Astronomers: 'Bullet star' shines 350 times brighter than the sun(Forwarded)

Georgia State University

CONTACT:
Hal McAlister, Center for High Angular Resolution Astronomy
404/651-1390

Robin Tricoles, University Relations
404/651-3574

Jan. 19, 2004

Astronomers: 'Bullet star' shines 350 times brighter than the sun

ATLANTA -- For decades, scientists have observed that Regulus, the brightest
star in the constellation Leo, spins much faster than the sun. But thanks to a
powerful new telescopic array, astronomers now know with unprecedented clarity
what that means to this massive celestial body.

A group of astronomers, led by Hal McAlister, director of Georgia State
University's Center for High Angular Resolution Astronomy, have used the
center's array of telescopes to detect for the first time Regulus' rotationally
induced distortions. Scientists have measured the size and shape of the star,
the temperature difference between its polar and equatorial regions, and the
orientation of its spin axis. The researchers' observations of Regulus represent
the first scientific output from the CHARA array, which became routinely
operational in early 2004.

Most stars rotate sedately about their spin axes, McAlister says. The sun, for
example, completes a full rotation in about 24 days, which means its equatorial
spin speed is roughly 4,500 miles per hour. Regulus' equatorial spin speed is
nearly 700,000 miles per hour and its diameter is about five times greater than
the sun's. Regulus also bulges conspicuously at its equator, a stellar rarity.

Regulus' centrifugal force causes it to expand so that its equatorial diameter
is one-third larger than its polar diameter. In fact, if Regulus were rotating
about 10 percent faster, its outward centrifugal force would exceed the inward
pull of gravity and the star would fly apart, says McAlister, CHARA's director
and Regents Professor of Astronomy at Georgia State.

Because of its distorted shape, Regulus, a single star, exhibits what is known
as "gravity darkening" -- the star becomes brighter at its poles than at its
equator -- a phenomenon previously only detected in binary stars. According to
McAlister, the darkening occurs because Regulus is colder at its equator than at
its poles. Regulus' equatorial bulge diminishes the pull of gravity at the
equator, which causes the temperature there to decrease. CHARA researchers have
found that the temperature at Regulus' poles is 15,100 degrees Celsius, while
the equator's temperature is only 10,000 Celsius. The temperature variation
causes the star to be about five times brighter at its poles than at its
equator. Regulus' surface is so hot that the star is actually nearly 350 times
more luminous than the sun.

CHARA researchers discovered another oddity when they determined the orientation
of the star's spin axis, says McAlister.

"We're looking at the star essentially equator-on, and the spin axis is tilted
about 86 degrees from the north direction in the sky," he says. "But, curiously
enough, the star is moving through space in the same direction its pole is
pointing. Regulus is moving like an enormous spinning bullet through space. We
have no idea why this is the case."

Astronomers viewed Regulus using CHARA's telescopes for six weeks last spring to
obtain interferometric data that, combined with spectroscopic measurements and
theoretical models, created a picture of the star that reveals the effects of
its incredibly fast spin. The results will be published this spring in The
Astrophysical Journal.

The CHARA array, located atop Mt. Wilson in southern California, is among a
handful of new "super" instruments composed of multiple telescopes optically
linked to function as a single telescope of enormous size. The array consists of
six telescopes, each containing a light-collecting mirror one meter in diameter.
The telescopes are arranged in the shape of a "Y," with the outermost telescopes
located about 200 meters from the center of the array.

A precise combination of the light from the individual telescopes allows the
CHARA array to behave as if it were a single telescope with a mirror 330 meters
across. The array can't show very faint objects detected by telescopes such as
the giant 10-meter Keck telescopes in Hawaii, but scientists can see details in
brighter objects nearly 100 times sharper than those obtainable using the Keck
array. Working at infrared wavelengths, the CHARA array can see details as small
as 0.0005 arcseconds. (One arcsecond is 1/3,600 of a degree, equivalent to the
angular size of a dime seen from a distance of 2.3 miles.)

In addition to Georgia State researchers, the CHARA team includes collaborators
from the National Optical Astronomy Observatories in Tucson, Ariz., and NASA's
Michelson Science Center at the California Institute of Technology in Pasadena.

The CHARA array was constructed with funding from the National Science
Foundation, Georgia State, the W. M. Keck Foundation, and the David and Lucile
Packard Foundation. The NSF also has awarded funds for ongoing research at the
CHARA array.

IMAGE CAPTION:
[http://www.nsf.gov/od/lpa/newsroom/i...FCCAkCaiyR.jpg (14KB)]
A computer-generated model of Regulus based on CHARA data, with a similar model
of the Sun shown to scale. This view shows the model almost equator-on, with its
axis tilted 86 degrees from celestial north -- just as Regulus appears in the
sky. That makes the axis almost horizontal in this picture, and the equator
almost vertical. The star's frenetic spin has distorted it into the shape of a
squashed beach ball. Credit: Wenjin Huang, CHARA, Georgia State University