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A Distant Sun, Revealed

1 June 2007 (All day)
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Ming Zhao/University of Michigan

Alien sun.
This plain-looking blue globe is an actual false-color image of the star Altair, with its axis of rotation superimposed.

Although still a bit blobby to the untrained eye, astronomers have captured the most detailed images yet of a star outside our solar system. The breakthrough could pave the way to observing solar systems in the process of forming--and even visualizing extrasolar planets--years ahead of the launch of spacecraft specifically designed for such missions.

Despite advances in telescope technology, stars have remained mysterious pinpoints of light. The best instruments have teased out some details about the sun's galactic neighbors--such as the gravitational pull and dimming effects of planets, and the motion of dust and gas orbiting around them (ScienceNOW, 28 September 2006)--but they have not been able to obtain any direct images. The only exception has been supergiant Betelgeuse, about 427 light-years away in the constellation Orion, whose diameter is so large it would extend nearly to the orbit of Mars. But even those images look like little more than bright blobs.

Hoping to snap a better picture of a distant sun, an international team of astronomers focused its attention on Altair, a star only about twice as large as our sun and a mere 17 light-years away. Even so, the challenge is like taking a photo of a child's plastic swimming pool on the moon. Yet that's exactly what the researchers accomplished employing the four-telescope CHARA array on Mount Wilson in California. Using a new instrument called the Michigan Infrared Combiner, or MIRC, which collects light from all four telescopes via fiber optics, the team created a virtual telescope 20 times bigger than any single telescope on the planet and pointed it in Altair's direction.

The resulting images show a whirling star, shaped more like a pumpkin than a basketball. It sports a large, dark bulge at its equator, something existing computer models did not predict. This puzzling feature could be connected to Altair's spin rate, which the images reveal is 90% of the velocity required to break the star apart. The bulge is about 20% farther from the star's core than the rest of its surface. That compares with Mount Everest, which protrudes from Earth's surface by only 0.025%. The big bulge could be cooler and therefore darker in the infrared than expected by the models, says MIRC developer John Monnier, an astronomer at the University of Michigan, Ann Arbor, and lead author of the study, which appeared online yesterday in Science.

Monnier suspects that rapid spinning might be quite common among stars, many of which, like Altair, lack a strong magnetic field, something that puts the brakes on rotational speed. "I think the real question is why [our] sun is rotating so slowly," he says. Next, Monnier's team plans to use MIRC to look at binary star systems and eventually hopes to scrutinize large extrasolar planets.

Actual images of stars represent a significant advance, says astrophysicist David Buscher of Cambridge University in the United Kingdom. Unlike spectral data, for example, astronomers can interpret them directly, without relying on models. Buscher agrees that the new technology could be used eventually to image extrasolar planets, but he says "a much more immediately enticing prospect" is imaging the disks of planetary systems in the process of forming, something traditional telescopes can't do.

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