How big is a black hole?
There are at least two different ways to describe how big something is. We can say how much mass it has, or we can say how much space it takes up. Let’s talk first about the masses of black holes. There is no limit in principle to how much or how little mass a black hole can have. Any amount of mass at all can in principle be made to form a black hole if you compress it to a high enough density. We suspect that most of the black holes that are actually out there were produced in the deaths of massive stars, and so we expect those black holes to weigh about as much as a massive star. A typical mass for such a stellar black hole would be about 10 times the mass of the Sun, or about 10^{31} kilograms. (Here I’m using scientific notation: 10^{31} means a 1 with 31 zeroes after it, or 10,000,000,000,000,000,000,000,000,000,000.) Astronomers also suspect that many galaxies harbor extremely massive black holes at their centers. These are thought to weigh about a million times as much as the S
