Question

How big are black dwarfs?

Answer 1

Somewhere between `2.765 × 10^30` kg and `1.0 × 10^30` kg. Roughly.

Explanation:

In the evolution of stars, in order for a star to become a white dwarf it cannot be too big. If it is bigger than `2.765 × 10^30` kg (called the Chandrasekhar limit) it will be unstable and become either a neutron star or a black hole or something. The lowest mass of a white dwarf is assumed to be about half the mass of the sun or `1.0 × 10^30` kg (roughly).

The reasons for having these limits are all due to gravity. If a white dwarf is greater than the Chandrasekhar limit it's gravity will be to great and cause the electrons of the atoms to collapse. In order for a star to end up as a white dwarf less than half the mass of the sun it would have to start at a mass with too little gravity to have proper fusion. These objects do exist, but are not real stars. We call them Brown Dwarfs.

What does the mass of a white dwarf have to do with a Black dwarf?

Well a Black Dwarf is the theoretical end product of a star that has evolved to the white dwarf stage and then subsequently radiated all of it's heat energy away, so that it is the temperature of the background of space (about 2-3 degrees K), since a white dwarf no longer generates any energy. A white dwarf does not lose any mass in transitioning to a black dwarf, so the mass of a black dwarf would be the same as a white dwarf.

It will be about a trillion years before we know if white dwarfs actually evolve into black dwarfs, since that is how long it is estimated to take before the coolest known white dwarf would radiated all it's remaining heat.