Black holes are objects so dense that not even light can escape their gravity, and since nothing can travel faster than light, nothing can escape from inside a Loosely speaking, a black hole is a region of space that has somuch mass concentrated in it that there is no way for a nearby object toescape its gravitational pull. Since our best theory of gravity at the moment isEinstein's general theory of relativity, we have to dig into some results of thistheory to understand black holes in detail, by thinking about gravity underfairly simple circumstances. Suppose that you are standing on the surface of aplanet. You throw a rock straight up into the air. Assuming you don't throw ittoo hard, it will rise for a while, but eventually the acceleration due to theplanet's gravity will make it start to fall down again. If you threw the rockhard enough, though, you could make it escape the planet's gravity entirely. Itwould keep on rising forever. The speed with which you need to throw therock in order that it just barely escapes the planet's gravity is called the"escape velocity." As you would expect, the escape velocity depends on themass of the planet: if the planet is extremely massive, then its gravity is verystrong, and the escape velocity is high. A lighter planet would have a smallerescape velocity. The escape velocity also depends on how far you are fromthe planet's center: the closer you are, the higher the escape velocity . TheEarth's escape velocity is 11.2 kilometers per second (about 25,000 M.P.H.),while the Moon's is only 2.4 kilometers per second (about 5300 M.P.H.).Wecannot see it, but radiation is emitted by any matter that gets swallowed byblack hole in the form of X-rays. Matter usually orbits a black hole beforebeing swallowed. The matter spins very fast and with other matter forms anaccretion disk of rapidly spinning matter. This accretion disk heats up throughfriction to such high temperatures that it emits X-rays. And als...
