UF, NASA astronomers eye star with midriff bulge

July 25, 2001

GAINESVILLE, Fla. — Using an ultra-high-resolution telescope, astronomers have observed a star spinning so fast its mid-section is stretched out, a phenomenon that had been suspected but never before measured.

The star, named Altair, is a well-known member of the Summer Triangle and clearly visible in the summer night sky. Using the California-based Palomar Testbed Interferometer, which links multiple telescopes at Palomar Mountain near San Diego, scientists at the University of Florida and NASA measured the star’s radius at different angles on the sky. They noticed the size of the star varied with changing angles, which was the tip-off that Altair is not perfectly round.

“Altair is the twelfth-brightest star in the sky — you’d think that everything there is to know about this star would have been discovered already,” said David Ciardi, an assistant scientist at UF’s astronomy department. “It’s a good example of the surprises you’re going to encounter when you are able to look at even familiar stars with unprecedented resolution.”

Previous studies of Altair raised the prospect that the star might have this type of midriff bulge, but never before had the shape been measured directly. Earlier measurements of the star’s spectrum, or light wave pattern, had hinted that Altair was rotating very fast. When a gaseous orb, like a star, spins fast enough, it tends to expand at the middle, making it look like a beach ball that is squeezed at the top and bottom.

Altair is a perfect example. It rotates at least once every 10.4 hours, and the new Palomar observations reveal the diameter at its equator is at least 14 percent greater than at its poles. On the other hand, if a star spins slowly, this effect is miniscule. A good example is our Sun, which rotates once every 30 days and has an equator only .001 percent greater in diameter than its poles.

The Palomar Testbed Interferometer uses three 20-inch telescopes in pairs to simulate the resolution of a much larger single telescope. To study Altair, one of the telescopes was joined with a twin 360 feet to the south for some observations, and with a second twin 280 feet to the southeast for others. Each of the two pairs provided resolution equivalent to a telescope as big as a football field. By measuring Altair’s size at separate positions along its edge, Ciardi and Gerard van Belle of NASA’s Jet Propulsion Laboratory in Pasadena determined that Altair rotates at a speed of at least 470,000 mph at the equator. Future studies may pin down the speed more precisely.

“We measured the size of another star, Vega, at the same time, which didn’t change with angle, so we knew this wasn’t just a fluke of the telescope,” van Belle said.

The Palomar Testbed Interferometer is paving the way for such missions as the Keck Interferometer, Space Interferometry Mission and Terrestrial Planet Finder that are part of NASA’s Origins program. The program will hunt for Earthlike planets that might harbor life around other stars.

“The development of these interferometric capabilities is, in the long run, one we’ll apply to search for planets around nearby stars. This is an important first step,” said Charles Beichman, chief scientist for astronomy and physics at JPL.

Van Belle and Ciardi co-authored the Altair paper, scheduled to appear in the Oct. 1 issue of the Astrophysical Journal, with Elizabeth Lada of UF; Robert Thompson of JPL and the University of Wyoming, Laramie; and Rachel Akeson of the JPL/Caltech Infrared Processing and Analysis Center in Pasadena, Calif.

Their research was funded by NASA’s Office of Space Science in Washington, D.C., and the National Science Foundation. More information on the Palomar Testbed Interferometer is available at http://huey.jpl.nasa.gov/palomar. More information on NASA’s Origins Program is available at http://origins.jpl.nasa.gov.