What is a black hole?
Loosely speaking, a black hole is a region of space that has so much mass concentrated in it that there is no way for a nearby object to escape its gravitational pull. Since our best theory of gravity at the moment is Einstein’s general theory of relativity, we have to delve into some results of this theory to understand black holes in detail, but let’s start of slow, by thinking about gravity under fairly simple circumstances. Suppose that you are standing on the surface of a planet. You throw a rock straight up into the air. Assuming you don’t throw it too hard, it will rise for a while, but eventually the acceleration due to the planet’s gravity will make it start to fall down again. If you threw the rock hard enough, though, you could make it escape the planet’s gravity entirely. It would keep on rising forever. The speed with which you need to throw the rock in order that it just barely escapes the planet’s gravity is called the “escape velocity.” As you would expect, the escape v
A. Very rare, very massive stars collapse under their own weight, and crush their cores into “black holes” – objects so dense that nothing – not even light – can escape. There are only a handful of black holes in our Milky Way galaxy, and the nearest is thousands of light years away. These black holes are “visible” because they have normal stars orbiting around them. The existence, and the mass, of the black hole can be deduced from the motion of the normal star. A small amount of material from the atmosphere of the normal star can fall into the black hole. Before it does, it gets very hot, and produces X-rays. Black holes can therefore be detected by X-ray telescopes in space. The material of the black hole, and any additional material which falls into it, becomes very dense. Astronomers do not know its exact state. The black hole can exist almost forever. It will not explode, or change into something else, or go somewhere else.
