FOCUS ON THE X-RAY SKY Page 16 MARCH/APRIL 2012 $5 Scaling the Universe Celebrating the centennial of a celestial yardstick THE UNIVERSITY OF TEXAS AT AUSTIN Mc DONALD OBSERVATORY S TAR D ATE 1 STARDATE STAFF EXECUTIVE EDITOR Damond Benningfield EDITOR Rebecca Johnson ART DIRECTOR MARCH/APRIL 2012 • Vol. 40, No. 2 C.J. Duncan TECHNICAL EDITOR Dr. Tom Barnes CONTRIBUTING EDITOR Alan MacRobert CIRCULATION MANAGER Paul Previte FEATURES DEPARtmENts MARKETING MANAGER Vincent Perez, III McDONALD OBSERVATORY MERLIN 3 ASSISTANT DIRECTOR, 4 Henrietta & the Cepheids EDUCATION AND OUTREACH Sandra Preston A hundred years ago, one woman’s patience and tenacity in pouring SKY CALENDAR MARCH/APRIL 10 STARDATE ADVISORY BOARD over photographic plates packed with Dr. David Lambert, Director stars changed our view of the cosmos THE STARS IN MARCH/APRIL 12 McDonald Observatory By Barbara Ryden Dr. George F. Benedict Dr. Karl Gebhardt ASTROMISCELLANY 15 Dr. Daniel T. Jaffe 16 A New Focus on the X-Ray Sky ASTRONEWS 20 For information about StarDate or other NuSTAR will scan the heavens to programs of the McDonald Observatory Education and Outreach Office, contact better understand high-energy X-ray Five Moons in Four Days us at 512-471-5285. For subscription phenomena orders only, call 800-STARDATE. Two Ways to Make a Star Go Boom StarDate (ISSN 0889-3098) is published By Rebecca Johnson Ready, Aim, Fire! bimonthly by the McDonald Observatory Don’t Miss the Big Show Education and Outreach Office, The Uni- versity of Texas at Austin, 1 University Milky Way Teeming with Billions of Planets Station A2100, Austin, TX 78712. © 2012 18 Join the Club! The University of Texas at Austin. Annual Bedding Down for a Long Winter’s Nap subscription rate is $24 in the United Astronomy clubs provide opportu- States, $29 in Canada and Mexico, $40 nities for learning, friendship, and for foreign addresses. Subscriptions may be paid for using credit card or money volunteering orders. The University of Texas cannot accept checks drawn on foreign banks. By Linton G. Robertson Direct all correspondence to StarDate, The University of Texas at Austin, 1 Univer- sity Station A2100, Austin, TX 78712, or call 512-471-5285. POSTMASTER: Send change of address to StarDate, The Univer- On The Cover sity of Texas at Austin, 1 University Station Henrietta Swan Leavitt’s studies A2100, Austin, TX 78712. Periodicals Postage Paid at Austin, TX. StarDate is a of variable stars in the Small registered trademark of The University of I Magellanic Cloud, seen here in c Texas McDonald Observatory. a recent composite image from the Herschel and Spitzer infrared Visit StarDate Online at space telescopes, changed how we stardate.org measure the cosmos. To learn how, see Page 4. ESA/NASA/JPL-CALTECH/STS This Page Coming Up Comet Lovejoy, which survived a plunge near The May/June issue will bring you detailed the surface of the Sun, rises above Earth’s information on the May 20 solar eclipse for atmosphere in this December 22 view from the the western United States, as well as the International Space Station. Lovejoy passed scoop on the transit of Venus in early June . StarDate less than one million miles from the Sun on We’ll also bring you Merlin’s answers to your StarDate Magazine December 15. The encounter stripped away its tough astronomy questions and the latest Frank N. Bash Visitors Center original tail but left the comet itself intact. A new astronomy news. NASA tail quickly sprouted as more ice at Lovejoy’s surface vaporized. 2 MARCH/APRIL 2012 S TAR D ATE 2 MERLIN Dear Merlin, Actually, over the last My husband and I disagree few decades, the Sep- about the motion of the Interna- tember equinox, which tional Space Station across the marks the beginning of night sky. I think it is not moving autumn in the northern but fixed at a place in orbit and it hemisphere, has most is the spinning Earth that makes often fallen on the 22nd. it look like it is moving. My hus- The difference is caused band thinks it is propelled. by the difference between Tina Lipman the calendar year (either Port Angeles, Washington 365 or 366 days) and the true year, which is The space station is in a low 365 days, 5 hours, 48 orbit (about 230 miles/370 LUNDSTRÖM LAYNE minutes, and 45 seconds km), so it is, in fact, its own long. motion that you are seeing fast airplane. It shines almost with a large core made of iron On average, the time (more than 17,000 miles per as brightly as Venus, the bril- and nickel. It’s reasonable to of the equinox advances by hour). For it to remain at a liant morning or evening star. assume that it has smatter- those extra few