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. 26
MAY 2011
Spinning
Hearts of
Darkness
Astronomers
probe whirling
black holes. p. 20
4 Planets Dance at Dawn p. 48
Explore the Virgo Cluster p. 66
See Superthin Galaxies p. 62
Visit SkyandTelescope.com

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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 Showcase
COVER o Darkness
of By Fred Schaaf
STORY
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-
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4 May 2011 sky & telescope

6. Robert Naeye
Spectrum
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.
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Subscription Rates: U.S. and possessions: $42.95 per year (12 issues);
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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
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expect this to happen in my lifetime. Skies, Night Sky, and ESSCO.
Editor in Chief
6 May 2011 sky & telescope

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8. Letters
Astro Outreach in the Canaries round-table discussion on our future in
We 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 Institute
June 20–25, 2011. of the Canary Islands
Starmus will bring astronauts, cosmo- La Laguna, Spain
nauts, astronomers, biologists, astropho- Astronomy,” page 6).
tographers, musicians, and artists together Astronomer Heroes Remembered In the early fifties, when I was roughly
in 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 the
in 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. I
Jack Szostak, and George Smoot, along remember the May 1958 issue of Sky & would spend the weekend enjoying the
with Jill Tarter of the SETI Institute, evo- Telescope very well. run of the Mount Wilson Observatory
lutionary biologist Richard Dawkins, and Today, readers can enjoy all the back under the watchful eye of my friend Joe
many other leading scientists. Pioneer- issues by obtaining the Complete Sky & Hickock, who did the routine work on
ing cosmonauts and astronauts Valentina Telescope: Seven Decade Collection on DVDs. the solar towers and who maintained the
Tereshkova, 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 the
humanity’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 on
guitarist and astrophysicist Brian May, That issue’s center spread was a photo- the 60-inch or the 100-inch Hooker with
Tangerine 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 Newtonian
astronomy exhibition, and an astropho- montage reached the incredible magnitude platform — 80 feet off the observatory
tography exhibition and competition. The of 17.5, far beyond what was expected for floor with no rails, thank you very much
winner receives 60 minutes of observing amateur astrophotography in those days. — with Dr. Sandage, who at that time may
time 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 dressed
largest 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, Arizona
8 May 2011 sky & telescope

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10. 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
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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 astronomer
WWW.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’s
Mon-Sat more and more rapid, comet. Whipple’s theory
9am-6pm (PST) See us at the: with the temperature that cometary nuclei
Toll Free:
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1012 suns, but practically spacecraft.”

11. See us at the:
20th Annual
NEAF
NORTHEAST
ASTRONOMY
FORUM
Suffern, NY / April 16-17

12. News Notes
Kepler’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, as
depicted 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 modest
are 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 zones
so far, large and small, including weird transiting stars. Team member Geoff range from about twice Earth’s diameter
worlds 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 all
based largely on data from just the fi rst compares to 513 exoplanets discovered by as seen from our line of sight, the statistics
four months (May 12 to Sept. 17, 2009) of all other projects since 1993. indicate that some 20% of stars are closely
Kepler’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. Adds
themes. One was the sheer number and team member Daniel Fabrycky (Univer- The Six-Transit System
variety 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 attention
small ones in their stars’ habitable zones. doubles, 45 triples, 8 quadruples, 1 quin- to Kepler-11, a near-copy of the Sun about
The team also highlighted one system in tuple, and 1 sextet.” 2,000 light-years away under the eastern
particular. A 14th-magnitude star dubbed Sixty-eight are roughly the diameter of wing of Cygnus. Six small bodies orbit
Kepler-11 seems to have six super-Earths Earth. The total includes 288 more with it with periods from 10 to 118 days. The
and Neptunes transiting it. All are orbit- super-Earth diameters, 662 in the Nep- amount of the star’s light that each blocks
ing in nearly the same plane, five of them tune class, 165 about the size of Jupiter, during its transits tells its size. Counting
in 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, and
edly 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 the
12 May 2011 sky & telescope

