Jane Platt (818) 354-0880
Jet Propulsion Laboratory, Pasadena, Calif.

William Steigerwald (301) 286-5017
Goddard Space Flight Center, Greenbelt, Md.

The local celestial neighborhood just got more crowded with a
discovery of a star that may be the third closest to the Sun. The
star, “SO25300.5+165258,” is a faint red dwarf star estimated to be
about 7.8 light-years from Earth in the direction of the constellation
Aries.

“Our new stellar neighbor is a pleasant surprise, since we weren’t
looking for it,” said Dr. Bonnard Teegarden, an astrophysicist at
NASA’s Goddard Space Flight Center, Greenbelt, Md. Teegarden is lead
author of a paper announcing the discovery to be published by the
Astrophysical Journal. This work has been done in close collaboration
with Dr. Steven Pravdo of NASA’s Jet Propulsion Laboratory, Pasadena,
Calif.

If its estimate of distance is confirmed, the newfound star will be
the Sun’s third-closest stellar neighbor, slightly farther than the
Alpha Centauri system, actually a group of three stars a bit more than
four light-years away, and Barnard’s star, about six light-years away.
One light-year is almost six trillion miles, or nearly 9.5 trillion
kilometers.

The new star has only about seven percent of the mass of the Sun, and
it is 300,000 times fainter. The star’s feeble glow is the reason why
it has not been seen until now, despite being relatively close.

“We discovered this star in September 2002 while searching for white
dwarf stars in an unrelated program,” said Teegarden. The team was
looking for white dwarf stars that move rapidly across the sky.
Celestial objects with apparent rapid motion are called High Proper
Motion objects. An object of this type can be discovered in successive
images of an area of sky because it noticeably shifts its position
while its surroundings remain fixed. Since either a distant star
moving quickly or a nearby star moving slower can exhibit the same
High Proper Motion, astronomers must use other measurements to
determine its distance from Earth.

During its star search, the team used the SkyMorph database for NASA’s
Near Earth Asteroid Tracking program, to search for asteroids that
might be on a collision course for Earth. Pravdo is project manager of
the asteroid tracking program and is principal investigator for
SkyMorph, which was separately supported by NASA’s Applied Information
Systems Research Program. Like High Proper Motion stars, asteroids
reveal themselves when they shift their position against background
stars in successive images. Automated telescopes scan the sky,
accumulating thousands of images for the Near Earth Asteroid Tracking
program, which have been incorporated into SkyMorph, a web-accessible
database, for use in other types of astronomical research.

Once the star revealed itself in the Near Earth Asteroid Tracking
images, the team found other images of the same patch of sky to
establish a rough distance estimate by a technique called
trigonometric parallax. This technique is used to calculate distances
to relatively close stars. As 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.

The team refined their initial distance estimate with another
technique called photometric parallax. They used the 3.5-meter (11.5
feet) Astrophysical Research Consortium telescope at the Apache Point
observatory, Sunspot, N.M., to observe the star and separate its light
into its component colors for analysis. This allowed the team to
determine what kind of star it is. The analysis indicates it’s similar
to a red dwarf star (spectral type M6.5) that’s shining by fusing
hydrogen atoms in its core, like our Sun (called a main sequence
star).

Once the type of star is known, its true brightness, called intrinsic
luminosity, can be determined. Since all light-emitting objects appear
dimmer as distance from them increases, the team compared how bright
the new star appeared in their images to its intrinsic luminosity to
improve their distance estimate.

Although the star resembles a M6.5 red dwarf, it actually appears
three times dimmer than expected for this kind of star at the initial
distance estimate of 7.8 light-years. The star could therefore really
be farther than the rough trigonometric distance indicates; or, if the
initial estimate holds, it could have unusual properties that make it
shine less brightly than typical M6.5 red dwarfs. A more precise
measurement of the new star’s position to establish an improved
trigonometric parallax distance is underway at the U.S. Naval
Observatory. This will confirm or refute its status as one of our
closest neighbors by late this year. Either way, we might get even
more company soon: “Since the survey only covered a band of the sky
(about 25 degrees in declination), it is entirely possible that other
faint nearby objects remain to be discovered,” said Teegarden.

In addition to Teegarden and Pravdo, the team includes Dr. Thomas
McGlynn of Goddard Space Flight Center; Dr. Michael Hicks and Dr.
Stuart Shaklan of JPL; Dr. Suzanne Hawley, Kevin Covey and Oliver
Fraser, of the University of Washington, Seattle; and Dr. Iann Reid of
the Space Telescope Science Institute, Baltimore, Md. An image and
more information are available at
http://www.gsfc.nasa.gov/topstory/2003/0520newstar.html.

JPL manages the Near Earth Asteroid Tracking system and SkyMorph for
NASA’s Office of Space Science, Washington, D.C. JPL is managed by
the California Institute of Technology in Pasadena.