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Astronomers discover a stellar neighbor

By PHIL BERARDELLI, UPI Deputy Science and Technology Editor

GREENBELT, Md., May 20 (UPI) -- Astronomers said Tuesday they have found a new star that might be the third closest to the sun.

The star, given the unexciting but temporary name of SO25300.5+165258, is a faint red dwarf estimated to be only about 7.8 light-years from Earth, in the direction of the constellation Aries.

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"Our new stellar neighbor is a pleasant surprise, since we weren't looking for it," said Bonnard Teegarden, an astrophysicist at NASA's Goddard Space Flight Center.

If its distance estimate is confirmed, it would place the new star slightly farther than Alpha Centauri -- actually a group of three stars about 4 light-years away -- and Barnard's star, about 6 light-years away.

One light-year is almost 6 trillion miles or nearly 9.5 trillion kilometers.

The tiny new star has only about 7 percent of the mass of the sun and is 300,000 times fainter, which explains why something so near, relatively speaking, to our solar system has gone undetected despite nearly 300 years of astronomical observations.

"We discovered this star in September 2002 while searching for white dwarf stars in an unrelated program," Teegarden said, explaining the team actually was looking for white dwarf stars that move rapidly across the sky.

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Such stars are called High Proper Motion objects, or HPMs. Astronomers search for them by analyzing successive images of an area of sky and watching objects that shift their positions noticeably while their surroundings remain fixed.

Teegarden and colleagues from NASA's Jet Propulsion Laboratory in Pasadena, Calif., found the star using images from the Near Earth Asteroid Tracking program. NEAT is run by JPL to search for asteroids that might be on a collision course with Earth. Like HPM stars, asteroids reveal themselves in successive images when they shift their position against static background stars. NEAT's automated telescopes have accumulated thousands of such images.

Because the same HPM can be exhibited by either a distant star moving quickly or a slower moving nearby star, astronomers need other measurements to determine an object's distance from Earth. So when SO25300.5+165258 was discovered among the NEAT images, Teegarden's team used a technique called trigonometric parallax, which is employed to calculate the distances of relatively close stars. As the Earth progresses in its orbit around the sun, the position of a nearby star will appear to shift compared to background stars much farther away -- the larger the shift, the closer the star.

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The team refined their initial distance estimate with another technique called photometric parallax. Using the 3.5-meter Astrophysical Research Consortium telescope at Apache Point Observatory in Sunspot, N.M., they separated the star's light into its component colors for analysis. This allowed the team to determine what kind of star it is. The analysis indicates although SO25300.5+165258 is much smaller, it is similar to our sun because it shines by fusing hydrogen atoms in its core.

Once the type of star is known, astronomers can determine its true brightness, called intrinsic luminosity. Because all light-emitting objects appear dimmer as they get farther away, the team refined their distance estimate by comparing how bright the new star appeared in their images to its intrinsic luminosity.

Although SO25300.5+165258 is a red dwarf, it actually appears three times dimmer than expected, the team reported. Therefore the star is either a bit farther away than estimated, or it could have unusual properties that make it shine less brightly than similar red dwarfs.

The U.S. Naval Observatory in Washington, D.C., is taking a more precise measurement of the new star's position to establish its distance. The measurement will either confirm or refute its status as one of the sun's closest neighbors later this year.

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"Since the NEAT survey only covered a (narrow) band of the sky ... it is entirely possible that other faint nearby objects remain to be discovered," Teegarden added.


(Editors: UPI photo WAX2003052001 is available)

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