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Looking Down the Barrel: Finding Jets Aimed at Earth p. 70
THE ESSENTIAL MAGAZINE OF ASTRONOMY
S&T Test Report: “Do-It-All” Software for Observers p. 34 APRIL 2010
Hanging Balance
Could our solar system go haywire? p.26
Space Artists Envision Alien Worlds p. 56
See Mercury and Venus at Sunset p. 53
Black Hole Blowtorches p. 20 Visit SkyandTelescope.com
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"Apogee has a well-deserved reputation of being a leader in the field of CCD imaging technologies. The quality of their products is second to none, and I have been extremely pleased with their service." --Steve Cannistra
NGC2237-9 Image Courtesy Steve Cannistra Apogee Instruments Alta U16M camera Takahashi FSQ106 f/5 scope on Takahashi NJP mount AFW50 filter wheel with Baader SII, Ha, OIII filters
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Great Image! The Rest of the Story: For more than 15 years we’ve been refining our designs to meet the increasing needs of our most demanding customers. From the beginning, we’ve offered professional shutters from Vincent and Melles Griot and coated fused silica windows. Along the way, we’ve added deeper cooling to 75C below ambient, knife edge baffles, IR pre-flash, and high precision filter wheels unequaled in astronomy. With our lifetime guarantee on the inner chamber seal and two year parts and labor warranty, you can feel confident that you’re buying a time-tested platform from a company that stands behind their products. And with expert advice on your whole system from our Tim Puckett, you’ll be certain that you’re getting the best system for your needs. 1020 Sundown Way, Ste 150 Roseville CA 95661 Tel: 916-218-7450 Fax: 916-218-7451 www.ccd.com ©2010 Apogee Instruments Inc. Alta is a registered trademark of Apogee Instruments Inc.
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April 2010 VOL. 119, NO. 4
On the cover: An orrery symbolizes the solar system’s stability. But modern computer models show we can’t take that for granted.
40
Northern Hemisphere’s Sky
43
April’s Sky at a Glance
45
Binocular Highlight
8 10
Letters 50 & 25 Years Ago By Leif J. Robinson
46
Planetary Almanac
12
News Notes
48
Sun, Moon, and Planets
18
Cosmic Relief
20 The Universal Jet Set
By David Grinspoon
By Fred Schaaf
50
Exploring the Moon By Charles A. Wood
53 26 Hanging in the Balance COVER STORY
Spectrum By Robert Naeye
By Gary Seronik
FE ATURE S
Wi Will the orbits of the planets ever go haywire? Mr. Newton, meet chaos theory. hay By Greg Laughlin
6
By Fred Schaaf
S&T: CASEY REED
Somehow, a wide variety of objects shoot out laser-like beams of matter, sometimes at near-light speed. By C. Renée James
AL S O IN THI S I S S U E
THI S M O N TH ’ S S K Y
65
38
New Product Showcase
68
Telescope Workshop By Gary Seronik
Celestial Calendar By Alan MacRobert
76
Gallery
Deep-Sky Wonders
86
Focal Point
By Sue French
By Mark Stevenson
56 Imagining Other Worlds Space artists help us visualize planets and moons that humans have only glimpsed. By Michael Carroll
70 Blazar, Blazar,
Burning Bright These exotic galaxies are some of the most distant objects visible through backyard telescopes. By Steve Gottlieb
S &T TE S T R E P O R T
Skyhound’s SkyTools 3 From generating observing lists to logging your observations, this program promises to do it all. By Rod Mollise
36
Quick Look: Explore Scientific’s 14mm 100° Eyepiece By Dennis di Cicco
4 April 2010 sky & telescope worldmags
56 SKY & TELESCOPE (ISSN 0037-6604) is published monthly by Sky & Telescope Media, LLC, 90 Sherman St., Cambridge, MA 02140-3264, 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. © 2010 Sky & Telescope Media, LLC. All rights reserved. Periodicals postage paid 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.
