How is a black hole formed?

   To end up as a black hole, a star must be at least two or three times the mass of our Sun. Any star will collapse once its nuclear fuel is all used up. The reason is that the force of the nuclear fusion process pushing outward from the star's core balances its immense gravity. An average-mass star, like the sun, will end up as a white dwarf star. A star five to eight times the mass of the sun will explode to produce a supernova, shedding much of its mass, and end up as a densely packed neutron star. A star ten to forty times the mass of the sun will produce a gravitational collapse so complete that, after the supernova, only a black hole remains. As a giant star collapses, its mass gets so concentrated that the force of gravity becomes completely overpowering. The collapsed star's surface, called the event horizon, becomes the point of no return. Anything crossing the event horizon gets drawn in and cannot escape.