The theory of General Relativity produces a formula for the radius of the event horizon of a black hole, R = 2*G*M/c
2. Here, G is Newton\'s gravitational constant, M is the mass of the object and c, as always, is the velocity of light. For a black hole with a mass of our sun this radius turns out to be about 3 kilometers. Of course our sun is not a black hole so the calculation is not relevant. Even for a real, super-massive black hole of a million solar masses, the radius is only a few million kilometers, a puny distance on the scale of our galaxy.
It is believed by most astronomers that the star Sagittarius A near the middle of our galaxy is a black hole. Take a look at http://en.wikipedia.org/wiki/Sagittarius_A* for some more information.
LeeH
(published on 05/16/2013)