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Wikipedia dicit:

A brown dwarf is a type of substellar object that has a mass between the most massive gas giant planets and the least massive stars, approximately 13 to 80 times that of Jupiter (MJ).

Unlike main sequence stars, brown dwarfs do not acquire enough mass to trigger sustained nuclear fusion of ordinary hydrogen (1H) into helium in their cores. For this reason brown dwarfs are sometimes referred to as failed stars. They are, however, thought to fuse deuterium (2H), and to fuse lithium (7Li) if their mass is > 65 MJ. The minimum mass required to trigger sustained hydrogen-burning forms the upper limit of the definition currently used by the International Astronomical Union, while the deuterium-burning minimum mass of ~13 MJ forms the lower limit of the class, below which lie the planets.

It is also debated whether brown dwarfs would be better defined by their formation process rather than by theoretical mass limits based on nuclear fusion reactions. Under this interpretation brown dwarfs are those objects that represent the lowest-mass products of the star formation process, while planets are objects formed in an accretion disk surrounding a star. The coolest free-floating objects discovered such as WISE 0855, as well as the lowest-mass young objects known like PSO J318.5−22, are thought to have masses below 13 MJ, and as a result are sometimes referred to as planetary mass objects due to the ambiguity of whether they should be regarded as rogue planets or brown dwarfs. There are planetary mass objects known to orbit brown dwarfs, such as 2M1207b, MOA-2007-BLG-192Lb, and 2MASS J044144b.

Astronomers classify self-luminous objects by spectral class, a distinction intimately tied to the surface temperature, and brown dwarfs occupy types M, L, T, and Y. As brown dwarfs do not undergo stable hydrogen fusion they cool down over time, progressively passing through later spectral types as they age.

Video credit: NASA’s Goddard Space Flight Center/Scott Wiessinger (USRA): Animator/Scott Wiessinger (USRA): Producer/Ashley Balzer (ADNET): Science Writer/Claire Andreoli (NASA/GSFC): Public Affairs Officer