge, MA super-Earths, rocky exoplanets, etc. etc. 02140-3264, USA. Phone: 617-864-7360. Fax: 617-864-6117. E-mail: editors@ I marvel at how astronomers have employed clever SkyandTelescope.com. Website: SkyandTelescope.com. Unsolicited proposals, manuscripts, photographs, and electronic images are welcome, but a stamped, NASA / JPL-CALTECH / ROBERT HURT (SSC-CALTECH) observational techniques and modeling to not only self-addressed envelope must be provided to guarantee their return; see our detect these distant worlds, but to actually learn something about their atmo- guidelines for contributors at SkyandTelescope.com. spheres, internal structures, and physical characteristics. When I attend sci- Advertising Information: Peter D. Hardy, Jr., 617-864-7360, ext. 2133. Fax: 617-864-6117. E-mail: peterh@SkyandTelescope.com ence conferences, it’s immediately obvious that exoplanet research is attract- Web: SkyandTelescope.com/advertising ing many of the brightest young minds, not only because of the field’s obvious Customer Service: Magazine customer service and change-of-address notices: “sex” appeal, but also because budding researchers see it as a dynamic field custserv@SkyandTelescope.com Phone toll free U.S. and Canada: 800-253-0245. that is making incredibly rapid progress. If I were an astronomy graduate stu- Outside the U.S. and Canada: 515-462-9286. dent right now, I’d go into this field. And as I have reported in previous issues, Product customer service: skyprodservice@SkyandTelescope.com Phone toll free: 888-253-0230. many amateurs have volunteered to help as well, and have made important Subscription Rates: U.S. and possessions: $42.95 per year (12 issues); contributions (for example, see the December 2009 issue, page 22). Canada: $49.95 (including GST); all other countries: $61.95, by expedited What excites me the most is not the latest discoveries, but what Sara Seager delivery. All prices are in U.S. dollars. wrote at the end of her October 2010 issue cover story: “the best in exoplanet Newsstand and Retail Distribution: Curtis Circulation Co., 730 River Rd., New Milford, NJ 07646-3048, USA. Phone: 201-634-7400. research is yet to come.” There’s no doubt that Kepler’s most exciting results No part of this publication may be reproduced by any mechanical, photographic, or electronic lie in the future. And who knows, after the exoplanet count has risen into the process, nor may it be stored in a retrieval system, transmitted, or otherwise copied (with the exception of one-time, noncommercial, personal use) without written permission from the thousands, and astronomers have characterized planets and systems to the publisher. For permission to make multiple photocopies of the same page or pages, contact the Copyright Clearance Center, 222 Rosewood Dr., Danvers, MA 01923, USA. point where they have a decent understanding of the full diversity of what’s Phone: 978-750-8400. Fax: 978-750-4470 Web: www.copyright.com. Specify ISSN 0037-6604. out there, we’ll be able to put our solar system in its proper context, and know The following are registered trademarks of Sky & Telescope Media, LLC: whether or not it’s a freak. Unlike the quest to understand dark energy, I Sky & Telescope and logo, Sky and Telescope, The Essential Magazine of Astronomy, Skyline, Sky Publications, SkyandTelescope.com, http://www.skypub.com/, SkyWatch, Scanning the expect this to happen in my lifetime. Skies, Night Sky, and ESSCO. Editor in Chief 6 May 2011 sky & telescope
  6. 6. ONLY MEADE INSTRUMENTS PUTS AN©2011 Meade Instruments Corp. All rights reserved. Specifications subject to change without notice. 30-11006 ASTRONOMER INSIDE YOUR TELESCOPE LOOK & LISTEN AS ASTRONOMER INSIDE™ GUIDES YOU THROUGH THE UNIVERSE. Originally developed for the ground breaking series of Meade LS™ LightSwitch® telescopes, Astronomer Inside is built into the new AudioStar™ controller (with speaker) included with Meade’s LX90™, LT™ and LXD75™ lines of telescopes. Now the intelligence integrated into these Meade telescopes delivers all the astronomical knowledge and experience of a professional astronomer. The incredible Astronomer Inside does all this: Knows the sky so you don’t have to Controls the telescope to find and track any of over 30,000 celestial objects automatically Takes you on a guided tour Shows you the best objects in your sky from your location and time and then plays audio descriptions of those objects so you learn the details of what you are seeing Has over 4 hours of audio descriptions that bring astronomy to life Includes planets, stars, constellations, clusters, nebulae, galaxies and more It’s like having a personal astronomer standing next to you. To find out more about Astronomer Inside and the entire line of Meade Instruments products, or to buy online, visit meade.com. LS LX90 LT LXD75 6" & 8" 8", 10" & 12" 6" & 8" All models with LightSwitch Technology with AudioStar controller with AudioStar controller with AudioStar controller Starting at $1,299 Starting at $1,799 Starting at $799 Starting at $599 You can find the complete line of Meade products at the authorized dealers below: Telescopes.com OPT Telescopes Woodland Hills B & H Photo 800.303.5873 800.483.6287 888.427.8766 800.482.8143 Optics Planet Scope City Canada • Khan Scopes 800.504.5897 800.235.3344 800.580.7160
