The black hole in the galaxy M82 has a mass about 500 times the mass of our Sun. It has about the same volume as Earth's moon. What is the density of this black hole?

mass sun = 1.9891 x 10^30 kg
volume moon = 2.1968 x 10^10 km3

1 Answer
Mar 26, 2016

The question is incorrect in the values, since black holes do not have volume. If we accept that as true then the density is infinite.


The thing about black holes is that in the formation the gravity is such that all particles are crush under it. In a neutron star you have gravity so high that protons are crushed together with electrons creating neutrons. Essentially this means that unlike "normal" matter which is 99% empty space, a neutron star is almost 100% solid. That means that essentially a neutron star is about as dense as you can possibly get.

Due to the larger mass and gravity, a black hole is denser than that. If you consider that the quasar and the event horizon are actually produced by the black hole but not part of it, then what is the black hole is the singularity. As the name suggests, a singularity has such a small volume it might as well be zero. The formula for density is Mass/Volume, and any number divided by zero is infinity.

You can calculate a sort of density based on the volume enclosed by the event horizon, but this density is not uniform. With the exception of the singularity itself, the entire volume within the event horizon is empty space as all the matter in that volume has been crushed into the singularity.