A Cool New Class of Stars

SAN DIEGO--Astronomers have revised a century-old system of categorizing stars to include a new class of objects so cool that they barely shine in visible light. Based on a survey that has spotted 20 of the tiny stars, called L dwarfs, scientists believe they outnumber all other stars in the Milky Way, according to research presented here yesterday at a meeting of the American Astronomical Society. However, the cosmic runts may not deserve the appellation "star," because some do not fuse hydrogen at their cores.

The classic method of sorting stars by temperature and color dates to 1890. Ranging from hottest to coolest, each star earns one of the following letters: O, B, A, F, G, K, and M. For instance, O stars are blazingly hot blue giants that burn quickly, while M stars are small, faint, red, and long-lived. Astronomers knew of a few shadowy brown dwarfs, which are failed stars with less than 7.5% of the sun's mass, but there weren't enough of them to study their properties in detail.

That changed with results from the 2 Micron All Sky Survey (2MASS), which uses 1.3-meter telescopes in Arizona and Chile to scan the sky at infrared wavelengths. The survey has spied 20 nearby objects with temperatures between 1200 and 1700 degrees Celsius, cooler than M stars and about as hot as an old coal glowing an extremely dull red. Spectra of the dwarfs, collected with the 10-meter Keck II Telescope in Hawaii, revealed that six of them contain lithium--an element that disintegrates when a star is big enough to ignite fusion. "Those six objects clearly are brown dwarfs," says astronomer J. Davy Kirkpatrick of the California Institute of Technology in Pasadena. However, he suspects other L dwarfs may be bona fide stars, albeit faint ones, if they are massive enough to spark hydrogen fusion at their centers. His team hopes to confirm that by finding a dwarf orbiting another star, which would reveal its precise mass.

Because 2MASS has seen so many L dwarfs in a small volume of space, Kirkpatrick says, they may compose at least half of all stellar objects in our galaxy. Retired astronomer Philip Keenan of Ohio State University in Columbus, an expert on K stars, applauds the new designation. "They certainly are different beasts," he says, and thus deserve a letter all their own.