  7. 7. LettersAstro Outreach in the Canaries round-table discussion on our future inWe would like to invite Sky & Telescope space, and a webcast live from the dome of Write to Letters to the Editor, Sky & Telescope,readers to join us for the Starmus Festival, the GTC. Full information is available at 90 Sherman St., Cambridge, MA 02140-3264,a major science outreach event celebrat- www.starmus.com. or send e-mail to letters@SkyandTelescope.com.ing astronomy and the space sciences to Garik Israelian Please limit your comments to 250 words.take place on the Canary island of Tenerife Astrophysical InstituteJune 20–25, 2011. of the Canary Islands Starmus will bring astronauts, cosmo- La Laguna, Spainnauts, astronomers, biologists, astropho- Astronomy,” page 6).tographers, musicians, and artists together Astronomer Heroes Remembered In the early fifties, when I was roughlyin presenting to the public, in a vibrant I greatly enjoyed Albert Boudreau’s Focal 14, I used to bum a ride from La Cañada,and informative way, the latest advances Point column “How Good We Have It” in California, to Mount Wilson with thein our study of the cosmos. Participating the March issue (page 86). The column KTTV crews who manned the pre-satel-will be Nobel laureates George E. Smith, was particularly poignant for me, as I lite-era TV towers on the mountain. IJack Szostak, and George Smoot, along remember the May 1958 issue of Sky & would spend the weekend enjoying thewith Jill Tarter of the SETI Institute, evo- Telescope very well. run of the Mount Wilson Observatorylutionary biologist Richard Dawkins, and Today, readers can enjoy all the back under the watchful eye of my friend Joemany other leading scientists. Pioneer- issues by obtaining the Complete Sky & Hickock, who did the routine work oning cosmonauts and astronauts Valentina Telescope: Seven Decade Collection on DVDs. the solar towers and who maintained theTereshkova, Alexei Leonov, Buzz Aldrin, When I read the May 1958 issue as a mere “monastery,” where the astronomers slept.Jim Lovell, and Charlie Duke will celebrate youth of 12, I was simply awed by the When not having exclusive use of thehumanity’s first half-century in space. incredible astrophotography performed by observatory’s 6-inch Alvin Clark refractor,From the musical world will be Queen Dr. Custer with his 12½-inch telescope. I was often invited to share the night onguitarist and astrophysicist Brian May, That issue’s center spread was a photo- the 60-inch or the 100-inch Hooker withTangerine Dream, and Graeme Revell. montage of M31, “The Great Nebula in the astronomers. I remember at least one There will also be a space art and 3-D Andromeda” as it was then called. The night sharing the 100-inch Newtonianastronomy exhibition, and an astropho- montage reached the incredible magnitude platform — 80 feet off the observatorytography exhibition and competition. The of 17.5, far beyond what was expected for floor with no rails, thank you very muchwinner receives 60 minutes of observing amateur astrophotography in those days. — with Dr. Sandage, who at that time maytime on the 10.4-meter Gran Telescopio In 1986 I was fortunate to meet have been a graduate student.Canarias (GTC) in La Palma, the world’s Dr. Custer on Sky & Telescope’s tour to Sandage and his assistant were dressedlargest optical telescope. Australia to view Comet Halley. He was in WW2 Air Force heated flight suits. The Festival will be topped by a star a spry octogenarian still working on his I was not, and I was freezing. A busy, party on the peaks of Tenerife, a astrophotography skills, experimenting humorous repartée went on through the with hyper-sensitization of Kodak 2415 night, mostly regarding the ghosts some- film. He still had original prints of his times seen passing ominously through the On the Web splendid mosaic, one of which he sent me. building in the wee hours. After midnight I proudly display it framed in a position of lunch, replete with raccoons, foxes, and Tips f Beginners: for honor in my astronomy den. other critters begging at the door for SkyandTelescope.com/gettingstarted Times have changed. While our skies treats, we returned to the dome and the Almanac for Your Location: are not as dark, our equipment is far remaining night’s work. SkyandTelescope.com/almanac better. What remains unchanged is that Towards dawn, Sandage turned the tele- amateur astronomy is fun and exciting, scope to Jupiter, and what a sight! It seemed S&T AstroAlerts & Weekly Newsletter: SkyandTelescope.com/newsletters and Dr. Custer is still one of my heroes. to fill the dome with its magnificence. It Tim Hunter truly was the most memorable astronomi- Outdoors at Night Podcasts: Tucson, Arizona cal event of my life and, whenever I turn SkyandTelescope.com/podcasts my own puny instrument on that giant, I Thank you for your comments in the fondly remember that night and the kind- Find us on March 2011 issue on the passing of several ness and humor of Allan Sandage. Facebook & Twitter illustrious astronomers, particularly Allan Bill Deák Sandage (“A Quadruple Whammy for Prescott Valley, Arizona8 May 2011 sky & telescope
  8. 8. RINGSFor your personal closeencounter with Saturn,take advantage ofCelestron’s Spring SCTSales Event! 10% to 15%off select Schmidt-Cassegrain telescopesfrom April 1 – 30, 2011! spring OFVisit www.celestron.comor participating dealersfor more information. Your Adventure Starts Here www.celestron.com