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Robert Naeye Spectrum Founded in 1941 by Charles A. Federer, Jr. and Helen Spence Federer
The Essential Magazine of Astronomy
Astronomy’s Best-Kept Secret
EDITORIAL
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 Editors Emeritus Richard T. Fienberg, Leif J. Robinson Senior Contributing Editors J. Kelly Beatty, Roger W. Sinnott
CRISTIAN VALENZUELA
In early December I toured northern Chile for five days, my second trip to the South American nation. I visited several amateur and professional observatories, along with other tourist venues. The trip was a blast, and I came home thinking that many more astronomy aficionados from the Northern Hemisphere would enjoy visiting Chile. Here are some of the reasons why: 1. Chile is in the Southern Hemisphere. If you’re a deep-sky fanatic and haven’t seen 47 Tucanae, the Tarantula Nebula, or Eta Carinae, for example, you must travel south sometime in your life to see these treasures. I’m blown away by these objects, especially when I view them through large scopes. 2. In northern Chile, the skies are exceptionally dark and the seeing is usually very good. 3. Having a clear night sky at the astronomy sites in northern Chile is about as close as it gets to a “sure thing” in life. 4. During my two trips combined I have visited three amateur observatories: Observatorio Cerro Mamalluca (www.mamalluca.org/ Observatorio del Pangue, near Vicuña, ingles/inicio.htm), Observatorio del Pangue Chile, houses a 25-inch Obsession Dob (www.observatoriodelpangue.blogspot. and several other scopes. Posing left to com), and Observatorio Cruz del Sur (www. right are Carolina Medina of ProChile, observatoriocruzdelsur.cl). All three feature Cristóbal Benítez of Astronomica Chile, large telescopes that are staffed by expert and staff astronomer Cristian Valenzuela, friendly observers who can speak English. S&T Editor in Chief Robert Naeye, and Please make arrangements before your visit. staff astronomer Eric Escalera. 5. Chile has at least five world-class professional observatories, including Europe’s Very Large Telescope at Paranal and the U.S. site at Cerro Tololo. You’ll have to make prior arrangements for tours. 6. Since Chile is due south of the eastern U.S., North Americans can fly there with little or no jet lag. It’s also not a particularly expensive place to visit. 7. Chile is a relatively prosperous nation with a modern transportation infrastructure. Excellent food is in abundance. Chile is a politically stable democracy and crime is extremely rare in the areas of interest to astro-tourists. 8. Chile features beautiful natural scenery and interesting cultures. I particularly love the Valle de la Luna (Valley of the Moon) and the Valley de Marte (Valley of Mars) in the Atacama Desert. What names could be more astronomical? I plan to visit Chile again in the future, and I encourage you to do the same.
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, David H. Levy, Jonathan McDowell, Fred Schaaf, Govert Schilling, Ivan Semeniuk, Gary Seronik, William Sheehan, Mike Simmons, Charles A. Wood, Robert Zimmerman Contributing Photographers P. K. Chen, Akira Fujii, Robert Gendler, Tony & Daphne Hallas ART & DESIGN
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Chief Executive Officer Stephen J. Kent Executive Vice President / CFO Mark F. Arnett Corporate Controller Jordan Bohrer Office Administrator Laura Riggs Editorial Correspondence: Sky & Telescope, 90 Sherman St., Cambridge, MA 02140-3264, USA. Phone: 617-864-7360. Fax: 617-864-6117. E-mail: editors@ SkyandTelescope.com. Website: SkyandTelescope.com. Unsolicited proposals, manuscripts, photographs, and electronic images are welcome, but a stamped, self-addressed envelope must be provided to guarantee their return; see our guidelines for contributors at SkyandTelescope.com. Advertising Information: Peter D. Hardy, Jr., 617-864-7360, ext. 2133.
