Q&A: Miscellaneous X-ray Sources

Q:
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:
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.