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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 a High-Energy Astronomer
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