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added more information about brown dwarf definition and detection
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.
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|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 isthe 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.<ref>http://www.dtm.ciw.edu/boss/definition.html</ref> 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.<ref name="bdref">http://www.daviddarling.info/encyclopedia/B/browndwarf.html</ref>
==History==
==History==
==Detecting brown dwarfs==
==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. 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.
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-ray|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 dwarf|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 of some 500 million years.<ref name="bdref">http://www.daviddarling.info/encyclopedia/B/browndwarf.html</ref>
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]].<ref>http://arxiv.org/abs/astro-ph/0608417</ref>
[[Category:Astronomy]]
[[Category:Astronomy]]