Re: Can anyone enlighten me?
Size, and whether or not hydrogen fusion occurs within it.
Taking some figures from our favourite on-line encyclopaedia, a stellar body needs to have a mass of approximately 8% of that of our Sun for the core pressure and temperature to get high enough for gravity-induced hydrogen fusion to occur; this also tends to "burn" lithium as well.
However, the mass of a sub-stellar body may yet be high enough for other forms of fusion to occur, using "heavy" isotopes of hydrogen - primarily deuterium (hydrogen-2) - and also possibly "burning" lithium, usually when the it is younger and hotter (from gravitation condensation/contraction).
If anyone can correct any mistakes I've made, please do so! :)