... a star that was sufficiently massive and compact would have such a strong gravitational field that light could not escape: any light emitted from the surface of the star would be dragged back by the star's gravitational attraction before it could get very far... Such objects are what we now call black holes...
- Stephen Hawking, A Brief History of Time
As the star contracts, the gravitational field at its surface gets stronger and the light cones get bent inward more. This makes it more difficult for light from the star to escape, and the light appears dimmer and redder to an observer at a distance. Eventually, when the star has shrunk to a certain critical radius, the gravitational field at the surface becomes so strong that the light cones are bent inward so much that light can no longer escape. According to the theory of relativity, nothing can travel faster than light. Thus if light cannot escape, neither can anything else...
- Stephen Hawking, A Brief History of Time
The event horizon , the boundary of the region of space-time from which it is not possible to escape, acts rather like a one-way membrane around the black hole... One could well say of the event horizon what the poet Dante said of the entrance to Hell: "All hope abandon, ye who enter here." Anything or anyone who falls through the event horizon will soon reach the region of infinite density and the end of time.
- Stephen Hawking, A Brief History of Time
We also now have evidence for several other black holes in systems like Cygnus X-1 in our galaxy and in two neighbouring galaxies called the Magellanic Clouds. The number of black holes, however, is almost certainly very much higher; in the long history of the universe, many stars must have burned all their nuclear fuel and have had to collapse. The number of black holes may well be greater even than the number of visible stars, which totals about a hundred thousand million in our galaxy alone.
- Stephen Hawking, A Brief History of Time
Black holes are enormously massive stars. The gravitational attraction of these stars is so high that anything which goes into it cannot come out. Even light cannot escape their gravitational pull. Hence they do not emit light.
A German astronomer, Karl Schwarzschild predicted the existence of black holes in 1907. He theoretically showed that black holes are the end results of all stars whose mass is much greater than that of the sun.
In 1939 J.Robert Oppenheimer and Hartland S.Synder showed that black holes are a consequence of Einstein's General Theory of Relativity. Let us consider a star whose mass is greater than that of the sun.
Its size remains normal due to the balance between the two forces- one being the expansion force caused by the enormously high temperature which tends to expand the star's material, and the other being the enormous gravitational pull which tends to contract the star's substance.
At some stage in the star's life, after thousands of millions of years, its nuclear fuel decreases causing a fall in it's core temperature. As a result, the gravitational pull becomes stronger than the expansion force. Gradually, the star begins to collapse.
In this process, the atoms present in the star break into electrons, protons and neutrons. The mutual repulsion between the electrons prevents further contraction.
The star, at this stage, is known as a 'White Dwarf'. In this process, the star is reduced to one-hundredth of its original size, thereby the gravitational pull in the White Dwarf becomes about 10,000 times more than the original value. Under certain conditions the gravitational pull becomes strong enough to overcome the electron repulsion.
The star begins to contract further and in this process of contraction, electrons and protons combine to form neutrons. The star at this stage is called a "Neutron Star". It's size is now reduced to five hundredth part of the dwarf star and the gravitational attraction becomes about 100,000,000,00 times that of the original star.
... the lower the mass of the black hole, the higher its temperature. So as the black hole loses mass, its temperature and rate of emission increase, so it loses mass more quickly. What happens when the mass of the black hole eventually becomes extremely small is not quite clear, but the most reasonable guess is that it would disappear completely in a tremendous final burst of emission, equivalent to the explosion of millions of H-bombs.
- Stephen Hawking, A Brief History of Time
A black hole with a mass of a few times that of the sun would have a temperature of only one ten millionth of a degree above absolute zero... If the universe is destined to go on expanding forever, the temperature of the microwave radiation will eventually decrease to less than that of such a black hole, which will then begin to lose mass. But, even then, its temperature would be so low that it would take about a million million million million million million million million million million million years to evaporate completely.
- Stephen Hawking, A Brief History of Time
... One such black hole could run ten large power stations, if only we could harness its power. This would be rather difficult, however: the black hole would have the mass of a mountain compressed into less than a million millionth of an inch, the size of the nucleus of an atom! If you had one of these black holes on the surface of the earth, there would be no way to stop it from falling through the floor to the center of the earth... So the only place to put such a black hole, in which one might use the energy it emitted, would be in orbit around the Earth - and the only way that one could get it to orbit the earth would be to attract it there by towing a large mass in front of it...
- Stephen Hawking, A Brief History of Time
One can therefore say that the observations of the gamma ray background do not provide any positive evidence for primordial black holes, but they do tell us that on average there cannot be more than 300 in every cubic light-year in the universe. This limit means that primordial black holes could make up at most one millionth of the matter in the universe.
- Stephen Hawking, A Brief History of Time
