Asked by: Martin Hearne, Chorley
Black holes are freezing cold on the inside, but incredibly hot just outside. The internal temperature of a black hole with the mass of our Sun is around one-millionth of a degree above absolute zero.
Just outside the hole, however, the material being pulled into the hole’s gravity well is accelerated to near the speed of light. The molecules of the material collide with such vigour that it is heated up to a temperature of hundreds of millions of degrees. When astronomers study black holes, this is the material that they see.
The radiation from the material masks the tiny amount of radiation escaping from the hole itself, and so what the astronomers observe is the very hot outside environment, rather than the freezing cold environment inside.