  9. 9. Letters Alien Misinformation of life on Earth and play it again, nothing After reading the letters in the January approaching our level of consciousness issue responding to your recent SETI and intelligence would occur again.” article, I was stunned by the lack of under- No. For Gould’s actual opinion, turn standing of this endeavor by the writers to page 290 of Gould’s book Wonderful (“Finding E.T.,” page 8). Life: “And so, ultimately, the question of Two letters argued that “contact” with questions boils down to the placement of aliens is dangerous, because they may the boundary between predictability under come and destroy us or make us “a footnote invariant law and the multifarious possibil- in [their] history book.” SETI is one-way ities of historical contingency. . . . Whether contact, not communication. It only listens, the evolutionary origin of self-conscious Exclusive Lunt Stocking Dealer it does not transmit. That’s done by our intelligence in any form lies above or below 60mm Ha Telescope with Doubles-Stacked External Etalon System and B600 with Pressure Tuner $2,242 TV and radio transmissions and, more the boundary, I simply do not know.” powerfully, by NORAD and our air-traffic- Dr. Gould recognized the folly of gener- control system. It’s too late to worry about alizing from a statistical sample of one. E.T. detecting our presence. We have been Steven Morris transmitting 24/7 for at least 60 years! Professor of Physics LX200 ACF GPS Peter Burkey Los Angeles Harbor College In Stock! Holland, Michigan Los Angeles, California Historical Correction For the Record Chester Hollaway’s letter (January issue, ✹ The double star Struve 742 lies ½° east of page 8) misstates Stephen Jay Gould’s M1, not west as stated on page 62 of Deep- opinion about evolution’s capacity to form Sky Wonders in the February 2011 issue. intelligent life, here or anywhere. Mr. ✹ Halley’s Comet will next reach perihelion Hollaway writes, “As Stephen J. Gould in 2061, not 2062 as stated in the caption to LightSwitch said, if you rewind the tape of the history the photo on page 42 of the March issue. Scope In Stock! 50 & 25 Years Ago Leif J. Robinson May 1961 none of this escapes, because of the high opacity CGE Pro 1400 HD The Helium Flash “After a star of Population of [an overlying] hydrogen shell. In Stock! II has evolved away from the main sequence of “At such a high temperature, the core is the Hertzsprung-Russell diagram and become finally forced to expand, and . . . the thermal a red giant star, a remarkable ‘thermal runaway’ runaway terminates.” occurs deep in its interior. This had been This depiction is still basically sound. The predicted by E. Mestel, and is now confirmed in calculations involved were only practical because step-by-step calculations . . . by R. Härm and M. of an electronic computer. By today’s standards, Schwarzschild. . . . the computer they used would be regarded as “The helium burning sets in when the tem- primitive indeed. 82° Water Proof perature in the contracting core reaches about 80 Eyepieces for $99.00 million degrees Kelvin. The energy released can May 1986 only raise the temperature further, as the high Well-Deserved Victory “Harvard astronomerWWW.TELESCOPES.NET pressure on the degenerate matter prevents its Fred Whipple signals success after the Giotto 5348 Topanga Canyon Blvd. expansion. The helium spacecraft glimpsed Woodland Hills, CA 91364 burning thus becomes the nucleus of Halley’sMon-Sat more and more rapid, comet. Whipple’s theory9am-6pm (PST) See us at the: with the temperature that cometary nucleiToll Free: 20th Annual continuing to rise to a are ‘dirty snowballs,’(888) 427-8766 NEAF maximum of about 350 advanced in 1950, wasLocal: NORTHEAST million degrees! At that confirmed in March by ASTRONOMY(818) 347-2270 FORUM time, the core is liberat- a spectacular seriesFax: Suffern, NY / April 16-17 ing as much energy as of visits to Halley by(818) 992-4486 1012 suns, but practically spacecraft.”
  10. 10. See us at the:20th AnnualNEAF NORTHEAST ASTRONOMY FORUMSuffern, NY / April 16-17
  11. 11. News NotesKepler’s Latest Planet Bonanza Kepler Planet Candidates (as of Feb. 1, 2011) 20 S&T: GREGG DINDERMAN, DATA: WILLIAM BORUCKI / KEPLER SCIENCE TEAM Jupiter- Size (Earth diameters) 10 size 4 Neptune- size 2 1 Earth- size NASA / TIM PYLE 1 2 4 10 20 40 100 Orbital period (days)Left: On August 26, 2010, the Kepler spacecraft observed the Sun-like star Kepler-11 being transited by three of its six planets simultaneously, asdepicted in this artist’s concept. Right: As of its latest data release, NASA’s Kepler science team had identified 1,235 planet candidates (yellow dots),far more than the number of transiting exoplanets known prior to the mission (red dots).Torrents of likely new exopl anets 1,235 Likely Worlds exposed on the surface under modestare pouring in from NASA’s Kepler space The new data release brings Kepler’s total atmospheric pressure, as on Earth. Forty-telescope — more than 1,200 of them to 1,235 planet candidates apparently nine other worlds within habitable zonesso far, large and small, including weird transiting stars. Team member Geoff range from about twice Earth’s diameterworlds and systems that no one expected. Marcy (University of California, Berke- to larger than Jupiter.The Kepler science team unveiled its ley) estimates that “90% to 95% of these Overall, when you consider the unlikeli-latest batch of findings on February 2nd, candidates are bona fide planets.” This hood that an object will transit its star at allbased largely on data from just the fi rst compares to 513 exoplanets discovered by as seen from our line of sight, the statisticsfour months (May 12 to Sept. 17, 2009) of all other projects since 1993. indicate that some 20% of stars are closelyKepler’s planned 3½-year mission. Of the new candidates, dozens are orbited by planets Earth-size and up. The Kepler scientists highlighted two roughly Earth-diameter or smaller. Addsthemes. One was the sheer number and team member Daniel Fabrycky (Univer- The Six-Transit Systemvariety of likely planets being found, sity of California, Santa Cruz), “There are Of the 156,000 stars that Kepler is watch-especially small ones, including several a ton of multiple-planet candidates: 115 ing, the team drew particular attentionsmall ones in their stars’ habitable zones. doubles, 45 triples, 8 quadruples, 1 quin- to Kepler-11, a near-copy of the Sun aboutThe team also highlighted one system in tuple, and 1 sextet.” 