Q&A: Miscellaneous X-ray Sources
When a brown dwarf has cooled to a black dwarf, is the black dwarf still gaseous or is it a solid now that it's cold?
A black dwarf that evolved from a white dwarf should be a solid, crystal
object, kind of like a very dense form of diamond. Although astronomers
expect that these objects will eventually form, none so far exist because
the universe isn't old enough for them to have formed (the evolution of
low-mass stars that form white dwarfs is relatively slow). A brown dwarf
should also eventually form into a solid object, but we are not sure if the resulting body is also called a black dwarf.