Black hole basics.

If you put a lot of mass in a small volume, you will create a very strong gravitational field. Even light is bent toward the center of the object by its strong gravity.

With enough mass in the small volume, gravity is so strong that nothing, not even light, can escape. That's a black hole.

Einstein's theory of gravity predicts the possibility of black holes. For any radius R that you have in mind, the theory tells how much mass M you have to have inside radius R in order for nothing to be able to escape.

For R = 30 km, the required mass is M = 10 M_sun.
For R = 3 x 10⁶ km, the required mass is M = 10⁶ M_sun.

A large black hole with no matter around it (a rather theoretical object) is really quite benign.

Say you went to visit a black hole with a mass M = 10⁶ M_sun.
Then its ``Schwarzschild radius'' is R = 3 x 10⁶ km.
- That's about 4 times the radius of the Sun.
You just stop your starship a few AU away and drift in.
If you weren't careful, you will have too much angular momentum and you will simply go into orbit around it.
- You will stay in orbit forever, or until you fire your rocket engines and go away.
If you are very careful, your orbit will take you inside the Schwarzschild radius.
Nothing much happens as you cross the Schwarzschild radius.
Once you are inside the Schwarzschild radius, you cannot send messages to the outside.
You very quickly fall way inside and are crushed by the gravational forces.
What happens to the atoms that you were made of? No one really knows.

A large black hole in reality would probably have an ``accretion disk'' around it.

Gas is orbiting in a disk around the black hole.
It orbits slowly far away from the black hole, but very fast near the black hole.
It is heated by friction, losing gravitational potential energy and getting very hot.
As the gas loses losing gravitational potential energy it spirals toward the black hole.
Meanwhile, because it is so hot, it radiates photons.
Since the black hole gravity is so strong, the matter in the accretion disk has lots of gravitational potential energy to lose.
Thus a black hole that is growing quickly should radiate enormous amounts of energy.
Also, there is enormous pressure on the inside of the disk.
The pressure pushes matter out.
Matter can't get out in the direction of the disk.
But it can get out towards the poles.
This should lead to jets of matter shot out of the central region at high velocity.

Davison E. Soper, Institute of Theoretical Science, University of Oregon, Eugene OR 97403 USA soper@bovine.uoregon.edu