2,000 light-years away under the easternparticular. A 14th-magnitude star dubbed Sixty-eight are roughly the diameter of wing of Cygnus. Six small bodies orbitKepler-11 seems to have six super-Earths Earth. The total includes 288 more with it with periods from 10 to 118 days. Theand Neptunes transiting it. All are orbit- super-Earth diameters, 662 in the Nep- amount of the star’s light that each blocksing in nearly the same plane, five of them tune class, 165 about the size of Jupiter, during its transits tells its size. Countingin compactly nested orbits closer to the and 19 significantly larger than Jupiter. out from the star, they have 2.0, 3.2, 3.4,star than Mercury is to the Sun. Two or Of the 68 roughly Earth-size bodies, 4.5, 2.6, and 3.7 Earth diameters.three of these planets have such unexpect- five are within the habitable zones of The six worlds are too lightweight, andedly low average densities that they would their host stars: the not-too-hot, not-too- the star is too far and faint, for astrono-float in water. cold region where liquid water could lie mers to measure their masses by the12 May 2011 sky & telescope
  12. 12. News Notesgravitational wobbles that they induce in conclusions of some studies.”the star. But in a tour de force of celestial The Crab Nebula is the remnant of amechanics, the Kepler team measured supernova seen in 1054 A.D. The explod-the slight delays and speedups in their ing star’s core became a pulsar, a hyper-observed transit times due to their gravita- dense neutron star now spinning 30tional influences on one another. The team times per second. The pulsar is slowingwas able to untangle all the interactions. down by magnetic braking, and the lost CXC / HST / NASA / J. HESTER ET. AL.The transit-timing variations yielded spin energy ends up exciting the nebulamasses for the five inner bodies: 4, 13, 6, 8, to glow. The nebula’s inner portion “isand 2 Earth masses. dominated by the pulsar’s magnetic field, Combining the masses with the diam- which we suspect is organized precari-eters gives each object’s average density: ously,” says Roger Blandford, director ofabout 3, 2, 0.9, 0.5, and 0.7 grams per cubic the Kavli Institute for Particle Astrophys-centimeter. The last three are less dense The inner part of the Crab Nebula is imaged ics and Cosmology. “The X-ray changesthan water (1.0). Earth’s average density, here in X rays (blue) and visible light (red). The may involve some rearrangement of theby comparison, is 5.5. They probably pulsar at the center powers the nebula to glow magnetic field, but just where this hap-have rock-iron cores surrounded by thick at all wavelengths. The rings mark where a “pul- pens is a mystery.”envelopes of “ices” (mainly water, meth- sar wind” of charged particles, flying away in theane, and ammonia, either solid or liquid) plane of the pulsar’s equator, plows into nebular Jupiter Swallowed an Asteroidand gas (mainly hydrogen and helium). material and heats it to high temperatures. The On July 19, 2009, Australian planetaryIn other words, they’re miniature giants. innermost ring is about one light-year across. imager Anthony Wesley spotted a newSays team member Jonathan Fortney (UC, dark smudge near Jupiter’s south pole. ItSanta Cruz), “I think of them as being like The Inconstant Crab Nebula looked like something had hit the gianta marshmallow with a ball bearing in the Astrophysicists have long assumed that planet for the first known time sincecenter. Most of the mass is in the core, but the Crab Nebula, the strongest perma- Comet Shoemaker-Levy 9’s celebratedmost of the volume is in the atmosphere.” nent X-ray source in the sky, is a steady death plunge in 1994. In the next few Says exoplanet researcher David Char- “standard candle” at these wavelengths, days astronomers at several observatoriesbonneau (Harvard-Smithsonian Center for and they have used it to calibrate X-ray recorded as much information as theyAstrophysics), “It’s quite simply one of the telescopes accordingly. The brightnesses could before the ashes faded from view.most beautiful data sets I have ever seen.” of other X-ray sources are often stated in It’s taken a while, but two articles in Computer simulations show that the “millicrabs.” But a NASA team now con- January’s planetary-science journal Icarusconfiguration can be gravitationally stable firms what others have been unwilling to detail what infrared observers learned inover the star’s estimated age of 8 billion admit: this “standard candle” varies by at the days and weeks following the impact.years. Surprisingly, none of the planets least several percent. The group compared Five days after the black spot appeared itare locked in orbital resonances with their observations from four separate X-ray was still glowing in the infrared, indicatingneighbors. They almost certainly formed satellites and found the same variations that stratospheric gases were still slightlyfarther out and migrated inward, due to over the years. These include a steadygravitational interactions with a massive decline totaling 7% in the past two yearsgas-and-rubble disk when the system was in four high-energy X-ray bands rangingyoung. The disk would