What are Black Holes?
According to Einstein a black hole is a region of space in which the gravitational field is so powerful that nothing can escape its pull after it has fallen past its event horizon. It's gravitational pull is so strong that no light can escape so this renders the hole's interior invisible, and indistinguishable from the black space around it. This is why we call them Black Holes.
How many types of black holes are there?
The simplest type of black hole is one that has mass but no charge nor angular momentum. These are often called Schwarzschild black holes.
What are the parts of a black hole?
A black hole consists of:
An Event horizon:
The event horizon of a black hole is the point of no return. Once an object passes this point there is no escape from the black holes gravitational pull.
A gravitational singularity
According to general relativity, there is a space-timesingularityat the centre of a spherical black hole.
A Photon sphere
This is a spherical boundary that traps photons in a circular orbit. Light can escape from a photon sphere.
An eliptical region located outside a rotating black hole.How can a black hole be found?
A black hole may reveal its presence despite its invisible interior, by it's interaction with matter orbiting around it's event horizon and it emitting Hawking radiation.
How are they formed?
There are believed to be different ways in which black hole are formed.
It is believed that most Black holes are formed when some very dense stars go nova. When a star's fuel is running low and it is cooling down it's internal pressure is insufficient to resist it's own gravity so it collapses in on it's self. This is known as gravitational collapse. During the creation of the universe just after the big bang there is believed to have been objects with enough density to collapse in this way and form black holes.
It is believed by some that they could be formed in high energy collisions. (This is the theory that made people believe that the Large Hadron Collider could create a black hole and destroy the earth.) There is no evidence to support or disprove this theory. We will have to wait and see what happens.
Do they grow?
Once a black hole has formed it pulls in every thing that is in it's gravitational pull, as it does so it will grow in mass. It will in mass more if the black hole was formed when one star of a binary system goes nova, as it can take matter from the other star and so expand it's self. In this case it could even swallow the other star.