Press Release

Star with midriff bulge eyed by astronomers

By SpaceRef Editor
July 25, 2001
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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

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:

Information on the Palomar Testbed Interferometer is
available at:

Information on NASA’s Origins Program is available at:

SpaceRef staff editor.