# Black Hole Questions

dmt740
Ok, these may be silly questions, but when learning new things I figure you won't know if you don't ask. So here goes:

1. When a star goes supernova and the core collapses into a neutron star, the new star spins fast to conserve angular momentum (I think) and it rotates rapidly. Well if that star is sufficently massive to continue its collapse into a BH, does the rotation increase as well? And wouldn't that mean that the BH created would rotate at an extreme rate almost approaching c? Or is it that the warping of space around the BH is such that the meaning of "rotation" is not applicable any longer?

2. Would all BHs be the same "size"? If they all collapse into a point of zero dimensions, it shouldn't matter how much mass is in it because zero dimensions is zero dimensions. The only difference of effect would be the remaining gravitational pull of the mass that originally created the BH.

## Answers and Replies

Science Advisor
Second question first. The size of a black hole is generally considered to be the size of the Event Horizon (EH), which is the distance from center at which escape velocity > c (where the "black" begins). The distance at which this happens is dependant upon the strength of the gravitational field, which is of course dependant upon the mass of central object (the singularity). So, a more massive BH is much "bigger" than a less massive one, in terms of the radius of the EH, often refferred to as the Schwartschild Radius. But you are correct to say that the object at the center should be the same size for all BH's, assuming they do indeed collapse to a point of zero dimensions.

Phrak
After collapse there are just three values that completely describe a BH. Mass, angular momentum and charge. The radius is related to the charge, so it doesn't enter separately. The angular momentum remains. It, and the charge effect the shape of the event horizon. You might find some perdy pictures if you google Kerr Solution and stuff like that.

(to be complete, you really need a few more values to describle a black hole, like it's position in space, but these values never seem to quoted.)

Second question first. The size of a black hole is generally considered to be the size of the Event Horizon (EH), which is the distance from center at which escape velocity > c (where the "black" begins).

I understood the event horizon to be that (imaginary) boundry for which any particle within the boundry could not escape to infinity.

rubecuber
since black holes keep on getting denser (unless you believe that jargon in the NULLPHYSICS) book, it depends on the time that they collapse. look at it like this" neutron star A collapses at time 0, then neutron star collapses at time 2, both turn into black holes. Both black holes will keep on collapsing upon themselves at the "same" speed. So, at time n, black hole a will be however many tons it gained in the first two seconds before star B collapsed. this is assuming that they collpase at the "same" rate