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The Question

(Submitted May 12, 1997)

What is the surface temperature of a white dwarf star?

The Answer

As you may know, a white dwarf is the cinder of a star which used to be like the Sun. At the end of its life, such a star expels much of its atmosphere, and the nuclear fusion stops. The hot core, about the size of the Earth but much denser, becomes exposed: this is the white dwarf.

When a star has just become a white dwarf, it is hotter than 100,000 K (about 180,000 F). It then gradually cools --- after many billions of years, it can become cooler than the Sun (which is about 6,000 K). So there is no particular temperature associated with the white dwarfs.

They are called 'white dwarfs', but not all are actually white; the first few that were discovered are white, with temperatures near 10,000 K. These are still the easiest to discover, so perhaps you can say that 10,000 K is the typical temperature of white dwarfs that we know of.

Blue (say 50,000 K) 'white dwarfs' are rare because they cool quickly; yellow and red (cooler than, say 6,000 K) ones are dim and very hard to discover, but there may be many if we look hard enough. Some astronomers look for these cool white dwarfs and estimate how long they have been cooling, so they can say something about the age of the Universe.

Koji Mukai and Jim Lochner
for Ask an Astrophysicist

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