UCB Astronomer Salvages Disabled NASA Satellite for Stellar Research
July 27, 1999
FOR IMMEDIATE RELEASE<
Contact: Robert Sanders
UC Berkeley Public Information Office
510-643-6998
rls@pa.urel.berkeley.edu
BERKELEY, CA: When the Wide-Field Infrared Explorer (WIRE) satellite went haywire early this year, NASA and the science community wrote it off as a complete and costly loss. Where NASA saw disaster, however, Derek Buzasi saw opportunity. He immediately approached NASA about using the one instrument aboard the spacecraft that still worked, a measly two-inch telescope originally intended to aid in pointing the satellite. While a two-inch diameter telescope on the ground is little more than a toy, in space and above Earth’s atmosphere it is capable of doing real astronomical research, reasoned Buzasi, a research physicist at the Space Sciences Laboratory at the University of California, Berkeley. He was right. Not long after NASA gave him access, he was able to observe a nearby cool star and record starquakes--the first time anyone has observed such vibrations on a normal, cool star. “People have been trying to do this for 30 years,” Buzasi said.
Detailed recording of the frequency and amplitude of brightness changes that accompany starquakes can give astronomers a wealth of information, from the star’s mass and age to its chemical composition deep into the core. “It is a spectacular thing he has done,” praised Pierre Demarque, professor emeritus of astronomy at Yale University. “What he has done can only be done from space--it’s a first. It is the beginning of a new field of stellar astronomy: stellar seismology.”
Buzasi has submitted two papers about his technique and the discovery of starquakes on the star, alpha Ursae Majoris, to the Astrophysical Journal. Alpha Ursae Majoris is a bright yellow giant sitting at the lip of the Big Dipper, known since ancient times by the Arabic name Dubhe. Part of a two-star system, it lies about 100 light years from Earth, or about 600 trillion miles.
“It’s a really nice instrument,” Buzasi said about the star tracker, “I was lucky that WIRE retained the full capability of the star tracker and that the CCD camera attached to it is better than most star trackers need. And WIRE points amazingly well.” NASA has agreed to let him use WIRE’s star tracker through October at least. Buzasi is now the sole user of the $73 million satellite, after NASA finished some engineering studies of the spacecraft last month.
His next target is one of Earth’s nearest neighbors, alpha Centauri, 4.4 light years distant and a yellow star similar to our own sun. “For us this is the most interesting star,” said Demarque, who is eager to get Buzasi’s data to compare to predictions from his model of how stars evolve. “We have made lots of calculations about alpha Centauri in hopes someday someone would make these observations.” Two satellites set to fly in 2001 will carry instruments to look at stellar pulsations: the French satellite COROT, designed to study stellar COnvection and ROTation as well as search for new planetary systems, and the Canadian satellite MOST. Neither would have been able to look at alpha Centauri, however. “Derek has scooped both of them,” Demarque said.
To study starquakes, Buzasi has only to measure the brightness fluctuations of the star, taking measurements ten times each second. However, to pluck the periodic variations out of the noise, which is a thousand times brighter than the fluctuations, he needs to look for a long time, on the order of a month. A typical ground- or space-based instrument would never have enough available time to allow this. Faced with such difficulties, Buzasi saw the tracking telescope aboard WIRE as ideal. Plus it is faster for this purpose than many ground telescopes, including the world’s largest, the 10-meter Keck Telescope in Hawaii. “A two-inch aperture above the atmosphere can be better than ten meters below it,” he said.
Scientists first observed periodic vibrations or solar quakes on the sun in the 1960s, and today they have a very detailed knowledge of its regular pulsations. These vibrations are like the ringing of a bell, and the longer the period the deeper into the sun’s interior the pulsation reaches. By studying the brightness fluctuations at many frequencies, it is possible to reconstruct the interior composition and movement all the way down to the core. The field of helioseismology has provided nearly all that’s known today about the internal composition of the sun, its internal rotation and dynamics.“This is the only way we can see inside a star, and thus the only way to test various models of the interior of the sun,” Buzasi says.
For the past thirty or more years, astronomers have been trying to measure similar periodic vibrations in stars. To date, asteroseismology has succeeded with fewer than 50 stars, all of them atypical, unstable and rapidly pulsing stars, or with white dwarfs, which can have very large oscillations in brightness. Now that Buzasi has been able to measure vibrations in a normal star, he is collaborating with astrophysicists at Yale, who have created a computer program to model stellar evolution.
Demarque and colleague David Guenther, associate professor of astronomy at Saint Mary’s University in Halifax, Nova Scotia, have been building models of stellar interiors for years in hopes of getting data like this for comparison. “We calculate the evolutionary track of stars from their beginning until it matches their current observed brightness and temperature,” he said. “With alpha Ursae Majoris we weren’t too optimistic, but we were able to reproduce most of the vibration modes he (Buzasi) observed.” Demarque, Guenther and Buzasi estimate the star’s mass at 4.25 times that of Earth’s sun, with a precision far greater than could be obtained by other means.
The masses of most stars are unknown. Only when they are part of a binary star system, as is alpha Ursae Majoris, can astronomers use the principles of mechanics to estimate their masses. Alpha Ursae Majoris was estimated before to be between four and five times as massive as the sun. “With Derek’s data we derived the mass independently. That’s exciting!” Demarque said.
WIRE, put together by scientists at the Jet Propulsion Laboratory in Pasadena, was launched by NASA March 4, 1999 into a perfect orbit. It was outfitted with a 30-centimeter, cryogenically cooled infrared imaging telescope designed to gather information on “starburst ” galaxies, those in which rapid star formation is taking place. It also was to study infant galaxies known as protogalaxies. Scientists had expected to detect at least 50,000 galaxies during a four-month survey of the sky. Unfortunately, the telescope was somehow uncovered early and the solid hydrogen needed to cool the telescope evaporated in the heat of the sun. Without the ability to cool the infrared telescope, it was useless. The mission was declared a failure March 8. When Demarque was asked recently by a senior advisor to NASA whether it was worth the money to let Buzasi continue to use the WIRE telescope, he was unequivocal. “It is well worth a million dollars,” he said. “Derek has made his point, his observations have been successful beyond all expectations.”
Editor’s Notes:
Derek Buzasi can be reached at 510-643-8244 or dbuzasi@ssl.berkeley.edu.
Pierre Demarque can be reached at the Center for Solar and Space Research at Yale University, 203-432-3024 or demarque@astro.yale.edu.
For more information on WIRE, check the web page at http://www.ipac.caltech.edu/wire/.