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Canada: $49.95 (including GST); all other countries: $61.95, by expedited delivery. All prices are in U.S. dollars. Newsstand and Retail Distribution: Curtis Circulation Co., 730 River Rd., New Milford, NJ 07646-3048, USA. Phone: 201-634-7400. No part of this publication may be reproduced by any mechanical, photographic, or electronic 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 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. Phone: 978-750-8400. Fax: 978-750-4470 Web: www.copyright.com. Specify ISSN 0037-6604. The following are registered trademarks of Sky & Telescope Media, LLC: Sky & Telescope and logo, Sky and Telescope, The Essential Magazine of Astronomy, Skyline, Sky Publications, SkyandTelescope.com, http://www.skypub.com/, SkyWatch, Scanning the Skies, Night Sky, and ESSCO.
Editor in Chief 6 April 2010 sky & telescope worldmags
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THE MEADE LX200 -ACF . A SERIOUS TELESCOPE FOR THE SERIOUS AMATEUR. ®
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Meade’s LX200 is the most widely used advanced amateur telescope on Earth. And for good reason. It combines the most sophisticated optical system available with rock-solid mechanical and electronic elements to create the most versatile astronomical system available. With its unbelievable list of features and capabilities — and the new Series 5000™ Auxiliary Equipment Mounting System — the advanced amateur or researcher can load it up to perform almost any astronomical task with ease. Every LX200 comes standard with: -
- Advanced Coma-Free™ Optics. ACF design produces a coma-free, flatter field of view with no diffraction spikes that equals traditional Ritchey Chrétien systems at a fraction of the cost. Combine this with Meade’s patented UHTC™ coatings that increase light transmission by 20% over standard coatings, for the sharpest, brightest images you can get.
5 4
. Primary Mirror Lock. Eliminates focus and mirror shift during long exposures. Like all of the features on this list, a Meade exclusive.
/ Smart Mount™ And Smart Drive™. The LX200 constantly refines pointing accuracy each time an object is centered and provides Permanent Periodic Error Correction on both axes over the course of one or more training periods. Works in both alt-az and equatorial modes.
0 Solid Fork Mount With Heavy Duty 5.75" Gears. Large, high-quality worm-gear drives in both axes provide smooth movements with low periodic error that are critical for long exposure astrophotography.
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horizon to horizon without any meridian flip, unlike German Equatorial Mounts.
2 AutoStar II Controller. Puts over 145,000 objects at your fingertips with the
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1Zero Meridian Flip. The inherent value of our fork mount is the freedom to go
3 Rock-Solid Field Tripod. Depending on aperture, each LX200 comes with an adjustable Standard Field Tripod, Giant Field Tripod or Super Giant Field Tripod for a solid, research-grade foundation.
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To customize your LX200, these optional accessories are also available:
4 NEW! Series 5000 Auxiliary Equipment Mounting System. Machined aluminum dovetail plates, rings and counterweight systems (shown here with the Series 5000 102mm ED APO refractor) designed specifically for the Meade LX series of telescopes. They join seamlessly to create the ideal custom configuration. Available for 8", 10", 12", 14" and 16" LX scopes.
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For more information about the LX200-ACF line of telescopes go to meade.com.
