Brown dwarfs are substellar 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(MJ). Brown dwarfs heavier than about 13 MJ are thought to fuse deuterium and those above ~65 MJ, fuse lithium as well.
Dwarfs are categorized by spectral classification, with the major types being M, L, T, and Y. Despite their name, brown dwarfs are different colours. Many brown dwarfs would likely appear magenta to the human eye according to A. J. Burgasser, whereas another source has noted orange/red. The term brown dwarf was not chosen to indicate their colour.
Another debate is whether brown dwarfs should 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.