There are two types of black holes:
- Schwarzschild - Non-rotating black hole
- Kerr - Rotating black hole
The Schwarzschild black hole is the simplest black hole, in which the core does not rotate. This type of black hole only has a singularity and an event horizon.
The Kerr black hole, which is probably the most common form in nature, rotates because the star from which it was formed was rotating. When the rotating star collapses, the core continues to rotate, and this carried over to the black hole (conservation of angular momentum). The Kerr black hole has the following parts:
- Singularity - The collapsed core
- Event horizon - The opening of the hole
- Ergosphere - An egg-shaped region of distorted space around the event horizon (The distortion is caused by the spinning of the black hole, which "drags" the space around it.)
- Static limit - The boundary between the ergosphere and normal space
If an object passes into the ergosphere it can still be ejected from the black hole by gaining energy from the hole's rotation.
However, if an object crosses the event horizon, it will be sucked into the black hole and never escape. What happens inside the black hole is unknown; even our current theories of physics do not apply in the vicinity of a singularity.
Even though we cannot see a black hole, it does have three properties that can or could be measured:
- Electric charge
- Rate of rotation (angular momentum)
As of now, we can only measure the mass of the black hole reliably by the movement of other objects around it. If a black hole has a companion (another star or disk of material), it is possible to measure the radius of rotation or speed of orbit of the material around the unseen black hole. The mass of the black hole can be calculated using Kepler's Modified Third Law of Planetary Motion or rotational motion.