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Letters
I would love to share a photo of a work that might interest enthusiasts of telescopes and quilts alike: my quilt of the U.S. Naval Observatory’s 26-inch Alvan Clark refractor at the old Foggy Bottom site in Washington, D.C. The USNO’s 26-inch Great Equatorial Telescope, the world’s largest refractor for more than a decade, saw first light on November 20, 1873. Made by Alvan Clark & Sons of Cambridgeport, Massachusetts, the telescope has had a distinguished career. It was with this instrument that Asaph Hall discovered the two moons of Mars in August 1877. In 1893 the telescope was relocated from Foggy Bottom to its present site on Washington’s Observatory Hill. It is still in use for measuring double stars and positions of moons of the outer planets. The inspiration for my quilt was an old engraving from a newspaper story celebrating the work of the telescope. The engraving shows Superintendent Rear Admiral Benjamin F. Sands (standing) and astronomer Simon Newcomb in the dome with the brand new telescope in late fall of 1873. The quilt took me a year to make! It is machine pieced and appliquéd, and machine quilted. I used many reproduction 19th-century fabrics to give a period feel. The tube of the telescope and the astronomers’ garments are shaded by fabrics with stars. I took the quilt to Stellafane last summer and entered it in the homemade telescope competition. It took first place in the category of Telescopes-Mechanical / Special-Other. Sara J. Schechner Cambridge, Massachusetts
Tackling Diversity I question why you think the following quote from Dara J. Norman’s article 8 April 2010 sky & telescope worldmags
SARA J. SCHECHNER (2)
Quilted Wonders
(“Expanding Diversity in Professional Astronomy,” February issue, page 86) is true: “Quite simply, astronomers need to secure a diverse workforce in the field in order to insure that the best scientific research is accomplished.” Show me the proof that “diversity” of itself has actually achieved anything at all. What we do need is a better educational process, one that actually inspires all students in the sciences. I have had personal experience with a current crop of public school teachers and, frankly, I’m not impressed; it seems as though my own teachers in the 1950s did a better job! I recently had a high-school teacher with a master’s degree ask me what caused the auroras. No wonder our public schools aren’t turning out scientists…but it has nothing at all to do with diversity. Let’s encourage the best and brightest, no matter what color or gender. John Bartucci Cuyahoga Falls, Ohio I was extremely pleased to see Dara Norman’s Focal Point and the accompanying editorial by Robert Naeye (“Diversifying the Ranks,” page 6) in the February 2010 issue of Sky & Telescope.
Astronomy as a field has made great strides in some areas of diversity. The fraction of women in advanced academic and research positions is improving, albeit slowly, but the gradient is positive. That is not to say that there still is not a long way to go, but progress is being made. However, the field of astronomy has fared extremely poorly in attracting underrepresented minorities. Until recently, the number of astronomy doctorates awarded to minorities each year could be counted on the fingers of one hand. The American Astronomical Society (AAS) recognized this problem 20 years ago and created a standing Committee on the Status of Minorities in Astronomy whose charge is “to enhance the participation of underrepresented minorities in Astronomy and Astrophysics at all levels and experience.” The CSMA has been active in running workshops, it publishes a newsletter, Spectrum, it arranges mentoring, it helps arrange partnerships between historically black colleges and doctoral institutions, and it helps represent the society at meetings like that of the National Society of Black Physicists and the Society of Hispanic Professional Engineers. The AAS Council also has recently adopted a mission and goals statement that includes a statement on promoting diversity: “The Society supports and promotes increased participation of historically underrespresented groups in astronomy.” The AAS is acting by improving communications, by promoting family-friendly activities, and by promoting programs that train minority scientists. We care deeply about this issue and are committed to enhancing the participation of minorities in the field at all levels. John Huchra President, AAS Harvard-Smithsonian Center for Astrophysics, Cambridge, Massachusetts I commend Dara Norman and Robert Naeye for pointing out the benefits of diversity in astronomy. Having been active
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M45, The Pleiades, taken with the Orion 8” f/4 Newtonian Astrograph, modified Canon EOS 350D DSLR, Orion StarShoot DSCI and Orion 100mm f/6 reflector for guiding, Tele Vue Universal Paracorr coma corrector. Orion Image.
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Letters
Sky & Tel called the AT106 “Too Good To Be True” at $1995.
AT106
$1995
Wonder what they’ll say about it at $1495?
The headAT106LE line on the $1495 Sky & Tel cover said the $1995 Astro-Tech AT106 4.2” ED triplet refractor was “A Scope Too Good To Be True.” And the four-page review inside said the AT106 “provides all the benefits of a first-class 4-inch apo but without the premium price.”
And now, there’s a Limited Edition Astro-Tech AT106LE available with the same gorgeous f/6.5 FPL-53 triplet optics as the AT106 (simply fewer mechanical bells and whistles) for only $1495. An on-line comment called it “one of the best deals I have ever seen offered in over 2 decades of this hobby.” The production run of the AT106LE is strictly limited to 50 pieces at $1495. When they are gone, they are gone for good. Don’t miss out on this one.
