Extraterrestrial Civilizations - Isaac Asimov [68]
Of course, the heavier elements do form with time. As each Population II star burns over the course of the billions of years, heavy elements build up in its core through fusion reactions, including particularly those needed for life.
These heavier elements are, however, useless for the production of life as long as they remain at the core of stars.
Eventually a star leaves the main sequence, however, expands, and then collapses. If the star is a small one and not too much larger than our Sun, the process of collapse is not accompanied by an explosion, and a white dwarf is produced. In the process of collapse, however, up to one-fifth of the mass of the collapsing star is left behind as a cloud of gas surrounding the white dwarf. The result is what is called a planetary nebula. The expanding shell of gas slowly spreads through space until it becomes too rarefied to detect visually, and left behind is a bare white dwarf.
If a star is more massive than 1.4 times the mass of the Sun, it explodes as it collapses. The more massive the star, the more violent the explosion. Such a supernova explosion can eject up to nine-tenths of the mass of a star into space as swirls of gas.
The gas spreading into space, whether it started as the product of a planetary nebula or of a supernova, contains appreciable percentages of the more complicated elements. The process of supernoval explosion would, in particular, manufacture the really complex elements, which do not form in the center of stars that are quietly maturing on the main sequence. In the center of those stars, nothing past iron is produced, whereas in the comparatively brief episode of the supernova explosion, elements up to uranium and beyond are produced.
The Population II stars, however, are not very massive and, containing as they do a high percentage of hydrogen to begin with, they remain on the main sequence for a long time. Even in the 15 billion years that have elapsed since the big bang, almost all of those stars are still on the main sequence and the heavy elements remain tucked inside their cores.
From all this we might deduce that the centers of galaxies are quiet, uneventful places—and we would be wrong.
In 1963, quasars were discovered. These are starlike objects; indeed, when first discovered they were thought to be dim stars of our own Galaxy. They turned out, instead, to be located at distances of over a billion light-years, farther than any of the visible galaxies. To be visible at that distance, quasars had to be shining with the luminosity of 100 ordinary galaxies. Yet they are small objects, at most one or two light-years across, as compared with the diameters of many thousands of light-years that characterize ordinary galaxies.
The evidence now seems to favor the thought that quasars are bright galactic centers, surrounded, of course, by the outer structure of an ordinary galaxy. At the huge distance of the quasars, however, only the bright center is visible.
The question, then, is: what makes a galactic center blaze so brightly?
It would appear that the very centers of galaxies are quite commonly the sites of violent events. Some are visibly exploding; some give off vast streams of radio waves from sources on either side of the center as though an explosion has ejected material in opposite directions.
All galactic centers are bright; some are brighter than others. As we consider galaxies that are more and more distant, we reach a point where we see only the brightest of the bright galactic centers—the quasars.
What happens to the quiet Population II stars to initiate such violence?
If they were left to themselves, nothing; but they are not left to themselves. In the crowded precincts of the galactic centers, the stars are a million times as densely packed as in our own area of the galactic outskirts. The stars at the galactic center may be separated by average distances of only 70 billion kilometers (45 billion