Cooling of White Dwarfs
The adjacent image shows a white dwarf embedded in a planetary nebula that may be the hottest star
known.
Its surface appears to have a temperature of about 200,000 degrees C, which is 30 times the surface
temperature of the Sun
(Ref).
Even though the temperature is so high, this is a dying star. It has used all its
nuclear fuel and can no longer produce energy by thermonuclear fusion.