A white dwarf starts out life as a very hot, planet-sized body with a surface temperature of 150,000 K or more. It VERY quickly cools down to 30 - 50,000 K over the course of a few million years, but after that its cooling rate becomes very much slower because the cooling time diminishes as the 5/7 power of the luminosity, which initially is very large. When the white dwarf gets dim enough, significant changes require BILLIONS of years but they are inevitable because the poor thing no longer has any source of internal energy that it can tap to keep its surface temperature elevated. The Extreme Ultraviolet Explorer has detected hundreds of white dwarfs within a few hundred light years of the Sun.
Eventually, a white dwarf will cool to become dull red, then a visibly black cinder of matter. But this is expected to take, literally, 10 billion years or more. After a billion years, their luminosities are typically 1/1000 of the Sun, yet their temperatures may still be 10 - 15,000 K so they are far from 'dark' and still emit copious quantities of visible light.
One of the hottest known white dwarfs is the central star of the planetary nebula NGC 7027, with a temperature of 180,000 K and a mass of 0.7 times the Sun's ( See Jacoby 1988, Astrophysical Journal v 333, p. 193). The white dwarf BPM 4729 described by Wickramasinghe and Bessell ( 1979, Monthly Notices of the Royal Astronomical Society, v.147, p. 123) has one of the lowest temperatures of 5500 K and a second object, G128-7 described by Wehrse and Leibert ( 1980: Astronomy and Astrophysics, v 83, 184) with T = 5800 K.
A review of the theory of white dwarf cooling by Francesca D'Antonia and Italio Mazzitelli in the 1990 edition of the Annual Review of Astronomy and Astrophysics ( p. 139) describe this business in detail. The cooling time varies as the inverse 5/7 power of the white dwarf luminosity, and there are several important stages in this cooling process including white dwarf 'crystallization' which occurs near temperatures of 4,000 K.
Essentially all of the white dwarfs that have evolved during the last 10 billion years are still visible, optically. There are as yet no 'black dwarfs' . We will have to wait a few more 10's of billions of years for THAT!
The above photograph was made with the Hubble Space Telescope of the core of the globular cluster Messier 4. The circles show the locations of white dwarfs in this 14 billion years old cluster. They are probably the oldest, coolest white dwarfs known.
Copyright 1997 Dr. Sten Odenwald
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