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Asked by rachaell to Adam, Catherine, Karen, Leila, Nazim on 15 Mar 2012.
Question: What is the radius of a blackhole?
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Adam Stevens answered on 13 Mar 2012:
The radius of a black hole depends on the mass of the object that formed it. There’s no reason why you couldn’t have a black hole 1mm across, or thousands of kilometres across.
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Leila Battison answered on 13 Mar 2012:
Like Adam says, it can vary a lot, but black holes that form from stars collapsing in on themselves will be about the same size of the stars that they were once.
Most black holes that started from collapsing stars are about 3 times the size of the sun, but the supermassive black hole in the middle of the galaxy is about 100 million times the size of our sun. Scary big!
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Karen Masters answered on 14 Mar 2012:
Well that depends on its mass. There’s a really simple relationship actually – R=3M (seriously). You just have to get the right units – the mass, M, is in solar masses (number of times the mass of our sun) and the radius, R is in kilometres. So a solar mass black hole has a radius of 3km. An Earth mass black hole has a radius of about 1 cm. Supermassive black holes in the centres of galaxies have masses of millions of times that of the sun, so their radius is millions of kilometres, and one cool fact is that eventually they get so large (in radius) that their average density can be less than water.
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Nazim Bharmal answered on 15 Mar 2012:
It varies enormously, like Adam says, but it has been predicted that blackholes can loose energy and so slowly shrink. At first this is slow, but it speeds up and eventually the blackhole vanishes in a burst of light. So the radius could always be changing.
I don’t think the prediction of the burst of light, called Hawking Radiation, has been seen yet but keep your eyes peeled.
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