In brown dwarfs, as in stars, thermonuclear reactions occur early in their lives. However, there is a difference between these objects: the fact is that brown dwarfs cool down very quickly, and the temperature in their subsoil is too low to provide a continuous transformation reaction hydrogen into helium, accompanied by the release of heat and light. Incidentally, the very color of these celestial bodies is due to their comparatively low temperature, which is less than 2,000 degrees Kelvin. There is also no radiant transfer zone in brown dwarfs, and heat transfer is done only by convection. In particular, lithium, which in stars either burns at the beginning of life or remains in the upper layers, in brown dwarfs gradually transitions from the cold upper layers to the hot inner, $ providing mixing of substances and relative uniformity of the structure of the celestial body.
Brown dwarfs were long thought to be planets, as their diameter on average is roughly equal to that of Jupiter. Also, they are incapable of maintaining thermonuclear reactions for a long enough time. However, there are also significant differences between these celestial bodies. First, brown dwarfs differ from planets in density and mass. As noted above, their mass can be 80 times the mass of the gas giant Jupiter. Second, brown dwarfs, unlike planets, are capable of radiating in infrared and sometimes X-ray, which has allowed astronomers to detect many of these celestial bodies far beyond limits of the solar system.