BROWN DWARF

hypothetical astronomical object that is intermediate between a planet and a star. Brown dwarfs have a mass less than 0.08 solar mass, and their surface temperatures are below 2,500 K (the lower limit of the temperature of red dwarfs). Sometimes described as failed stars, brown dwarfs are believed to form in the same way that stars form; fragments of an interstellar cloud contract into smaller, denser clouds. Unlike stars, however, brown dwarfs do not have mass enough to generate the internal heat that in stars ignites hydrogen and establishes thermonuclear fusion reactions, which are the source of stellar energies. Though they generate some heat and some light, brown dwarfs also cool rapidly and shrink. Similar in appearance to high-mass planets, brown dwarfs may be distinguishable from planets only in their formation mechanism.