hours and “fixed” point in the sky, it ings of other heavy metals, minutes each year (the exact would have to be in geosyn- Dear Merlin, including gold, silver, and timing varies somewhat be- chronous orbit, at an altitude I understand that the surface platinum. The planet has also cause of the gravitational of 22,300 miles (35,700 km). of Mercury has some of the most had abundant volcanic activ- pull of the Moon and the If that were the case, though, valuable real estate in the solar ity (and may have some on- other planets in the solar sys- it would appear to remain at system, its surface covered with going activity today), which tem). Leap Year essentially a fixed point in the sky, al- heavy metals like gold, silver, and is involved in concentrating pushes the equinox back to though it would be moving platinum. Why are these metals such elements. its starting position every in orbit around Earth at the so abundant on Mercury yet rare Yet the amounts of these four years. So today, for three same rate as Earth’s rotation on Earth? And how do I stake a elements, their distribution, consecutive years the equi- on its axis. claim in “them thar hills?” and many other factors are nox occurs on the 22nd for It’s not correct to say that Roy completely unknown. The the Lower 48 states, then it the station is propelled, how- Reading, Pennsylvania MESSENGER spacecraft is jumps to the 23rd the year ever; it is “falling” around studying Mercury’s composi- before Leap Year, as it did in Earth, with its orbital speed Merlin suggests holding off tion from orbit, but it doesn’t 2011. keeping it in space. Astro- on filing a claim just yet. First, have the ability to detect indi- This arrangement doesn’t nauts do occasionally fire its such things are subject to in- vidual deposits of interesting completely balance the books, thrusters to tweak its orbit, ternational treaties, which metals or other materials. A however. If uncorrected, it which decays as the station means lots of paperwork. more detailed understanding would cause the date of the passes through thin wisps of And second, no one has yet will have to wait for orbiters equinox to move about one the atmosphere. measured the composition of with more sensitive instru- day earlier every 128 years. For those who haven’t Mercury in enough detail to ments, or, better yet, landers (In the first half of the 20th seen it, the station can ap- determine how much of these that can actually dig into the century, for example, the pear shortly before sunrise or precious metals it contains. surface to see what’s there. equinox occurred most fre- after sunset, moving across Like Earth, Mercury is quently on the 23rd, not the the sky fairly quickly, like a a dense, metal-rich world, Dear Merlin, 22nd.) The current calen- I have noticed over the years dar system drops Leap Year SEND QUESTIONS TO that the first day of autumn was in “century” years (such as Merlin generally either September 21 1800 or 1900), unless those StarDate or 22. In 2011, however, it was years are divisible by 400 Merlin is unable to send per- University of Texas at Austin September 23. What causes this (such as the year 2000). sonal replies. Answers to many 1 University Station, A2100 variation? For the most part, these ad- astronomy questions are avail- Austin, TX 78712 able through our web site: [email protected] Jerry Hequembourg justments keep the September stardate.org/astro-guide stardate.org/magazine Eastham, Massachusetts equinox on the 22nd or 23rd. S TAR D ATE 3 HARVARD COLLEGE OBSERVATORY (2) COLLEGE OBSERVATORY HARVARD Above: Henrietta Swan Leavitt, about 1910. Background image: This 1905 view of the Large Magellanic Cloud (LMC) taken at Harvard’s Boyden Station in Arequipa, Peru, is one of many Leavitt studied in her search for Cepheid variable stars. Today we know the LMC is a satellite galaxy of the Milky Way. 4 MARCH/APRIL 2012 S TAR D ATE 4 century ago this month, a human computer announced a finding that eventually expanded the size of our universe as much as Galileo’s first telescopic look at the stars. A hundred years ago, It was on March 3, 1912, that astronomer one woman’s patience Henrietta Swan Leavitt’s finding about the and tenacity in pouring behavior of a specific kind of pulsing star was published in the Harvard College Observatory over photographic plates Circular. Her finding allowed astronomers packed with stars changed to discover that the Milky Way is not the whole of the universe, and today remains our view of the cosmos an important rung on the “extragalactic distance ladder” that allows astronomers to measure distances across the cosmos.