Color-free scopes in glorious living Technicolor™! The Astro -Tech AT72ED 2.8” f/6 ED doublet reAT72ED fractor is a larger $379 aperture 2” dualspeed focuser version of the AT66ED, one of the most popular Astro -Tech scopes ever. The AT72ED was first available in black only. Then we started getting requests for the AT72ED body in color, the same way the AT66ED was once available. You spoke, we listened. The Astro-Tech AT72ED is now available in your choice of five tube colors: Celestron black, Meade blue, white, deep red, and forest green. You can now have great 72mm Astro-Tech ED optics, free from spurious color, in a glorious living Technicolor™ tube! Enjoy!
ASTRO-TECH from Astronomy Technologies 680 24th Avenue SW, Norman, OK 73069 Astro-Tech is available in the U. S. from these dealers:
astronomics.com ● highpointscientific.com auroraastro.com ● optcorp.com ● camerabug.com sunrivernaturecenter.org ● greatredspot.com apmamerica.com ● tetontelescopes.com In Canada, from: escience.ca ● astronomieplus.com a-p-o.ca In Europe, from: altairastro.com scopemania.eu In Asia, from: kkohki.com Dealer inquiries welcome:
[email protected] Prices as of 1/1/10. All prices plus freight.
10 April 2010 sky & telescope worldmags
in international astronomy programs for several years, I have seen the enormous potential for contributions to astronomy from currently marginalized communities. As co-chair of the 100 Hours of Astronomy Cornerstone Project of the International Year of Astronomy 2009, I found that the most successful efforts were often in the countries with the least resources. It is the passion, determination, and abilities of individuals who make the difference and actually represent the most valuable resource for these efforts. But among these enthusiasts are many students who will never have the opportunity to employ those abilities at high levels of professional astronomy. As Dr. Norman points out, this represents incalculable loss to the world’s scientific community. The need to expand astronomy beyond academia and communicate with the public has been recognized for years, and that recognition and the ensuing effort led to the unprecedented IYA2009, in which 148 nations participated. That effort will expand further with the recent adoption
50 & 25 Years Ago
Write to Letters to the Editor, Sky & Telescope, 90 Sherman St., Cambridge, MA 02140-3264, or send e-mail to
[email protected]. Please limit your comments to 250 words.
of the International Astronomical Union’s 10-year strategic plan, “Astronomy for the Developing World.” Adopted at the General Assembly in Rio de Janeiro in August 2009, the plan calls for the development of astronomy awareness, education, and professional development in countries that now lack adequate programs. A century ago, women were excluded from the male-dominated field of astronomy, and some Europeans saw the United States as little more than an upstart. Imagine the impact on astronomy if those prejudices had prevailed. The continued success of the pursuit of diversity will determine the future of the field. The untapped human resources yet to be discovered are to be ignored at our peril. Mike Simmons Agoura, California
Leif J. Robinson
April 1960 Hemisphere Imbalance “In his address . . . [Dirk] Brouwer discussed some consequences of the fact that the world’s observatories and astronomers are strongly concentrated in the Northern Hemisphere. This unbalance, which leaves the southern skies less well studied, is harmful to observational astronomy, and special measures are necessary to overcome it. . . . “Today the situation in Southern Hemisphere astrometry is alarming, for the Cape Observatory [in South Africa] is the only one below the equator now making fundamental determinations of star positions. This makes it practically impossible to provide adequate checks on possible systematic errors in the positions and proper motions of southern stars.” Brouwer’s specialty was the calculation of orbits, thus his emphasis on positional astronomy at the dawning of the Space Age.
April 1985 Hubble Heritage “Although U. S. officials rarely acknowledge the existence of photo-reconnaissance satellites, these ‘spies in the sky’ have played an indispensable role in military surveillance for more than two decades. Thus, a fair question might be whether NASA’s Hubble Space Telescope is simply an existing satellite redesigned to look up, not down. . . . “NASA and industrial managers [indicate] when interviewed that the Hubble telescope has little direct inheritance from past or current programs. . . . “But it has certainly drawn from state-ofthe-art military advances in detectors, control systems, data encoding, and data transmission — even the methods used to assemble spacecraft of this size.” So wrote space-exploration expert J. Kelly Beatty in an issue whose feature articles were largely devoted to the future Hubble telescope.