also tend to circu- from 12,000 to 500,000 electron volts.larize their orbits, keep the orbital planes Checking further back, the group foundwell aligned, and help prevent the planets that the Crab brightened and dimmed infrom locking one another into resonances. X rays as much as 3.5% from year to year The planets generally become less since 1999.dense the farther they orbit the star, a rela- Previous observers had assumed thattionship that hardly comes as a surprise. these variations were in their equipment.The star’s heat and wind are likely to be “Since we haven’t gone back yet and re-slowly stripping the atmospheres away, analyzed previous results, we can’t say forwith the innermost planets suffering the sure what the impact will be,” said teammost. “This is exactly the kind of system member Colleen Wilson-Hodge (NASA/you want in order to study this mass-loss Marshall Space Flight Center). “For some Anthony Wesley’s image of Jupiter on July 19,process,” says Fortney. “Six planets around instruments and observations the effect 2009, showed a new black marking like thosethe same star — it’s ideal for comparative would be slight, but there may be instru- left when pieces of Comet Shoemaker-Levy 9 hitplanetary science.” ments for which re-analysis could change Jupiter in July 1994. South is up.14 May 2011 sky & telescope
  13. 13. News Noteswarm (by a few degrees). Leigh Fletcher Deep Impact Stardust / NExT(Oxford University) and colleagues con-clude that the incoming object delivered apunch that would have come from a body70 to 500 meters (230 to 1,600 feet) across.The spectra also showed soot rich in iron,silicate minerals (rock dust), and silica(SiO2) lying atop the stratosphere — evi-dence that the impactor was an asteroid,not a comet nucleus. Similar conclusions appear in a secondreport. Glenn Orton (Jet PropulsionLaboratory) and others conclude that theinterloper was 200 to 500 meters across,and their spectra also show silicates andsilica. Silica wasn’t detected during themuch more energetic Shoemaker-Levy 9comet strikes. “We weren’t expecting to find thatan asteroid was the likely culprit in this Above: The nucleus of Cometimpact,” said small-body specialist Paul Tempel 1 got a second close-up scrutiny (right) when NASA’sChodas. But a little dynamical digging Stardust spacecraft paid it a visitturned up asteroids with unstable orbits on February 15th. The same cra-that could make close brushes with Jupiter. ters are indicated for comparison. Right: Before-and-after imagesComet Tempel 1 of the spot hit by the impactor NASA / JPL / CORNELL UNIVERSITYGets a Second Look from Deep Impact. The left frameWhen NASA’s Stardust spacecraft shows the target zone just beforereturned samples of Comet 81P/Wild 2 to it was struck in July 2005. At right,Earth in early 2006, its mission seemed a circle draws attention to the 200 mcomplete. However, planetary scien- resulting crater, about 500 feettists soon hatched a plan to redirect the (150 m) across.still-healthy craft to a second target. At3:40 Universal Time on February 15th, sity). He describes the crater as subdued that appears to have shrunk since DeepStardust became a successful two-timer and about 150 meters across with a small Impact’s visit.when it skirted 110 miles (178 km) past the central mound. “It looks as if stuff from Stardust’s dust detector also recordednucleus of periodic Comet 9P/Tempel 1. the impact went up and came back down.” several thousand hits as it cruised through It was a second time for the comet That the impact zone was seen at all, the comet’s coma of gas and dust. Butalso. Tempel 1 is the comet that the Deep with excellent lighting just as Stardust rather than slowly rising to a crescendoImpact spacecraft flew by and whacked came closest, is a testament to a years-long and then falling smoothly, the recordedwith an 815-pound (370-kg) copper projec- effort by ground-based observatories to nail hits reveal that dust peppered the craft intile in mid-2005. down the comet’s rotational state and spin rapid-fire bursts with silence in between, Stardust’s sole camera, pieced together period. This allowed Jet Propulsion Labo- a consequence of small “clods” disinte-from an old lens from the Voyager ratory dynamicists to time the encounter grating into countless smaller flecks afterprogram and a detector from Galileo, for optimum viewing. Indeed, the nucleus leaving the nucleus.recorded a diverse and structured dirty had rotated to within just a few degrees of NASA now can claim two spacecrafticescape that looked very familiar. where the team hoped it would be. that have each visited a pair of comets.Researchers were especially keen to This is the first time a comet has been Just two months ago, on November 4th,see Deep Impact’s strike point; the hit seen up close before and after its perihe- Deep Impact performed its own secondreleased an unexpectedly huge cloud of lion with the Sun. Besides spotting the rendezvous: a successful flyby of Cometgas and dust (S&T: October 2005, page 34). muted crater, the science team found 103P/Hartley 2 (February issue, page 14). The Deep Impact crater is not obvious, changes elsewhere on the 5-by-3-mile Sending Stardust to a second cometbut it’s there. “We did get it, there’s no (8-by-5-km) nucleus, particularly around was a bargain, adding only $29 million todoubt,” says Peter Schultz (Brown Univer- a flat tongue of debris, about 2 miles long, the mission’s $300 million total cost.16 May 2011 sky & telescope
