Two questions arise when discussing the astronomical consequences of the possibility of the existence of black holes .Where would we expect to find them, and how would we detect them ?

To form a black hole we need to find places in the Universe which are much denser than their surroundings. The matter in the Universe was at its most dense at the origin of the Universe the big bang. The earliest time at which we can hope to apply known gravitational theory is the so-called PLANCK TIMK which is 10 -43 seconds after the big bang. A black hole formed at this time has a mass of 10-8kg and a radius of l0-35m. Anything with a smaller radius would not form a black hole, and anything with a, much larger radius would not just form a black hole, but would cut itself off from our Universe and form a tiny Universe of its own ! There could be a large number of such mini black holes around without us noticing them. One possibility for finding them is that they could be mistaken for heavy particles in detectors of cosmic radiation.

When a star like the Sun has evolved so far that it has run out of nuclear fuel, it becomes a white dwarf which gradually cools and fades away. When a star more massive than about four solar masses has run out of nuclear energy, however, its core is too mass¬ive to form a white dwarf. According to present theories, the core collapses to form a neutron star – possibly a pulsar – and the energy released in the collapse blows off the outer layers of the star in the form of a supernova explosion. However, if a star exceeding about eight solar masses runs out of nuclear fuel, the compact remnant it would wish to leave behind is too massive to become even a neutron star. There are then two possibilities. Either the core of the star is blown apart in the supernova explosion or the core collapses to form a black hole. We would expect the masses of such black holes to be three solar masses and greater. If all the stars that have existed in the Galaxy with masses greater than eight solar masses left black holes as their remnants there could be as many as 108 black holes in the Galaxy at present. That is, about one in one thousand of all starry objects could be a black hole!

Another region of space where the density of material is higher than in the surroundings is in the core of a globular cluster. This has a much higher density of stars than its surroundings. It is quite possible that at the same time as the cluster formed, a large amount of gas collected in the centre of the cluster and collapsed to form a black hole. The mass of such a black hole could be as much as a thousand solar masses. Similarly, it is entirely possible that at the same time as a galaxy forms, a large quantity of gas collects in the nucleus and creates a black hole. Such a black hole could contain as much as 109 solar masses. It has also been suggested that some gas clouds, collapsing to form galaxies, may have overshot and formed black holes themselves – containing as much as 1012 solar masses. Perhaps a nascent cluster of galaxies could have overshot the mark, to form a black hole of some 1014 solar masses. It has even been suggested that the Universe itself is a giant black hole – after all nothing can escape from it, by definition! This is not however, a useful concept and we shall not pursue it further.

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