What determines the size of a black hole?

Question

This question has a few parts:
From the article here:
http://news.yahoo.com/s/afp/20071031/sc_afp/usastronomy2_071031201428
it is true that black holes vary in size - what determines the size of the hole and does size change over time?

Does light absorbed by a black hole contribute to its mass?

Is the gravity of a black hole the same as that of earth - differing only by strength/force?

Answer 1

Black holes do vary in size. The bigger the singularity (as in, the more massive it is), the deeper the gravity well, and the wider the dark area is where light cannot escape.

Black holes DO get more massive over time. They swallow objects that fall into their gravity wells, and this adds to the mass of the singularity.

Light DOES contribute to the mass of a black hole. Photons have energy, and energy and mass are interchangable in the environment of a black hole. They just don't contribute much.

Yes, gravity operates the same at a black hole as it does on earth. But gravity is created by mass - anything with mass has a gravitational field (called a gravity well). You, your toaster, your uncle Stan. All of us have gravity. The depth of a gravity well, the strength of the pull of gravity, increases as mass increases. So because black holes are HUGELY massive, they have lots of gravity.

Answer 2

Mass alone determines the size of a black hole's event horizon, which is what most people would think of as the size of the black hole. More mass means a bigger event horizon diameter.

I suppose the mass equivalent of the energy in light absorbed by the black hole does add to its mass, but that would be such a small amount that it could not be measured. E=MC^2 gives an astonishingly small amount of mass for a given amount of energy (which is the flip side of the astonishingly large amount of energy you can get from a small mass that makes atomic bombs so powerful).

Gravity of a black hole is exactly the same as gravity of any other object of the same mass. If there were a black hole with the same mass as Earth, then at a distance of 4,000 miles from its center gravity would be 1G, just like it is on the surface of the Earth, 4,000 miles from the Earth's center. But a black hole with that mass would be extremely small. Smaller than 1 mile in diameter I think. Since gravity gets 4 times stronger every time you cut the distance in half, by the time you got to withing 1 mile of the center of that black hole, gravity would be extremely strong. At 1 mile from the center of the Earth you would be deep inside the body of the planet, and 49.999% of the Earth's mass would be above you, pulling you the other way, so you would feel almost no gravity at all.

Answer 3

The actual "black hole" does NOT vary in size - it's *always* a dimensionless point in space.

What *does* vary, is what we see - the Event Horizon. The size of the event horizon is an indicator to how much mass is trapped in the black hole.

If Earth were crushed to a black hole, it would form an event horizon of about 1 1/2 inches - about 1/2 the size of a golf ball. If the sun were to collapse to a black hole, it's event horizon would be about 10-12 feet in diameter.

"New" black holes - those consisting of single stellar cores, are about 10-12 miles in diameter, but grow as they consume mass.

I would guess that light absorbed by a black hole does NOT add to it's mass, as light consists of essentially mass-less particles.

The gravitational force is the same as that of Earth - but remember - say your standing *at* the event horizon. This is the border between normal space, and a zone of such intense gravity that our universe can't support it. You're about 5 or 6 miles away from a point that contains all the mass of 3 to 8 suns - that's a LOT of mass VERY close to you - and, you end up weighing a few million (billion?) pounds.

Answer 4

the top contributor said it all he knows what he is talking about.