  14. 14. The Joy of Mathematics Taught by Professor Arthur T. Benjamin harvey mudd college IM ED T E OF lecture titles IT 1. The Joy of Math—The Big Picture LIM FE 70% R 2. The Joy of Numbers 3. The Joy of Primes 4. The Joy of Counting off 5. The Joy of Fibonacci Numbers 6. The Joy of Algebra 27 RD O 7. The Joy of Higher Algebra E R B Y M AY 8. The Joy of Algebra Made Visual 9. The Joy of 9 10. The Joy of Proofs 11. The Joy of Geometry 12. The Joy of Pi 13. The Joy of Trigonometry 14. The Joy of the Imaginary Number i 15. The Joy of the Number e 16. The Joy of Infinity 17. The Joy of Infinite Series 18. The Joy of Differential Calculus 19. The Joy of Approximating with Calculus 20. The Joy of Integral Calculus 21. The Joy of Pascal’s Triangle 22. The Joy of Probability 23. The Joy of Mathematical Games 24. The Joy of Mathematical MagicUnderstand the Fun and Beautyin Mathematical ConceptsHumans have been having fun and games with mathematicsfor thousands of years. Along the way, they’ve discovered the amazingutility of this field—in science, engineering, finance, games of chance,and many other aspects of life. The 24 half-hour lectures of The Joyof Mathematics celebrate the sheer joy of mathematics, taught by a The Joy of Mathematicsmathematician who is literally a Course no. 1411 | 24 lectures (30 minutes/lecture)magician with numbers. SAVE $185Professor Arthur T. Benjamin of Harvey Mudd College is renowned forhis feats of mental calculation performed before audiences at schools,museums, conferences, and other venues. His teaching has been repeatedlyhonored by the Mathematical Association of America, the nation’s largest DVD $254.95 NOW $69.95professional mathematical society. Throughout these lectures, Professor +$10 Shipping, Processing, and Lifetime Satisfaction GuaranteeBenjamin shows how everything in mathematics is magically connected— Priority Code: 52171how the beautiful and often imposing edifice that has given us algebra,geometry, trigonometry, calculus, probability, and so much else Designed to meet the demand for lifelongis based on nothing more than fooling around with numbers. learning, The Great Courses is a highly popular series of audio and video lectures ledOffer expires 05/27/11 by top professors and experts. Each of our more than 300 courses is an intellectually1-800-832-2412 engaging experience that will change how you think about the world. Since 1990,www.thegreatcourses.com/8sky over 9 million courses have been sold.
  15. 15. News Notes 2011’s Solar System Bonanza Recovery and Interior Laboratory) will Stardust’s encounter with Comet Tempel rocket Moonward to fly in tandem just 30 1 was just the start of a big year expected miles (50 km) above the lunar surface. for planetary science. In the next several By tracking their positions with respect MAXIME BOCCAS / BENOIT NEICHEL / GEMINI OBSERVATORY months NASA’s interplanetary exploration to each other extremely precisely as they is set to proceed at a rapid pace: orbit, scientists hope to derive, once and • The twin STEREO solar observatories for all, the detailed gravitational structure have already taken up their positions on of the lunar interior from crust to core. opposite sides of the Sun. For the first November 8: The Russian Space time, solar scientists can monitor all of the Agency hopes to launch its own much- Sun’s surface at once. That should pay big delayed Phobos-Grunt mission. (Grunt is dividends as solar activity ramps up. Russian for “ground.”) Conceived in 1996 March 18: After three brief flybys in to land on the larger of Mars’s two satel- 2008 and 2009, NASA’s Messenger space- lites and return a sample to Earth, Phobos- craft will brake into a permanent, looping Grunt has been redesigned several times. The Gemini South MCAO system creates a polar orbit around Mercury. Once per orbit In its latest configuration it will carry the tiny “constellation” of five artificial guide stars the craft will skim 125 miles (200 km) from small, Chinese-built spacecraft Yinghou by exciting sodium atoms about 60 miles up. the surface. With its seven cameras, spec- 1, which will detach and orbit Mars for up The guide stars look bright in this 30-second trometers, and other instruments, Mes- to a year. Plans still call for the main craft zoomed-in exposure but are invisible to the senger has already made many new to land on Phobos, collect 85 to 160 grams naked eye. The upward beam, however, is easy discoveries about the innermost (3 to 5½ ounces) of samples, and return to see from up to a few hundred yards away. planet. But the real science them to Earth by early 2013. breakthroughs, such as its November 25: Another delayed mis- Standard adaptive optics work by interior structure, will come sion, NASA’s Mars Science Laboratory, watching a single guide star for atmo- after the orbiter has made will launch toward Mars. The primary spheric distortions, taking readings as long-term observations. scientific objective for this large rover is rapidly as 1,000 times a second and adjust- T. IN S July 16: The Dawn space- to advance the assessment of whether ing a flexible mirror just as rapidly to IE EGMessenger craft will reach 4 Vesta, the the Red Planet ever had an environment compensate. The star can be either real or RN CA L/ S A/ JH U AP asteroid belt’s most interesting conducive to life — or still does. The rover artificial: created by a laser that’s tuned to NA chunk of rock. It’s thought to have was named Curiosity following a Disney- excite the sodium atoms in a layer of atmo- an iron-nickel core, an olivine mantle, inspired contest in 2009. sphere about 60 miles (90 km) up. MCAO, a rock crust of nearly unique composi- Some dates are subject to change. on the other hand, requires monitoring