brown dwarf, astronomical object that is intermediate between a planet and a star. Brown dwarfs usually have a mass less than 0.075 that of the Sun, or roughly 75 times that of Jupiter. (This maximum mass is a little higher for objects with fewer heavy elements than the Sun.)  Many astronomers draw the line between brown dwarfs and planets at the lower fusion boundary of about 13 Jupiter masses. The difference between brown dwarfs and stars is that, unlike stars, brown dwarfs do not reach stable luminosities by thermonuclear fusion of normal hydrogen. Both stars and brown dwarfs produce energy by fusion of deuterium (a rare isotope of hydrogen) in their first few million years. The cores of stars then continue to contract and get hotter until they fuse hydrogen. However, brown dwarfs prevent further contraction because their cores are dense enough to hold themselves up with electron degeneracy pressure. (Those brown dwarfs above 60 Jupiter masses begin to fuse hydrogen, but they then stabilize, and the fusion stops.)

Brown dwarfs are not actually brown but appear from deep red to magenta depending on their temperature. Objects below about 2,200 K, however, do actually have mineral grains in their atmospheres. The surface temperatures of brown dwarfs depend on both their mass and their age. The most massive and youngest brown dwarfs have temperatures as high as 2,800 K, which overlaps with the temperatures of very low-mass stars, or red dwarfs. (By comparison, the Sun has a surface temperature of 5,800 K.) All brown dwarfs eventually cool below the minimum main-sequence stellar temperature of about 1,800 K. The oldest and smallest can be as cool as about 300 K.

Brown dwarfs were first hypothesized in 1963 by American astronomer Shiv Kumar, who called them “black” dwarfs. American astronomer Jill Tarter proposed the name “brown dwarf” in 1975; although brown dwarfs are not brown, the name stuck because these objects were thought to have dust, and the more accurate “red dwarf” already described a different type of star. Searches for brown dwarfs in the 1980s and 1990s found several candidates; however, none was confirmed as a brown dwarf. In order to distinguish brown dwarfs from stars of the same temperature, one can search their spectra for evidence of lithium (which stars destroy when hydrogen fusion begins). Alternatively, one can look for (fainter) objects below the minimum stellar temperature. In 1995 both methods paid off. Astronomers at the University of California, Berkeley, observed lithium in an object in the Pleiades, but this result was not immediately and widely embraced. This object, however, was later accepted as the first binary brown dwarf. Astronomers at Palomar Observatory and Johns Hopkins University found a companion to a low-mass star called Gliese 229 B. The detection of methane in its spectrum showed that it has a surface temperature less than 1,200 K. Its extremely low luminosity, coupled with the age of its stellar companion, implies that it is about 50 Jupiter masses. Hence, Gliese 229 B was the first object widely accepted as a brown dwarf. Infrared sky surveys and other techniques have now uncovered hundreds of brown dwarfs. Some of them are companions to stars; others are binary brown dwarfs; and many of them are isolated objects. They seem to form in much the same way as stars, and there may be 1–10 percent as many brown dwarfs as stars.

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