Why do white dwarfs have high surface temperatures?

Answer 1

Good question. Three part answer.

First, all stars convert gravitational potential energy into radiative energy. White dwarfs have a mass that's in the same order of magnitude as the stars that became them, but with a much higher density and a much smaller surface area. So the amount of energy radiated per inch would be higher, resulting in higher surface temperatures.

Second, the degenerate matter that makes up the bulk of a white dwarf has a very low opacity, because any absorption of a photon requires an electron transition to a higher empty state, which may not be available given the energy of the photon.

Third, since the heat-generating capacity of the white dwarf is not replenished by nuclear fusion - and IF there is no companion star present from which the dwarf gains new mass - the star will slowly cool; the high surface temperatures do not last.