Astronomers capture the first image of surface features on a sun-like star (Forwarded)

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May 31, 2007

U-M astronomers capture the first image of surface features on a sun-like
star

ANN ARBOR, Mich. -- University of Michigan astronomers combined light from
four widely separated telescopes to produce the first picture showing
surface details on a sun-like star beyond our solar system.

The image of the rapidly rotating, hot star Altair is the most detailed
stellar picture ever made using an innovative light-combining technique
called optical interferometry, said U-M astronomer John Monnier.

Beyond this technical milestone, the Altair observations provide
surprising new insights that will force theorists to revise ideas about
the behavior of rapid rotators like Altair.

"This powerful new tool allows us to zoom in on a star that's a million
times farther away than the sun," said Monnier, lead author of a paper to
be published online Thursday by the journal Science. "We're testing the
theories of how stars work in much more detail than ever before."

Monnier and U-M graduate student Ming Zhao led an international team that
made the Altair observations using four of the six telescopes at Georgia
State University's Center for High Angular Resolution Astronomy (CHARA)
interferometric array on Mount Wilson, Calif.

The four telescopes were separated by nearly 300 yards. Vacuum tubes
carried starlight from the four scopes to a U-M built device called the
Michigan Infrared Combiner, known as MIRC.

The combiner allowed researchers to merge infrared light from four of
CHARA's telescopes for the first time, simulating a single giant
instrument three football fields across. The result was an image of
unprecedented detail -- roughly 100 times sharper than pictures from the
Hubble Space Telescope.

While solar astronomers can view sunspots and storms on our home star's
roiling surface in exquisite detail, most other stars have -- until very
recently -- appeared as simple points of light through even the most
powerful telescopes.

But in the past decade, advances in the emerging field of optical
interferometry have launched a new era of stellar imaging.

Other research teams have used the technique to acquire surface images of
giant stars hundreds of times bigger than Altair. But the U-M-led study
provides the first picture of a so-called main sequence star, one that
generates energy mainly from hydrogen-to-helium nuclear fusion reactions
in its core. Main sequence stars include the sun and most of the stars we
see in the night sky.

"This is just a monumental stepping stone for us," said Harold McAlister,
director of CHARA and a regent's professor of astronomy at Georgia State
University in Atlanta. "Main sequence stars are far and away the largest
population of stars out there, and being able to make a picture of one
creates tremendous opportunities for future research."

One likely target for future studies: Imaging planets around stars beyond
our solar system, said U-M's Zhao. "Imaging stars is just the start," he
said.

Altair is the brightest star in the constellation Aquila, The Eagle, and
is clearly visible with the naked eye in Northern Hemisphere skies. The
nearby star is hotter and younger than the sun and nearly twice its size.
Altair rotates at 638,000 mph at its equator, roughly 60 times faster than
our home star.

"It's really whipping around and that's why, of course, it's spread out
like a twirling ball of pizza dough," said Monnier, an assistant professor
of astronomy.

Previous studies revealed that Altair, unlike most stars, is not a perfect
sphere. Instead, its rapid spin rate creates centrifugal forces that
flatten it into an oval: Its radius is significantly larger at the equator
than at the poles.

In 1924, astronomer Hugo von Zeipel predicted that rapid rotators would
display just this type of equatorial bulge. He also surmised that these
stars would sport a dark band along the equator called gravity darkening.
The bloated equator would appear dark because it is farther from the
star's fiery nuclear core, and therefore cooler than the poles.

The CHARA picture of Altair, the result of observations made on two nights
last summer, is "the first image of a star that allows us to visually
confirm that basic idea" of gravity darkening, Monnier said. But the
Altair image displays even more equatorial darkening than standard models
predict, pointing to flaws in current models, he said.

U-M astronomer David Berger was a co-author of the Science paper. The team
also included researchers from St. Andrews University, Cambridge
University, Georgia State University, California Institute of Technology,
Cornell University, the Laboratoire d'Astrophysique de Grenoble in France,
the Michelson Science Center, and the National Optical Astronomy
Observatory. Funding for the Altair project was provided by the National
Science Foundation and NASA.

Related Links:

* Animation and images
http://www.astro.lsa.umich.edu/~monnier/Local/altair2007.html


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