“A brown dwarf is an object which is made of the same stuff as stars, but does not have enough mass for hydrogen fusion (combining hydrogen atoms into helium atoms). Hydrogen fusion is what makes stars glow. Brown dwarfs are not massive enough to do this, so they are not stars. On the other hand, they are not regular giant planets, because they do glow.”
While very young they generate heat from both deuterium fusion but that only lasts millions to tens of millions of years. Energy from radioactive decay and their slow contraction lasts for much longer.
The nebulous space the Brown dwarf occupies in terms of classification is fascinating to say the least. I'm a passing not even hobbyist on the subject but that they glow but not like that is really interesting.
When I think of the brown dwarf I am also reminded of the hypothesized black dwarf. This would be the stage after white dwarf. It's hypothesized because it takes longer for the white dwarf bodies to cool than exists presently in the history of the universe. It's only a handful of sentences and will largely repeat the previous paragraph but there's the wiki page for black dwarf.