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Power Pairs!
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“Not only were the stars pinpoint to the very edges of the field, the contrast was greatly enhanced as well. I was blown away…” —Tippy D’Auria, Founder of the Winter Star Party
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The award-winning 100° Series™ Nitrogen-Purged Waterproof Eyepieces not only give you spectacular hyperwide visual views through your telescope, but they are sealed to protect the internal elements. To celebrate the addition of the new 9mm, you can “Power Pair” with a 9mm and a 14mm or a 9mm and a 20mm to expand your 100° eyepiece collection and save a bundle! To get your Power Pair, visit your Explore Scientific Dealer through June, 2010.
explorescientific.com | 888.599.7597 ©2010 Explore Scientific, LLC
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News Notes
All News Note stories are presented in greater depth, with links to further inforgre mation, at SkyandTelescope.com; search ma for the keyword SkyTelApr10.
This artist’s concept shows the Epsilon Aurigae dust disk, with a single B star inside it, starting to cross the face of the F supergiant.
UV
VISIBLE
NEAR IR
MID-IR
NASA / JPL-CALTECH
Epsilon Aurigae Solved at Last?
FAR IR
1000
10
F Staar D. HOARD (SPITZER SPACE CENTER / CALTECH)
Relative brightness
100
1
Disk
0.1
B Star 0.01
0.1
1
10
100
Wavelength (microns) The entire observed energy output from the Epsilon Aurigae system, from the far ultraviolet to the far infrared (colored line and dots), is now fully accounted for by just three objects: the bright F supergiant star, a single hot B star about 1,000 times dimmer, and the enormous, warm dust disk glowing in the mid- and far infrared.
12 April 2010 sky & telescope worldmags
A group of astronomers claims a breakthrough in the long-standing mystery of Epsilon Aurigae. This 3rd-magnitude star, a type-F supergiant 130,000 times brighter than the Sun, loses half its light for almost two years every 27.1 years — when a nearly opaque dust disk, seen edge-on, slides across its face (S&T: May 2009, page 58). The problem is that the disk must have a massive body in its middle, but nothing is seen. Astronomers are quite sure that the disk’s central object is at least as massive as the F stars, so it ought to shine at least about as brightly. It can’t be totally hidden by the disk itself; the disk is open in the middle, like a flattened doughnut tilted slightly from edge on. We know this because around mid-eclipse, we see part of the F star through the opening. So what is this unseen thing in plain sight? Many ideas have been proposed and ruled out. (No, it can’t be a black hole.) The key that unlocks the mystery, says Donald Hoard of Caltech, is to not assume that the disk means Epsilon Aurigae is a young star system that’s still forming. If instead it’s nearing the end of its life, the bright star can have a much lower mass than has been presumed for it. Even a star with just 2 solar masses can shine with supergiant brilliance in its death throes. This would allow the disk’s central object to have only 6 solar masses, also much lower than formerly presumed. So it could be a normal, garden-variety type-B star with only a couple hundred times the Sun’s luminosity — bright by ordinary standards, but about 1,000 times dimmer than the supergiant and lost in its glare. Epsilon Aurigae began dimming again on schedule last August and reached its minimum brightness in December, where it will more or less remain until March 2011. The difference in Auriga’s familiar
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News Notes
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` Tauri Normally, Epsilon Aurigae is slightly the brightest of the three “Kids” stars south of Capella. But currently it’s only a match for Zeta (ζ) Aurigae, and the change is fairly plain to the eye. Stars are labeled with their visual magnitudes.
pattern (above) has become plain to careful skywatchers. NASA’s Spitzer Space Telescope has confirmed the presence and size of the warm dust disk: it’s 8 astronomical units wide. Far-ultraviolet satellites have found traces of B-star light coming from somewhere in the system. Hoard and his colleagues propose that the disk is material that the B star has gravitationally captured from the dying primary’s thick wind. “All of these intertwined parameters just sort of work out,” says Hoard. He estimates that the system is 10 million years old. If so, we’re catching it at a lucky time (which may be why it’s unique). Over the next thousands of years, the dying F star will puff off most of its remaining mass to form a planetary nebula. But the book is far from closed. Intensive observations continue worldwide, including by amateurs doing long-term photometry and spectroscopy.
