The idea of the black hole as a cosmological object dates back to 1783, when the Reverend John Michell from Britain realised that, from Newton s laws of gravity and motion, an object with a radius 500 times that of the sun, but of the same density would have an escape velocity greater than the speed of light. The size of the event horizon depends on the amount of mass in the hole. It is due to the escape velocity of a black hole being greater than the speed of light, that black holes are very hard to detect. We have discovered ways in which they can be detected however, through indirect means. Our sun is unusual in that it is alone.
Most stars occur in binary systems .
There are a number of stars in our observable universe that do not behave as we would expect, in the way they move. By knowing the mass of the star we can see, we can determine the mass of the object that it is orbiting. The only other possibility for the object then is a black hole. Finding Black Holes Through the Gravitational Lens Effect Black holes have enormously powerful gravitational fields . Because of the ability of gravity to bend light, a black hole can act as a gravitational lens.