Faint star may have a brown-dwarf glow.

Faint star may have a brown-dwarf glow

Astronomers have identified what they believe is the faintest star yet detected--a dim red star in the skies of the Southern Hemisphere. Preliminary evidence suggests it may have a mass small enough to qualify it as a brown dwarf. According to theory, a brown dwarf would have such a weak gravitational field that nuclear fusion reactions, which power larger, brighter stars, could not occur at its core.

The discovery stems from the work of Philip A. Ianna of the University of Virginia in Charlottesville and Michael S. Bessell of the Stromlo and Siding Spring Observatories in Australia. The researchers determined the distance from Earth to the dimmest of a number of very faint stars identified in a survey conducted by Michael R.S. Hawkins of the Royal Observatory in Edinburgh, Scotland. This particular star, they found, lies about 68 light-years from Earth and shines with a brightness only about one-four-thousandth that of the sun.

Whether a star falls in the brown-dwarf category depends on its mass, which astronomers cannot determine directly. To calculate the mass of such a celestial object, they rely on theoretical models that relate a star's brightness to its mass. But uncertainties in the models can lead to large errors.

"Theory tells us the brightness depends not just on the mass but also on the age of the object," says Donald W. McCarthy Jr. of the University of Arizona in Tucson. Young brown-dwarf stars would be much brighter than older stars of a similar mass. "A lot hinges on the object's age," he says.

"The redder stars we saw in our survey appear to be younger than the hotter stars," Bessell says. According to one theoretical model, that assumption would put the mass of the faintest of these stars as low as 5 percent of the sun's mass -- low enough for it to be a brown dwarf. But uncertainties in the data and theory mean that the star's mass could still exceed 8 percent of the sun's mass -- the minimum mass a star must have to sustain nuclear fusion.

Ianna and Bessell are now applying the same measurement techniques to other faint stars. By studying a population of such dim stars in detail, astronomers hope to refine their theories concerning the behavior and characteristics of brown dwarfs.

"It's all very encouraging," says Michael F. Skrutskie of the University of Massachusetts in Amherst. "At this point, it would be hard to detect a star that's fainter than the faintest known stars without it being a brown dwarf. But extraordinary claims require extraordinary evidence."