Dark Energy News One of the biggest surprises in the history of astronomy was the discovery that, contrary to all expected physics, the expansion of the universe is speeding up. The unknown “dark energy” that’s caus14 April 2010 sky & telescope worldmags
ing this repulsion is strong enough that it amounts to about 74% of the entire matterand-energy budget of the universe (see the pie chart below). More evidence of the dark energy keeps showing up. An international team recently measured its effect right here in our Local Group of galaxies. The Local Group includes the Milky Way, the Andromeda Galaxy, M33, and about 50 dim dwarf galaxies identified so far. The team analyzed measurements of Local Group galaxy motions (radial velocities and distances) with respect to the group’s gravitational center. They found that a boundary exists where the Local Group’s gravity gives way to dark energy’s “antigravity” effect on larger scales. Dwarf galaxies that lie beyond this boundary are moving outward — cosmic acceleration in miniature. Says team member Gene Byrd (University of Alabama), “We found a dark-energy outflow repulsion consistent with that found from studying galaxies that are billions of light-years away.”
74% dark energy 22% dark matter 3.6% intergalactic gas 0.4% stars, etc.
Dark Matter News The mysterious dark matter, which amounts to another 22% or so of the cosmic matter-and-energy density, is also revealing itself more clearly. Astronomers have long known that large galaxies, such as our Milky Way, are centered in larger pools of dark matter, like patches of scum on invisible ponds. A group led by David Law (UCLA) now finds that the Milky Way’s dark-matter halo is not spherical but flattened in three directions into a triaxial shape, sort of like a squashed football. Surprisingly, the flattest side is perpendicular to the Milky’s Way’s starry disk of normal matter.
The team reached this conclusion by analyzing the motions of thousands of stars in the “Sagittarius stream,” a sparse filament that loops all the way around the Milky Way well outside the galaxy’s fringes. The stream is the far-flung remains of a dwarf galaxy that was pulled apart by the Milky Way’s tidal effect during a close encounter. The motions of stars in the different parts of the stream allowed the astronomers to map the gravitational fields exerted by different parts of our dark-matter halo.
Exoplanet News Roundup With more than 420 planets now known outside our solar system, and better instruments yielding ever-finer data, exoplanet specialists have lots of material to digest. Some recent news: Massive Host Stars. A group using the MMT Telescope in Arizona surveyed disks around big young stars that have 1.5 to 15 times the Sun’s mass. These will DAVID A. AGUILAR / CFA settle down to be hot shiners of spectral types F, A, and B when their birth processes are finished. The group found that many such stars have protoplanetary disks, and the disks often show infrared signs of gaps that planets have already cleared in them. Evidently, planet formation is a robust process that occurs around stars over essentially the whole range of stellar masses, from dim red M dwarfs to hot, blue-white B dazzlers. Super-Volcanic Planet From Hell. The only exoplanet known for sure to be rock and metal like Earth is Corot7b, a super-Earth that has Earth’s density and about 5 times Earth’s mass. It orbits so close to L. CALÇADA (ESO) its star, a yelloworange dwarf, that its surface should be roasted to molten-lava temperatures (S&T cover story, May 2009).
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News Notes
For astronomy news as it breaks, see SkyandTelescope.com/newsblog.
light. This discovery method, however, has one big advantage: it’s most sensitive to giant planets that are far from their stars, at roughly Jupiter’s and Saturn’s distance from the Sun. Looking at the statistics so far, a team led by Andy Gould (The Ohio State University) estimates that a third of stars host at least one giant planet in a distant orbit: a “cold Jupiter.” A sixth of stars may have multiple giant planets in distant orbits, as our solar system does. This is
NASA / JPL / UNIV. OF ARIZONA / DLR
good news for the possible formation and survival of terrestrial planets closer in. In related news, a group tracking stars’ radial-velocity wobbles has announced that the Sun-like star 23 Librae seems to have a Jupiter-like planet in a circular orbit at a Jupiter-like distance. Cold Jupiters are just beginning to reach detectability by the wobble method, as technology improves and the time span covered by radial-velocity monitoring lengthens. Planets Everywhere. As for smaller worlds? Exoplanet hunter Paul Butler now says that, based on statistics, it’s likely that half of nearby stars possess at least one detectable planet with Neptune’s mass (17 Earths) or less.
