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The Star That Thinks It's a Comet

15 August 2007 (All day)
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NASA/JPL-Caltech/POSS-II/DSS

Seeding cloud.
Seen in ultraviolet light, the red giant star MIRA (bright dot at right) sports a cometlike tail stretching more than 13 light-years.

One of the hundreds of billions of stars in the Milky Way seems to be having an identity crisis. New images from NASA's Galaxy Evolution Explorer (GALEX) show the star trailing a tail 13 light-years long, making it look like a gigantic comet. Astronomers think the tail will help them probe the star's evolution over the past 30,000 years, and the discovery raises the possibility that other stars sport such features.

It may seem ordinary, but our local neighborhood of stars actually teems with unusual objects. Take Mira, a relatively cool, older star known as a red giant, which lies 350 light-years away in the constellation Cetus. Astronomers first spotted Mira in 1596 and noted its unusual ability to disappear and reappear every 332 days. Earlier this year, an American team took a closer look at Mira with the GALEX spacecraft, which is in the process of mapping the entire sky as it appears in ultraviolet (UV) light.

The astronomers noticed that one of the images appeared smudged. Further inspection revealed that the star was trailing a tail of material some 13 light-years long--or more than three times the distance between the sun and Proxima Centauri, its nearest neighbor. This is the first time anyone has seen a star with a tail, which spectral analysis reveals comprises mainly carbon, nitrogen, and oxygen molecules--some of the elements necessary for life. The atoms in our bodies could have been produced by a star just like Mira, says astronomer Michael Shara of the American Museum of Natural History in New York City. "We're seeing [the material] implanted across the galaxy right now!"

So why the long tail? Although Mira contains only about as much mass as the sun, its diameter is about 400 times greater, meaning that its gravity has a hard time holding on to its atmosphere. Moreover, Mira's course has been taking it through a relatively dense zone of interstellar space, and its interaction with molecules of gas and dust creates a bow-shock effect, similar to what happens when a bullet or a supersonic aircraft travels through air. Material from the bow shock gets heated by friction and then is whipped around and behind the star. Finally, Mira is streaking across the galaxy at 130 kilometers per second, more than twice as fast as most other stars, the team reports online today in Nature. For reasons not yet fully understood, the tail can be seen only in UV.

Based on Mira's speed and the size of its tail, the star has been sloughing off material for at least 30,000 years, says co-author Christopher Martin, NASA's principal investigator for the GALEX mission. "We believe this record will tell us how stars age over time," Martin told reporters during a teleconference today. Red giants like Mira are quite common, notes Shara, so it's probably not the only comet imposter out there. "There must be lots more of these things" in the galaxy, he says.

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