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Brown dwarfs are substellar objects that are not massive enough to sustain nuclear fusion of ordinary hydrogen (1H) into helium in their cores, unlike a main-sequence star. They have a mass between the most massive gas giant planets and the least massive stars, approximately 13 to 80 times that of Jupiter (MJ). However, they are able to fuse deuterium (2H), and the most massive ones (> 65 MJ) are able to fuse lithium (7Li).
Astronomers classify self-luminous objects by spectral class, a distinction intimately tied to the surface temperature, and brown dwarfs occupy types M, L, T, and Y. As brown dwarfs do not undergo stable hydrogen fusion, they cool down over time, progressively passing through later spectral types as they age.
Despite their name, to the naked eye brown dwarfs would appear different colors depending on their temperature. The warmest are possibly orange or red, while cooler brown dwarfs would likely appear magenta to the human eye. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.
Though their existence was originally theorized in the 1960s, it was not until the mid-1990s that the first unambiguous brown dwarfs were discovered. As brown dwarfs have relatively low surface temperatures, they are not very bright at visible wavelengths, emitting the majority of their light in the infrared. With the advent of more capable infrared detecting devices, thousands of brown dwarfs have been identified. The nearest-known brown dwarfs are located in the Luhman 16 system, a binary of L- and T-type brown dwarfs at a distance of about 6.5 light-years. Luhman 16 is the third-closest system to the Sun after Alpha Centauri and Barnard's Star.
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