A star-like object whose mass is too small to sustain hydrogen fusion in its interior and become a star. Brown dwarfs are substellar objects and occupy an intermediate regime between those of stars and giant planets. With a mass less than 0.08 times that of the Sun (about 80 Jupiter masses), nuclear reactions in the core of brown dwarfs are limited to the transformation of deuterium into Helium-3. The reason is that the cores of these objects are supported against gravitational collapse by electron degeneracy pressure (at early spectral types) and Coulomb pressure (at later spectral types). Brown dwarfs, as ever cooling objects, will have late M dwarf spectral types within a few Myrs of their formation and gradually evolve as L, T and Y dwarfs brown dwarf cooling. As late-M and early-L dwarfs, they overlap in temperature with the cool end of the stellar main sequence (M dwarf, L dwarf, T dwarf, Y dwarf). In contrast to the OBAFGKM sequence, the M-L-T-Y sequence is an evolutionary one.