How are some black holes’ event horizons bigger than others?

Question

If their mass is always contained in a singularity and their density is infinite, how are some black holes supermassive and others are not?

In: 5

Answer 1

Different black holes have different masses, and the size of the event horizon scales with the mass of the black hole. (Linearly, as it turns out: double the mass means double the radius of the event horizon, triple the mass means triple the radius, and so on.)

If it helps, the event horizon is not an object, and the “infinite density” at the singularity (which is probably a mathematical artifact, just in ways we don’t know how to resolve just yet) contains all of the mass found inside the hole. The event horizon is just the edge of the region from which it’s impossible to escape.