What is a Brown Dwarf?
A brown dwarf is a substellar object: heavier than any planet, lighter than any star. Between roughly 13 and 80 Jupiter masses, it can fuse deuterium for a while but never ignites ordinary hydrogen fusion — so instead of settling onto the main sequence, it simply cools and fades for the rest of time.
The lower boundary is the deuterium-burning limit near 13 Jupiter masses; below that an object is a planet by the usual convention. The upper boundary is the hydrogen-burning limit at about 80 Jupiter masses, or 0.08 solar masses, where core temperature finally reaches the roughly 3 million kelvin needed for sustained proton fusion. In between sits a class of object that was purely hypothetical until the first confirmed detection in 1995.
Remarkably, brown dwarfs are all about the same size — roughly the radius of Jupiter — across that entire mass range. Electron degeneracy pressure props them up, and it barely cares how much mass you add. A 70-Jupiter-mass brown dwarf is not much larger than a 15-Jupiter-mass one, just far denser.
Their atmospheres are the closest thing in nature to a planet's weather on a stellar body: banded, turbulent, and full of clouds of iron, silicate dust, and at the coldest end water ice. The Y-class dwarf WISE 0855-0714 has an effective temperature near 250 K, colder than the Arctic. Luhman 16, a binary pair only 6.5 light-years away, has been mapped well enough to show its clouds rotating in and out of view.