Brown dwarfs are substellar objects not massive enough to sustain hydrogen-1 fusion reactions in their cores, unlike main-sequence stars. They occupy the mass range between the heaviest gas giants and the lightest stars, with an upper limit around 75 to 80 Jupiter masses (MJ). Brown dwarfs heavier than about 13 MJ are thought to fuse deuterium and those above ~65 MJ, fuse lithium as well. Brown dwarfs may be fully convective, with no layers or chemical differentiation by depth.The defining differences between a very-low-mass brown dwarf and a giant planet (~13 MJ) are debated. One school of thought is based on formation; the other, on the physics of the interior.Part of the debate concerns whether ""brown dwarfs"" must, by definition, have experienced fusion at some point in their history.Stars are categorized by spectral class, with brown dwarfs being designated as types M, L, T, and Y. Despite their name, brown dwarfs are of different colors. Many brown dwarfs would likely appear magenta to the human eye, or possibly orange/red. Brown dwarfs are not very luminous at visible wavelengths.Some planets are known to orbit brown dwarfs: 2M1207b, MOA-2007-BLG-192Lb, and 2MASS J044144bAt a distance of about 6.5 light years, the nearest known brown dwarf is Luhman 16, a binary system of brown dwarfs discovered in 2013. One brown dwarf, DENIS-P J082303.1-491201 b, from an ultracool binary system, has a mass of about 28 MJ, making it the largest known exoplanet (as of March 2014).