Extraterrestrial Civilizations - Isaac Asimov [69]
Under such packed conditions, collisions and near-collisions may not be very rare. Transfer and capture of mass may serve to build up stars of great mass that quickly explode with a force that leads to a veritable chain reaction of explosions and to the formation of “black holes.” These are the ultimate in star condensations (see my book, The Collapsing Universe).
A black hole is matter at its ultimate density, and has a gravitational field so intense at its surface that nothing can escape it, not even light.
If a black hole is formed under conditions in which matter of all kinds surrounds it (as in galactic centers), such matter is constantly spiraling into the black hole, releasing x-rays and other energetic radiation in the process. (This radiation is released before the matter actually enters the black hole, so that it can escape into outer space.) The black hole gains in mass and may eventually be large enough to swallow stars whole.
There is a strong radiation source at the very center of our own Galaxy, and it may well be that a black hole is present there, one that has a mass of 100 million stars. The giant galaxy M87 was reported in 1978 to have a black hole in its center in all likelihood, one that has a mass as high as that of 10 billion stars. It may even be that every galaxy and every globular cluster has a black hole at its core.
Such violent events at the centers of galaxies may produce the massive atoms of complex elements and spread them through space, but of what use would that be? Those violent events are the sites of emission of enormous quantities of energetic radiation, and for many light-years in every direction, life (as we know it) might for that reason be impossible.
The Population II regions are therefore, considering chemical constitution or energetic radiation, doubly unsuitable for life.
Suppose we pass on to the outskirts now, regions where the violence and radiation of the center does not reach.
Here, the primordial gas was relatively thin and was distributed irregularly. For that reason, stars were formed irregularly, and giant stars were routinely formed in numbers that could not possibly have existed in the center. (Of course, many medium and small stars were also formed.)
The stars in the outskirts of a galaxy, rich in giants and spread out irregularly over much vaster volumes of space than exist in the central regions, are referred to as Population I stars.* What’s more, there were places in the outskirts where the gas was too thin to condense readily. To this day, therefore, the outer Population I regions of the galaxies are rich in clouds of gas and dust.
The original Population I stars were as entirely hydrogen-helium in constitution as were the Population II stars. There was this difference, however:
The giant stars that formed in the galactic outskirts didn’t remain on the main sequence long. A few hundred thousand years only, for the real monsters; a few million years for the mere titans; as much as a billion years for those that were simply giant.
And when they left the main sequence, expanded and finally collapsed, they exploded into supernovas of unimaginable violence. Vast volumes of gas, containing significant quantities of complex elements rolled out into space, adding themselves to the clouds of uncondensed gas that were already present.
Such explosions take place repeatedly in the outer regions of a galaxy, but so widely separated are the stars in those vast outer regions that supernovas do not seriously affect any stars other than (at most) their immediate neighbors.
As many as 500 million supernova explosions may have taken place in the outskirts of our own Galaxy since it came into being. The 500 million have enriched space enormously with complex elements, and have added to the density of the clouds of gas and dust that existed from the beginning. The outward force of the explosion may even have served as an initiation of swirls and compressions in nearby gas clouds that led to the formation