The Discovery of Brown Dwarfs; April 2000; Scientific American Magazine; by Basri; 8 Page(s)

A brown dwarf is a failed star. A star shines because of the thermonuclear reactions in its core, which release enormous amounts of energy by fusing hydrogen into helium. For the fusion reactions to occur, though, the temperature in the star's core must reach at least three million kelvins. And because core temperature rises with gravitational pressure, the star must have a minimum mass: about 75 times the mass of the planet Jupiter, or about 7 percent of the mass of our sun. A brown dwarf just misses that mark-it is heavier than a gas-giant planet but not quite massive enough to be a star.

For decades, brown dwarfs were the "missing link" of celestial bodies: thought to exist but never observed. In 1963 University of Virginia astronomer Shiv Kumar theorized that the same process of gravitational contraction that creates stars from vast clouds of gas and dust would also frequently produce smaller objects. These hypothesized bodies were called black stars or infrared stars before the name "brown dwarf" was suggested in 1975 by astrophysicist Jill C. Tarter, now director of research at the SETI Institute in Mountain View, Calif. The name is a bit misleading; a brown dwarf actually appears red, not brown. But the name "red dwarf" was already taken. (It is used to describe stars with less than half the sun's mass.)