tion, lava flows, and a giant crater. several guide stars at once. Hubble images have given only The Next Step On January 22nd engineers at the Gem- the vaguest of looks, but Dawn for Adaptive Optics ini South observatory in Chile test-fired a will open up this new world The giant telescopes of the future — the 50-watt laser to create a tight, 1-arcminute- in high resolution. Dawn planned Thirty-Meter Telescope (TMT), wide “constellation” of five sodium stars. will orbit Vesta for a year the 24-meter Giant Magellan Telescope Project leader Celine d’Orgeville says that before easing away and set- (GMT), and the 42-meter European Gemini’s MCAO system should allow theDawn ting course for 1 Ceres. Extremely Large Telescope (E-ELT) — will 8.1-meter telescope to see ultrasharp views August 5 is the scheduled need the most advanced adaptive-optics up to 2 arcminutes wide starting in 2012. NA SA launch date for NASA’s ambitious systems possible if they are to reach their Other observatories are pursuing adap- Juno mission. When it reaches Jupiter full potential. Adaptive optics work to tive optics with multiple guide stars. In in 2016, Juno is to slip into a looping polar cancel out the blurring effects of Earth 2007 the European Southern Observatory’s orbit that will subject it to dangerously atmosphere. The current cutting edge Very Large Telescope tested a system that intense radiation but allow it to of the art is “multi-conjugate adaptive uses natural stars, and a system using four answer, its science team hopes, optics,” or MCAO. This version models laser guide stars should be ready in 2015. key questions about Jupiter’s not just the two-dimensional wavefront of The MMT Observatory in Arizona has composition and interior light reaching the telescope, but the entire, tested a system that uses a green laser to structure — and from those three-dimensional column of air in front create multiple artificial stars in the lower results, how Jupiter and of the telescope’s aperture. This allows the atmosphere by scattered light. ✦ other giant planets formed. image-sharpening process to work over aJuno September 8: The twin much wider field of view; narrow fields are To get astronomy news as it breaks, visit H C TE AL A SA / JP L- C GRAIL spacecraft (Gravity a severe limitation of current systems. Sky SkyandTelescope.com/newsblog. N 18 May 2011 sky & telescope
  16. 16. Check our web site for a full line of: • Photography, ETX-LS LightSwitch MiniTower Advanced • MP3 Players, The revolutionary ETX-LS Portable GoTo Altazimuth • Computers, astronomical telescope Mount with Metal Case, • Home Office, features the most advanced Tripod, Supports 25 lbs. • Printers, electronics and optics ever • Video, applied to a consumer telescope. • Photo Lab, • Etc. STARTING AT STARTING AT$1299.99 $749.99 CGEM 925, 9.25” LX200-ACF SERIES Nexstar SE Series Schmidt-Cassegrain ADVANCED Coma-Free 5SE, 6SE, 8SE, Telescope SE TELESCOPES INCLUDES: • CGEM Computerized The 8, 10, 12 & 14” • XLT Coatings Equatorial Mount LX200-ACF • Orange Removable OTA, • Celestron’s premium With Ultra-High • 40,000 object database, StarBright XLT coatings Transmission Coatings • Nexstar Remote Software, • 6x30 finderscope to help STARTING AT (UHTC™) STARTING AT • The Sky Software STARTING AT accurately find objects$2599.99 $499.99 • Adjustable Tripod $2199.99 The David H. Levy Comet Hunter™ 8” Dobsonian Special Edition Telescope 1200mm Focal The Comet Hunter comes Length (f/5.9) standard with a two-inch, 100° Apparent 2 Image Control 30mm focal length eyepiece System with Special Field with 2” Crayford Style with a wide 70° MGA Device, apparent field. This eyepiece 6mm, 8mm 13-mm Advanced Reflex Focuser, combined with the short 17-MM Ethos Sky & Control 1.25” adaptor, 10mm & focal length of the telescope Telescope Ratings 25mm produce an amazing 2.86 Optical Performance Plossl Eyepieces, 8x50 VF, ° true field of view with a 6.2mm exit pupil. $569.95 $495.95 & Rocker Mount. Big Discounts Telrad TT320X-AG Tracking Mount With Auto-Guiding port for PST, The Imaging Source Digial SLR SolarMax 640 x 480 Monochrome Software Bisque Cameras includes Asto Trac FireWire Telescope 40 Telescopes, Illuminated Camera 60 & 90mm systems Rigel Systems Polar Scope, 12 Volt Car Battery Adapter $390.95
  17. 17. Nature’s Ultimate Whirlpools Spinning Hearts of Darkness Astronomers are measuring the rotation rates of black holes to deter- mine their formation and history. Disney must have had close ties with some of our local TV stations in the 1980s, because it seemed like The Black Hole was playing whenever I was home sick from elementary school. My love for this movie blos- somed in spite of its cartoonishly stereotypical characters, cheesy costumes, and laughable special effects. I didn’t care about the movie except for the black hole itself. The cosmic beast was hypnotic and omnipresent — a dark, gaping maw surrounded by an infalling funnel of fluo- rescent gas. I was mesmerized. The thought of such an extreme object existing out there was amazing, but even more so was the notion that humans could travel into one and emerge in some other dimension. I was later disappointed to learn that physicists deem it virtually impossible to journey into a black hole and exit elsewhere intact. In fact, black holes are nature’s simplest objects, described only by their mass and spin. So why do so many scientists study these objects, and why does the public gobble up fictional accounts of them? Perhaps it’s because black holes represent the ultimate unknown. Spinning Black Holes zon. This released energy can influence the