Pinpointing a Stellar Neighbor

By Ken Croswell

Published on (February 17, 2005).

A faint red star found five years ago is the 28th closest star system to the Sun, say astronomers in the United States and Chile. Their precise parallax yields the best distance ever determined for the star.

In 2000, Xavier Delfosse at the Institute of Astrophysics of the Canary Islands and Thierry Forveille of the Canada-France-Hawaii Telescope Corporation were conducting the Deep Near-Infrared Survey (DENIS) when they discovered a very red star in Antlia, an obscure constellation south of Leo and Hydra. The star's brightness suggested it might be nearby--if it were an intrinsically faint dwarf rather than a giant. Eduardo Martín at the University of Hawaii at Manoa then obtained the star's spectrum, which indicated a red or brown dwarf. From the spectral type and brightness, the astronomers estimated the star lay only 11 to 15 light-years from Earth.

Now Wei-Chun Jao and Todd Henry of Georgia State University and their colleagues have measured the star's actual distance. Using the 0.9-meter telescope at Cerro Tololo Inter-American Observatory in Chile, they detected the star's parallax--the small shift that results in the star's apparent position because astronomers view it from slightly different directions as Earth orbits the Sun. The larger the parallax, the closer the star. As Jao's team will report in the April issue of The Astronomical Journal, the star, named DENIS 1048-39, is 13.2 ± 0.1 light-years from Earth--three times farther than the Sun's closest neighbor, Alpha Centauri.

The star is spectral type M8.5, and its apparent visual magnitude is 17.39. Together with the newly determined distance, this means the star's absolute magnitude--the apparent magnitude it would have if it were 10 parsecs (32.6 light-years) away--is 19.36. The star emits only 0.00015 percent as much visible light as the Sun and, therefore, requires 1,800 years to generate the same amount of light the Sun throws off in a 24-hour day. If the star took the Sun's place, it would look dimmer than the Full Moon.

The star's exact nature is unknown. It may be a red dwarf, a dim main-sequence star that converts hydrogen into helium at its core, as the Sun does, but more slowly due to its lower mass. Or it may be a brown dwarf, which has even less mass--too little to sustain such nuclear reactions. In this case, the star shines by converting gravitational energy into heat and will slowly fade.

The star is as feisty as it is feeble. In 2002, Mary Putman, now at the University of Michigan, and Adam Burgasser, now at the American Museum of Natural History in New York, detected two giant radio flares the star had spewed into space. The activity likely arose from rapid rotation. The star's discoverers noted it spins at 27 kilometers per second (60,000 mph)--quite fast for a red star.

Meanwhile, astronomers continue to monitor another newfound star in the Sun's neighborhood. Scottish astronomers discovered the star, named SIPS 1259-4336, late last year in Centaurus. They estimate it lies roughly 12 light-years away. Determining a precise parallax will take about a year, but Henry tells Astronomy his group has already measured the star's visual magnitude: 18.0, about a magnitude brighter than originally estimated.

Ken Croswell is an astronomer in Berkeley, California, and the author of Magnificent Universe and Magnificent Mars.

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