Sunglint from a Titan Lake
Millisecond-Pulsar GPS
NASA
In addition, says Rory Barnes of the University of Washington, it probably has strong internal heating as well, leading to extreme volcanism. The planet orbits its star so closely that if its orbit is even slightly elongated (quite likely, given the presence of a second planet nearby that should perturb it), then Corot-7b will be tidally squeezed and flexed once per orbit. Internal friction will dissipate some of this tidal energy, heating the planet’s interior. Jupiter’s moon Io is heated in a similar way, causing Io’s vigorous volcanism (page 30). But for Corot-7b the effect should be many times greater. Bare Core of a Gas Giant? Another group finds that Corot-7b could be boiling off half an Earth mass of its material every billion years under the heat of its star. Brian Jackson (NASA/Goddard Space Flight Center) concludes that the object may have started life as a gas giant, and that its hydrogen and helium were long ago driven off to leave the present rocky core behind. Water World? A different super-Earth has about the same mass but a larger diameter, as revealed by its transits across its star. GJ 1214b has an average density of only 1.9 grams per cubic centimeter. A good guess is that this planet is water almost all the way through, overlaid by a massive atmosphere. Even though deep planetary interiors are very hot, the high pressures there should force the water into solid form: a hot, dense phase of ice. Familiar Solar Systems. Only 10 exoplanets have been found by their “microlensing” of a distant background star’s
Pulsars are amazing enough; they’re like giant atomic nuclei with more than the Sun’s mass squeezed into a sphere about 20 kilometers (12 miles) wide. Even wilder are the millisecond pulsars, which spin several hundred times per second. Until recently, astronomers knew of only 60 of them floating loose in the Milky Way, but NASA’s Fermi Gamma-ray Space Telescope is providing a treasure map for finding more. Fermi’s first-year map of the high-energy gamma-ray sky recorded 1,451 discrete sources. Radio astronomers following up at these points have picked up signals from at least 17 previously unknown millisecond pulsars. Finding more of them matters. Millisecond pulsars are the most precise “clocks” in nature, matching or beating the best artificial atomic clocks. Enough of them could offer a sensitive, GPS-like timing grid to watch for gravitational waves — ripples in spacetime predicted by Einstein — passing Earth’s vicinity.
This picture of Saturn’s big, smog-covered moon Titan, recently returned by the Cassini probe, is surely destined to be a classic astro-image. Taken in haze-penetrating infrared light, it shows a patch of sunlight reflecting at a very low angle from one of Titan’s large northern lakes. The lakes are a mixture of methane and ethane, similar to liquefied natural gas (pure methane). The case for lakes on Titan was already pretty ironclad, but this certainly clinches it. The reflection is from Kraken Mare, an irregular lake or sea mapped by radar that covers about 400,000 square kilometers, larger than the Caspian Sea on Earth.
Saturn’s Weird Prometheus In nearly six years of orbiting Saturn, Cassini has flown by many of the planet’s 61 moons, no two quite alike. Prometheus, 74 miles (119 km) long and seen in new high resolution below, appears to be thickly mantled with dust from Saturn’s F ring, which it dips in and out of. Note how the craters are fi lled in. Before the first landings on Earth’s Moon in the 1960s, science-fiction writers sometimes set tales on the Moon involving seas of deep, treacherous, fluff y dust. Perhaps these stories will yet play out on a different moon. ✦
NASA / JPL / SPACE SCIENCE INST. / S. WALKER
16 April 2010 sky & telescope worldmags
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