In simplest terms a black hole is an area of space that has so much mass concentrated in it that there is no way for nearby objects to escape its gravitational pull. To understand the magnitude of this gravitational pull we need to explore our best theory of gravity, at the present moment, Einstein's general theory of relativity. His premise states that anything with mass will bend the fabric of space and time. The greater the mass the more distortion in the fabric of the universe. Imagine the universe as a giant rubber sheet with objects, varying in weight, placed upon it, representing planets, asteroids, moons, etc. The objects with greater mass distort the fabric more and hence have a stronger gravitational field. For another object to escape the pull it must achieve something called escape velocity. Escape velocity is the speed you must achieve to get away from a planet and into space. The earth escape velocity is 25,000 mph, while the moon, with a smaller mass, is about 5,300 mph. A black hole has such a large concentration of mass in such a small area that the escape velocity is faster than the speed of light. Then, since nothing travels faster than light, nothing can escape a black holes gravitational field. Even a beam of light would be pulled back by the gravity and be unable to escape (http://www.signet.com).
             There are a lot of different types of black holes, but they all seem to be formed in pretty much the same manner. When a star has used up all of it fuel it can no longer stand the immense force of gravity that has been pulling on it for its entire life. The star expands quickly and then collapses in on its self. The tremendous weight of the outer layer cause the core to collapse and the star begins to form tremendous mass and dig deeper into the fabric of space and time. It continues to compress until it forms a singularity, which is an object with infinite density (http://www.geocities.com/).
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