What is a brown dwarf?
Brown dwarfs are starlike objects that, because of their extremely small size, never achieve the necessary conditions in their cores to sustain the nuclear fusion that powers normal stars. These objects, the extreme lower end of the stellar family, may in fact represent a bridge between the faintest of "true" hydrogen-burning stars and cold gas giant planets such as Jupiter.
When a star forms, its eventual size is determined by a wide variety of factors intrinsic to the dense gas cloud in which it was born, but a study of the numbers of different-sized stars clearly shows that stars somewhat smaller than the Sun are the favored model. Enormous stars, such as Eta Carinae -- 120 time as massive as the Sun -- are extremely rare, while the galaxy is practically brimming over with stars just over half of the Sun's size.
Stars are limited in how small they can be, however; the high pressures and temperatures necessary for the nuclear fusion that drives a star requires a mass of at least 8 percent of the Sun's -- about 80 times the mass of Jupiter. When a "star" forms below this mass limit, it fails to ignite its nuclear fuel, and glows a dull red color strictly from the heat of its pressurized gases. Such an object is called a "brown dwarf", for somewhat obvious reasons. Small and extremely dim, brown dwarfs have only recently begun to be detected by astronomers using the latest development in infrared telescope technologies.
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