Brown dwarfs are substellar objects that have more mass than the biggest gas giant planets, but less than the least massive main-sequence stars. Their mass is approximately 13 to 80 times that of Jupiter (MJ)—not big enough to sustain nuclear fusion of hydrogen into helium in their cores, but massive enough to emit some light and heat from deuterium fusion, 2H, an isotope of hydrogen with a neutron as well as a proton, that can undergo fusion at lower temperatures. The most massive ones (> 65 MJ) can fuse lithium (7Li). Astronomers classify self-luminous objects by spectral type, a distinction intimately tied to the surface temperature -- brown dwarfs occupy types M (2100–3500 K), L (1300–2100 K), T (600–1300 K), and Y (< 600 K). As brown dwarfs do not undergo stable hydrogen fusion, they cool down over time, progressively passing through later spectral types as they age. The "brown" in brown dwarf was meant to name a color between red and black. To the naked eye, most brown dwarfs would appear to be magenta, purple, or even black with others in different colors depending on their temperature. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth. Though their existence was initially theorized in the 1960s, it was not until 1994 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 most of their light in the infrared. However, 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 about 6.5 light-years (2.0 parsecs) from the Sun. Luhman 16 is the third closest system to the Sun after Alpha Centauri and Barnard's Star.
History
Early theorizing
In the 1960s Shiv Kumar theorized the existence of objects now called brown dwarfs; they were originally called black dwarfs, a classification for dark substellar objects floating freely in space that were not massive enough to sustain hydrogen fusion. However:
(a) the term black dwarf was already in use to refer to a cold white dwarf (b) red dwarfs fuse hydrogen (c) these objects may be luminous at visible wavelengths early in their lives. Because of this, alternative names for these objects were proposed, including planetar and substar. In 1975 Jill Tarter, as part of her PhD thesis at University of California at Berkeley was the first to suggest that the term to describe these objects should be brown dwarf, using brown as a color "somewhere between red and black", suggesting that the dwarfs appeared dim, dark, and dull, even though not exactly brown. The term black dwarf continues to be used to refer to a white dwarf that has cooled to the point that it no longer emits significant amounts of light. However, the time required for even the lowest-mass white dwarf to cool to this temperature is calculated to be longer than the current age of the universe; hence such objects are expected to not yet exist. Early theories concerning the nature of the lowest-mass stars and the hydrogen-burning limit suggested that a population I object with a mass less than 0.07 solar masses (M☉) or a population II object less than 0.09 M☉ would never go through normal stellar evolution and would become a completely degenerate star. The resulting brown dwarf is sometimes called a failed star. The first self-consistent calculation of the hydrogen-burning minimum mass confirmed a value between 0.07–0.08 solar masses for population I objects.
Deuterium fusion The discovery of deuterium fusion down to 0.013 M☉ (13.6 MJ) and the impact of dust formation in the cool outer atmospheres of brown dwarfs in the late 1980s brought these theories into question. However, such objects were hard to find because they emit almost no visible light. Their strongest emissions are in the infrared (IR) spectrum, and ground-based IR detectors were too imprecise at that time to readily identify any brown dwarfs. Since then, numerous searches by various methods have sought these objects. These methods included multi-color imaging surveys around field stars, imaging surveys for faint companions of main-sequence dwarfs and white dwarfs, surveys of young star clusters, and radial velocity monitoring for close companions.
First observations For many years, efforts to discover brown dwarfs were fruitless: they are very faint and emit most of their light in the near-infrared part of the spectrum. Then in the mid-1990s several brown dwarfs were found.
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