One of the most startling topics in astronomy is black holes. A black hole is an object that has such a large gravitational field that, that part of space becaomes separated from the rest of the universe. No radiation or matter that enters the black hole ever comes out. Because this includes light, black holes cannot be seen. Black holes can form in one of two ways. Either when a star collapses, or when neutron stars run into each other.
Black holes can be almost any size. The largest, or supermassive black holes, are millions of times more massive than the sun, there is thought to be one at the center of the Milky Way. Or, the smallest, called Micro Black holes. However the latter has not been proven to exist yet.
An object that falls into a black hole expierences, "Spaghettification." Because the side of the object that is closer to the black hole experiences more force, it is drawn to the black hole faster than the other side of the object. Thus the object itself stretches out.
A unique feature of black holes is gravitational time dialation. An obvserver would not see an object disappear behind the horizon of a black hole. The object would appear to the oberver as just above the horizon. The object would also grow redder and dimmer as it fells. This is because of gravitational red shift. Gravitational red shift is the loss of energy of light as it moves away from a large gravitational force. The light becomes red because red is the lowest frequency, or energy, of visible light. Eventually the object falling into the black hole would become too dim to see.
There is an intersting theory regarding black holes called worm holes. Think of space time as folded over itself. If there is a black hole on each side of space time, and a "throat" or connection, theoretically one could take a short cut through space time by going through a worm hole. The phrase, "worm hole," is a good analogy. Say a worm on an apple wants to get to the other side of the apple. It could either travel around the circumfrence, or it could take a shorter route by burrowing through the middle of the apple. In theory, worm holes could also connect to parallel universes.