A brown dwarf is a celestial body with mass but insufficient nuclear fusion reactions to produce energy, and thus not classifiable as a star. Brown dwarfs have fully convective surfaces and atmospheres, with no layers or division into core, radiative zone, and envelope. As a result, they have very little internal structure and do not rotate very rapidly. Brown dwarfs are thought to form like stars, through the gravitational collapse of molecular clouds of dust and gas. However, unlike stars, their cores never get hot enough for hydrogen fusion to begin. The highest-mass brown dwarfs (also called “failed stars”) fuse deuterium at temperatures of about 2 million K; objects below this mass limit are fully convective and do not sustain any deuterium burning whatsoever.
The term “brown dwarf” was coined in 1975 by Jill Tarter and Gordon Marcy, based on the spectral classifications originally proposed by Trumpler & Weaver (1930) and independently by Munch (1950). At that time, the lowest-mass stars were thought to have masses of about 0.08 solar masses (<1 Jupiter mass), while the most massive brown dwarfs were estimated to be only about 0.012 solar masses (>13 Jupiter masses). The first confirmed discovery of a brown dwarf was made in 1995 when an extrasolar planet was found orbiting the nearby star Gliese 229B; this object has a mass only slightly larger than that of Jupiter but is much cooler (T~1200 K). Since then, many more brown dwarfs have been discovered both in our own Solar neighborhood and in other galaxies.