Brown Dwarf
Brown dwarfs are sub-stellar objects too low in mass to sustain hydrogen-1 fusion reactions in their cores, unlike main sequence stars, which can. They occupy the mass range between the heaviest gas giants and the lightest stars, with an upper limit around 75 to 80 Jupiter masses . Brown dwarfs heavier than about 13 are thought to fuse deuterium and those above ~65, fuse lithium as well.
However, for some years now there has been debate concerning what criterion to use for defining the separation between a brown dwarf and a giant planet at very low brown dwarf masses (~13 Jupiter masses). One school of thought is based on formation, and another on interior physics.
Dwarfs are categorized by spectral classification, with the major types being M, L, T, and Y. Despite their name, most brown dwarf's would appear magenta to the human eye.
Another debate is whether brown dwarfs are required to have experienced fusion at some point in their history. Some planets are known to orbit brown dwarfs: 2M1207b, MOA-2007-BLG-192Lb, and 2MASS J044144b. Brown dwarfs may have fully convective surfaces and interiors, with no chemical differentiation by depth.
Read more about Brown Dwarf: History, Theory, Planets Around Brown Dwarfs, Superelative Brown Dwarfs
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