black hole’s Black holes were given their name because their gravity surroundings to great distances. is so strong that not even light can escape once it passes A black hole actively gorging itself on nearby gas will a critical outer boundary known as the event belch out prodigious energy. These objects also tend to horizon. If we sent a probe past the event hori- be the ones that offer us the best glimpse into the nature zon with instructions to beam back pictures, the of the innermost accretion disk, closest to the hole and beam would lack the necessary velocity to escape where most of the action is taking place. the hole’s gravitational pull, even though it would The action that I’m most interested in is how fast the travel at the speed of light. So black holes neither black hole is spinning. We can access this information emit light nor let incoming light out once it crosses only by observing radiation coming from very close to LAURA the event horizon. But we can observe the radiation from the event horizon, where material is so hot that energy isBRENNEMAN infalling gas that doesn’t quite make it past the event hori- emitted as X rays. Because Earth’s atmosphere blocks this20 May 2011 sky & telescope
  18. 18. Illustration by Mark Garlickhigh-energy radiation, orbiting telescopes perform the GAPING MAW Like almost all objects in space, black holeslion’s share of the work. rotate. Along with mass, the spin rate is one of only two fun- Why bother figuring out how fast a black hole is spin- damental characteristics of a black hole, and it has a profound influence on the surrounding spacetime and how the blackning? What do we hope to learn that will justify the effort? hole accretes matter. This artistic rendering shows a black hole First, most galaxies contain supermassive black holes accreting material from its surrounding disk.(SMBHs) at their cores, which can range from millionsto billions of solar masses. A SMBH plays a pivotal rolein shaping its host galaxy. It determines how large its through its unparalleled efficiency in releasing energygalaxy gets and how many stars it contains, and will do so from the gas it accretes, and the black hole’s spin is a keyin spite of having a relative size ratio to its galaxy equiva- cog in that engine. Jets released from SMBHs shoot outlent to that of a grain of sand in the center of the United like geysers from a galaxy’s core, keeping intragalactic gasStates. The black hole accomplishes this feat largely warm enough to stop star formation in the host galaxy, Sk yandTelescope.com May 2011 21
  19. 19. Nature’s Ultimate Whirlpools 1.5 1.5 1.5 Fast prograde No Fast retrograde spin spin spin Relative X-ray flux Relative X-ray flux Relative X-ray flux 1.0 1.0 1.0 0.5 0.5 0.5 S&T: LEAH TISCIONE 0 0 0 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 Energy (keV) Energy (keV) Energy (keV) SPIN Above: A black hole’s spin plays a pivotal role in determining how close material can orbit before falling into the abyss. Material orbits farther from a nonspinning black hole, and farther still from a retrograde hole. Simulated spectra of the Fe Kα line are below. Left: The faster a black hole spins, the larger the frame dragging (represented by blue lines). This effect of general relativity allows material to orbit very close to a fast-spinning, prograde black hole. thus regulating its size. Physicists think jet power is directly related to black hole spin. Second, measuring black hole spin can tell us about the recent growth history of a SMBH and its host galaxy. According to computer simulations by Marta Volonteri S&T: LEAH TISCIONE (University of Michigan) and her collaborators, black holes that grow primarily by accretion will spin faster than those that grow mostly by mergers with other black holes. Prolonged accretion usually funnels gas onto the THE POINT OF NO RETURN accretion disk in the same direction that it (and the black All black holes have an innermost stable circular orbit (ISCO), the hole) is already spinning, increasing the hole’s spin. In distance from the event horizon at which the energy and angular contrast, black hole mergers occur at random angles, momentum of orbiting material is no longer sufficient to counteract which can alter the spin direction and speed of the result- the inward pull of gravity. Once material crosses the ISCO, it plunges ing black hole. SMBH mergers occur millions of years into the black hole on a ballistic trajectory. after their host galaxies merge (S&T: April 2009, page 26). The ISCO’s distance from the event horizon depends on the black Spin, therefore, is an excellent diagnostic for assessing hole’s mass and spin. According to general relativity, a spinning a black hole’s recent history of mergers versus accretion. massive object drags the surrounding spacetime, an effect known As the number of SMBHs with reliable black hole spin as frame dragging. A fast-spinning black hole will spin up nearby measurements grows, so too will our knowledge of spin spacetime, imparting more angular momentum to material orbiting demographics and its relation to other physical properties in the inner accretion disk. This energy enables the material to resist of SMBH systems such as mass, accretion rate, jet power, the black hole’s gravity, keeping it from plunging into the hole at the and host-galaxy mass and type. distance it otherwise would have without the help of frame dragging. In the case of black holes that originate from dying massive stars (the so-called galactic black holes, or GBHs),22 May 2011 sky & telescope

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