Star with midriff bulge eyed by astronomers
Posted: July 26, 2001

For the first time ever, a star spinning so fast its mid-section is stretched out has been directly measured by an ultra-high-resolution NASA telescope system on Palomar Mountain near San Diego.

Red Spider
The Altair star. Photo: NASA/JPL
"Measuring the shape of this star, Altair, was as difficult as standing in Los Angeles, looking at a hen's egg in New York, and trying to prove that it's oval-shaped and not circular," said Dr. Charles Beichman, chief scientist for astronomy and physics at NASA's Jet Propulsion Laboratory (JPL), Pasadena, CA.

Altair is a well-known member of the Summer Triangle, clearly visible in the summer night sky across the United States. Scientists using the Palomar Testbed Interferometer, which links multiple telescopes, 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 first tip-off that Altair is not perfectly round.

"This surprising observation led to a bit of challenging detective work to properly interpret the data," said principal investigator Dr. Gerard van Belle of JPL. "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."

Previous studies of Altair raised the prospect that the star might have 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, 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. For a star that spins slowly, this effect is miniscule. For example, our Sun rotates once every 30 days and has an equator only .001 percent greater in diameter than its poles.

By measuring Altair's size at separate positions along its edge, van Belle and his colleagues determined that Altair rotates at a speed of at least 210 kilometers per second (470,000 mph) at the equator. Future studies may pin down the speed more precisely.

"Determining the shape of another star helps us learn about the forces that control the shape and structure of all stars, including our star, the Sun," Beichman said. "This tells us more about the Sun's behavior and ultimate fate."

The Palomar Testbed Interferometer has three 50-centimeter (20-inch) telescopes. To study Altair, the telescopes were used two at a time. The combined light from the telescope pairs provided sharpness comparable to a telescope as large as a football field.

"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 co-investigator Dr. David Ciardi of the University of Florida, Gainesville. "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."

The Palomar Testbed Interferometer is paving the way for the Keck Interferometer, Space Interferometry Mission and Terrestrial Planet Finder, all part of NASA's Origins program. The program will hunt for Earthlike planets that might harbor life around other stars. "In the long run, we'll use these interferometric capabilities to search for planets around nearby stars. This is an important first step," said Beichman.

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

Their research was funded by NASA's Office of Space Science, Washington, along with the National Science Foundation. Palomar Observatory is owned and operated by the California Institute of Technology in Pasadena, which manages JPL for NASA. The Palomar Testbed Interferometer was designed and built by a team of JPL researchers led by Drs. Mark Colavita and Michael Shao. Funded by NASA and managed by JPL, the interferometer is located at the Palomar Observatory near the historic 200-inch Hale Telescope.