A black hole is one of the most bizarre astronomical phenomena ever discovered: a star which has undergone gravitational collapse. It still has the same mass as a normal star but its size is incredibly tiny. It is infinitely dense, with such an enormous gravitational pull near its center that nothing can escape from it - not even light (hence the term black hole)!The basic idea behind black holes is not really that new. Theories that predicted such objects were suggested as long ago as the 18th century. However, it has not been until recently that we have finally collected enough evidence to make the existence of black holes almost unquestionable.
According to our current understanding, a black hole is formed when the fuel of a star (hydrogen) runs out. When this happens, the star’s structure is significantly weakened and it begins to collapse on itself, pulled together by its own gravity. A dead star can turn into three things: a white dwarf which is very small and hot, a neutron star which is even smaller and very dense, or a black hole which is infinitely small and infinitely dense. The more massive the star was originally, the more likely the chances that it will collapse into a black hole.
The problem with black holes of course is that once something gets within a certain distance of one it gets sucked in and never back comes out - at least as far as we know. When the gravitational pull of a star is very strong, the escape velocity (the speed needed to escape a gravitational pull) is also very strong. In the case of a black hole which has an infinitely strong gravitational pull, the speed needed to escape its pull is also infinite. The speed of light in a vacuum is about 300,000 kilometers per second, and according to the theory of general relativity, nothing can travel faster than the speed of light. That means nothing can escape the pull of a black hole once it is within what we call the event horizon. For more on escape velocity see Small Scale Effects Page 9.
Picture the event horizon as an imaginary ring drawn around a black hole that marks the point of no return. Planets and objects outside the event horizon are safe, and may orbit the black hole in the same way that planets orbit a star. But once an object or ray of light crosses the event horizon there is no force in existence that is strong enough to bring it back out!
Another interesting characteristic about black holes is that, at the very center of one, time comes to a stand still. Einstein’s general theory of relativity tells us that the deeper you travel into a gravitational field, the slower time runs for you when compared to the speed of time for people and objects outside that field. And since a black hole’s gravity is infinitely strong at its center, then anything at a black hole’s center is frozen in time so to speak. For more on gravity and time see Small Scale Effects Page 11.
For more on the theory of general relativity see What is Gravity? Pages 5, 6 and 7.
