Well, it's simple. If you pack a given mass, M, into a small enough space, you will create a black hole.
Do you know what defines a black hole? It's simple, really. First, imagine the good ol' Earth. You know that if you throw a baseball up into the air, it'll come to the ground, right? What happens if you throw it really hard, by strapping a big rocket to it? If you can accelerate it up to 11 km/s, the baseball can actually leave the Earth's gravitational field entirely, and never come home.
That speed, 11 km/s, is called the
escape velocity, because an object will have to go at least that fast to escape the Earth's gravity.
A black hole is an object with such intense gravity that even light cannot escape. In other words, at some distance from the object, the escape velocity exceeds the speed of light. The distance from the object at which this occurs is called the
event horizon, and the event horizon is at a distance of 2GM/c^2 from the object.
Let's put in a concrete example. How about the mass of the Sun? How small would you have to compress the Sun to turn it into a black hole?
Answer:
http://www.google.com/search?hl=en&ie=UTF-8&oe=UTF-8&q=2+G+(mass+of+sun)+/+c^2&btnG=Google+Search
Keep in mind that I haven't mentioned singularities at all. Why not? Because you don't have to have a singularity to have a black hole. All you have to do is get enough mass into a small enough space. Current physical models know of no forces that could prevent such a mass from collapsing all the way to a singularity, and most physicists feel that means that current physical models are wrong!
It is quite likely that a theory of quantum gravity like string theory will eliminate the singularity in our models of black holes.
- Warren