If their mass is always contained in a singularity and their density is infinite, how are some black holes supermassive and others are not?
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The Schwarzschild radius of black hole is a product of its mass. More mass = larger event horizon. Density is irrelevant. Also, the concept of a singularity is not really a valid things and hasn’t been for a while. There’s no actual point of infinite density. A singularity is a mathematical problem that arises because our physics breaks down at that point.
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.
Density and mass are not the same thing. The radius of the event horizon is proportional to the black hole’s mass, not its density. Remember that density is just an object’s mass divided by its volume, so the density of any object approaches infinity as the volume approaches zero, regardless of how much mass there is. The singularity’s density may be infinite, but its mass is not.
The supermassive refers to the mass of the singularity. That singularity can be four solar masses or four million solar masses without any trouble.
The event horizons change because they’re based on the mass of that singularity. As you get closer and closer to the black hole, it becomes harder and harder to escape. The event horizon is where light can’t escape anymore and we can’t see what’s happening inside–the horizon beyond which we can’t see events in other words. Where that transition happens depends on the mass of the black hole just like the gravitational attraction of the Earth changes with how close you are to it.
There’s no such thing as a singularity. A singularity is where the maths breaks down and can’t explain what is actually happening. It’s a placeholder.
The mass inside a black hole is not a singularity but instead is some non zero size. We just don’t have the theories to explain what is actually going on but it is not accepted that it’s in an actual singularity.
The size of a black hole depends on its mass.
A guy very fittingly named “Schwarzschild” (black shield) worked that out some time ago.
If you increase the mass of a black hole, by for example dumping some stuff into it, you increase its size by a corresponding factor.
The exact formula is that the radius of the event horizon equals the mass of an object times 2 times the gravitational constant divided by the square of the speed of light.
Since the speed of light, the gravitational constant and 2 are all constant, the radius grows proportional with the mass of an object.
You can do that in reverse to, to figure out how much you would need to compress an object to turn it into a black hole. Our planet would need to be compressed to a radius of 8.87 mm to turn into a very small and not very long lived black home.
“Massive” usually means a thing has a lot of volume, but in astrophysics it means it has a lot of mass.
When things have a lot of mass sometimes they become smaller because of their own gravity so we can’t use their size to measure how “big” they are.
Even though all the black holes are compressed to the max (infinite density), some have more mass in there than others, and since mass is what causes gravity and gravity is what causes event horizons, some have bigger event horizons.
Because some have more mass?
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