A Brown Dwarf is a sub-stellar object that has an intermediate mass between a star and a large gas giant planet. Sometimes described as "failed stars", they lack the necessary mass to start or maintain hydrogen burning nuclear fusion reactions in their cores. These objects are not actually "brown" in color, but a very dull red, as most of their radiation is emitted in the infrared.
Brown dwarfs are primarily identified by their mass and origin, with mass being the most common method of classification. According to current models of star formation, a mass of at least .0075 that of our Sun (or 75 times Jupiter's mass) is required to initiate core hydrogen fusion, establishing this is the upper limit of the mass of a brown dwarf. The lower limit of .0013 masses of the Sun (or 13 times the mass of Jupiter) is more of an arbitrary number as there is no clear transition between a high mass gas giant and a low mass brown dwarf. However around .0013 solar masses is what is required to fuse deuterium, and is the lower mass limit established by the International Astronomical Union for a sub-stellar object to be classified as a brown dwarf, any substellar object of lower mass that orbits a star or stellar remnant is considered a planet. Brown dwarfs are also seen as different from planets as they are believed to form the same was as stars do, through the compression of interstellar gas clouds, instead of forming in the circumstellar disk as a planet would.
The name "Brown Dwarf" was first coined by Jill Tarter in 1975 for a classification of sub-stellar objects in space which are unable to sustain hydrogen fusion.
The first located Brown Dwarf was discovered in 1988 as a companion to the star GD 165, which was too cool to be classified as an M class star. For almost a decade, GD 165B, today known as a L class brown dwarf was the only known object that could be classified as such. In 1995 this changed, using a combination of mass determination, spectroscopic studies, and direct imaging another confirmed brown dwarf was discovered as a companion to Gliese 229. The discovery would later help establish a new class of brown dwarfs known as "T dwarfs". Today hundreds of brown dwarfs are known.
There are three classifications of brown dwarfs today:
Spectral class L
L dwarfs are defined in the red opitcal region, much like M class stars, but have spectra defined by strong metal hydride bands (FeH, CrH, MgH, CaH) and alkali lines (Na I, K I, Cs I, Rb I). The surface temperature of of L class dwarfs typical average around 1,500K to 2,200K. Teide 1 is an example of a L dwarf.
Spectral class T
Cooler then the L class dwarfs, T class, or methane dwarfs are have a surface range of 1,500K to a mere 800K. Their spectra display strong absorption by methane (CH4), and water (H2O). If seen visually, they would appear as dark magenta. Gliese 229b is an example of a T dwarf.
Spectral class Y
Y dwarfs are believed to be even cooler than T dwarfs, and are considered theoretical. Such dwarfs would have surface temperatures under 700K. In March 2008, a brown dwarf called CFBDS J005910.90-011401.3 was discovered, which a surface temperature of only 620K, and is believed to be the first such Y dwarf found.
Detecting brown dwarfs
Because of the very limited amount of light brown dwarfs give, methods other then observations in the visual spectrum are used to discover them. One method of detection is through the large amount of infrared radiation such objects emit, allowing observations using infrared equipment, brown dwarfs are also distinguishable from planets through their emission of x-rays. They are also detected via the wobble that a brown dwarf would cause in the motion of a parent or companion star. Thus radial velocity is used to find a large amount of brown dwarfs. A telescope equipped with a coronagraph may also be used to detect brown dwarfs.
The most significant indicator used to differentiate between brown dwarfs, and very low mass, cool red dwarfs is the presence of lithium, often referred to as the Lithium test. This is due to red dwarf stars having sufficient temperature to quickly deplete any lithium available, which occurs when lithium-7 and a proton collide to produce two helium-4 atoms. As red dwarfs transport energy to the surface entirely through convection, all the lithium in the star is converted over time. Normally, brown dwarfs lack the mass and temperature to exhaust the available lithium, however higher mass brown dwarfs of .0065 solar masses or greater, have sufficient temperature to also expend their lithium (which is slightly lower then the temperature required to fuse hydrogen), abet over a longer period of time.
For the coolest and least massive brown dwarfs, the main method to distinguish them from large gas giants is density. Even though all brown dwarfs are around the same radius, which is close to the radius of the largest gas giants, they are many times more massive, and thus more dense. Typically the higher volume of a brown dwarf is controlled through electron degeneracy, although the least massive brown dwarfs' volume is controlled through Coulomb pressure.