Donald Savage
Headquarters, Washington, DC              July 25, 2001
(Phone:  202/358-1547)

Jane Platt 
Jet Propulsion Laboratory , Pasadena, CA
(Phone: 818/354-0880)

RELEASE:  01-150

STAR WITH MIDRIFF BULGE EYED BY ASTRONOMERS

     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.

"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. 

Images and animation of Altair are available at:
     http://www.jpl.nasa.gov/images/stars/index.html
Information on the Palomar Testbed Interferometer is 
available at:
            http://huey.jpl.nasa.gov/palomar
Information on NASA's Origins Program is available at:    
               http://origins.jpl.nasa.gov
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