... so we expect those black holes to weigh about as much as a massive star. Astronomers also suspect that other galaxies may harbour extremely massive black holes at their centres. These are thought to weigh about a million times as much as the Sun, or 1x1036 kilograms. The more massive a black hole is the more space it takes up. In fact, the if one black hole weighs ten times as much as another, its radius is ten times as large. A black hole with a mass equal to that of the Sun would have a radius of 3 kilometres. So a typical 10-solar-mass black hole would have a radius of 30 kilometres, and a million-solar-mass black hole at the centre of a galaxy would have a radius of 3 million kilometres. Three million kilometres may sound like a lot, but it's actually not so big by astronomical standards. The ...

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