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The Question

(Submitted November 11, 1997)

What is the temperature of a black hole?

The Answer

The temperature of a black hole is determined by the 'black body radiation temperature' of the radiation which comes from it. (e.g., If something is hot enough to give off bright blue light, it is hotter than something that is merely a dim red hot.)

For black holes the mass of our Sun, the radiation coming from it is so weak and so cool that the temperature is only one ten-millionth of a degree above absolute zero. This is colder than scientists could make things on Earth up until just a few years ago (and the invention of of a way to get things that cold won the Nobel prize this year). Some black holes are thought to weigh a billion times as much as the Sun, and they would be a billion times colder, far colder than what scientists have achieved on Earth.

However, even though these things are very cold, they can be surrounded by extremely hot material. As they pull gas and stars down into their gravity wells, the material rubs against itself at a good fraction of the speed of light. This heats it up to hundreds of millions of degrees. The radiation from this hot, infalling material is what high-energy astronomers study.

David Palmer
for Ask an Astrophysicist

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