14. News Notes
gravitational 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 a
mechanics, 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 30
tional influences on one another. The team times per second. The pulsar is slowing
was 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 nebula
masses for the five inner bodies: 4, 13, 6, 8, to glow. The nebula’s inner portion “is
and 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 of
about 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 changes
than water (1.0). Earth’s average density, here in X rays (blue) and visible light (red). The may involve some rearrangement of the
by 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 the
ane, and ammonia, either solid or liquid) plane of the pulsar’s equator, plows into nebular Jupiter Swallowed an Asteroid
and gas (mainly hydrogen and helium). material and heats it to high temperatures. The On July 19, 2009, Australian planetary
In other words, they’re miniature giants. innermost ring is about one light-year across. imager Anthony Wesley spotted a new
Says team member Jonathan Fortney (UC, dark smudge near Jupiter’s south pole. It
Santa Cruz), “I think of them as being like The Inconstant Crab Nebula looked like something had hit the giant
a marshmallow with a ball bearing in the Astrophysicists have long assumed that planet for the first known time since
center. Most of the mass is in the core, but the Crab Nebula, the strongest perma- Comet Shoemaker-Levy 9’s celebrated
most 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 observatories
bonneau (Harvard-Smithsonian Center for and they have used it to calibrate X-ray recorded as much information as they
Astrophysics), “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 Icarus
configuration can be gravitationally stable firms what others have been unwilling to detail what infrared observers learned in
over 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 it
are locked in orbital resonances with their observations from four separate X-ray was still glowing in the infrared, indicating
neighbors. They almost certainly formed satellites and found the same variations that stratospheric gases were still slightly
farther out and migrated inward, due to over the years. These include a steady
gravitational interactions with a massive decline totaling 7% in the past two years
gas-and-rubble disk when the system was in four high-energy X-ray bands ranging
young. 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 found
well aligned, and help prevent the planets that the Crab brightened and dimmed in
from 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 that
tionship 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 for
with the innermost planets suffering the sure what the impact will be,” said team
most. “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 those
the same star — it’s ideal for comparative would be slight, but there may be instru- left when pieces of Comet Shoemaker-Levy 9 hit
planetary science.” ments for which re-analysis could change Jupiter in July 1994. South is up.
14 May 2011 sky & telescope

16. News Notes
warm (by a few degrees). Leigh Fletcher Deep Impact Stardust / NExT
(Oxford University) and colleagues con-
clude that the incoming object delivered a
punch that would have come from a body
70 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 second
report. Glenn Orton (Jet Propulsion
Laboratory) and others conclude that the
interloper was 200 to 500 meters across,
and their spectra also show silicates and
silica. Silica wasn’t detected during the
much more energetic Shoemaker-Levy 9
comet strikes.
“We weren’t expecting to find that
an asteroid was the likely culprit in this Above: The nucleus of Comet
impact,” said small-body specialist Paul Tempel 1 got a second close-up
scrutiny (right) when NASA’s
Chodas. But a little dynamical digging
Stardust spacecraft paid it a visit
turned 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 images
Comet Tempel 1 of the spot hit by the impactor
NASA / JPL / CORNELL UNIVERSITY
Gets a Second Look from Deep Impact. The left frame
When NASA’s Stardust spacecraft shows the target zone just before
returned 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 m
complete. However, planetary scien- resulting crater, about 500 feet
tists soon hatched a plan to redirect the (150 m) across.
still-healthy craft to a second target. At
3:40 Universal Time on February 15th, sity). He describes the crater as subdued that appears to have shrunk since Deep
Stardust 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 recorded
nucleus 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. But
also. Tempel 1 is the comet that the Deep with excellent lighting just as Stardust rather than slowly rising to a crescendo
Impact spacecraft flew by and whacked came closest, is a testament to a years-long and then falling smoothly, the recorded
with an 815-pound (370-kg) copper projec- effort by ground-based observatories to nail hits reveal that dust peppered the craft in
tile 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 after
program 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 spacecraft
icescape 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 second
released an unexpectedly huge cloud of lion with the Sun. Besides spotting the rendezvous: a successful flyby of Comet
gas 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 comet
but it’s there. “We did get it, there’s no (8-by-5-km) nucleus, particularly around was a bargain, adding only $29 million to
doubt,” 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

17. The Joy of Mathematics
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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 Magic
Understand the Fun and Beauty
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18. 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
EG
Messenger 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 the
Dawn
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 a
Juno 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

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20. 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 is
BRENNEMAN infalling gas that doesn’t quite make it past the event hori- emitted as X rays. Because Earth’s atmosphere blocks this
20 May 2011 sky & telescope

21. Illustration by Mark Garlick
high-energy radiation, orbiting telescopes perform the GAPING MAW Like almost all objects in space, black holes
lion’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 black
ning? 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 millions
to billions of solar masses. A SMBH plays a pivotal role
in shaping its host galaxy. It determines how large its through its unparalleled efficiency in releasing energy
galaxy gets and how many stars it contains, and will do so from the gas it accretes, and the black hole’s spin is a key
in spite of having a relative size ratio to its galaxy equiva- cog in that engine. Jets released from SMBHs shoot out
lent to that of a grain of sand in the center of the United like geysers from a galaxy’s core, keeping intragalactic gas
States. The black hole accomplishes this feat largely warm enough to stop star formation in the host galaxy,
Sk yandTelescope.com May 2011 21

22. 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