Extraterrestrial Civilizations - Isaac Asimov [67]
Of the binary systems we have eliminated all Sunlike stars that have as a companion a giant star (or a small, dense star that is the shrunken and condensed remnant of a giant star that exploded).
Of the 18 billion Sunlike stars that are in binary association with another Sunlike star, we might estimate conservatively that only one-third have useful ecospheres. That would mean 6 billion stars in this category. At a guess (nothing more than that) I would say there would be 4 billion binaries with two Sunlike stars, in which only the larger would have a useful ecosphere; and one million binaries of this kind in which both Sunlike stars would have a useful ecosphere.
Finally, what of the binaries in which a Sunlike star is teamed with a midget star? We had estimated there were 25 billion such binaries in the Galaxy altogether. A midget star is far less likely to interfere with a planetary system, either gravitationally or radiationally, than a larger star would. We might estimate, again conservatively that two-thirds of these Sunlike stars have useful ecospheres, and this would mean approximately 16 billion stars.
We now have our fourth figure:
4—The number of Sunlike stars in our Galaxy with a useful ecosphere = 52,000,000,000.
STAR POPULATIONS
Yet we are not through. A Sunlike star may have a useful ecosphere and even so there may be no possibility of an Earthlike planet revolving within that ecosphere. As it happens, stars may differ in ways other than mass, luminosity, and state of association. They may also differ in chemical composition.
When the Universe first formed about 15 billion years ago, matter seems to have spread outward from an exploding central mass. To begin with, that matter consisted almost entirely of hydrogen, the simplest element, with a small admixture of a few percent of helium, the next simplest element. Virtually none of the still heavier elements existed.
This primordial matter, forming a Universe-sized mass of gas, split up into turbulent sections, each of galaxy size. Out of these protogalaxies, the stars of the various galaxies formed.
If we concentrate on any of the galaxy-sized masses of gas, the central regions were denser than the outer regions. The gas in the central regions split up into small, star-sized masses pretty evenly, each crowding the other so that no one star-sized mass had more chance than another to collect its share. The result was that very many stars were formed, all small and medium in size; virtually none of them giants. What’s more, nearly all the gas was collected by one star or another, so that the interstellar regions in a galactic center ended up almost gas free.
These stars, characteristic of the central regions of a galaxy, are called Population II stars.
For regions at moderate distance outside the center, there is not enough gas to form a steady, continuous packing of stars. The gas shreds into a couple of hundred smaller pockets of denseness, however, and out of each of them a tight group of some ten thousand to a million stars form. In this way, a “globular cluster” is formed. Globular clusters are arranged in a spherical shell about the galactic center, and are virtually dust free; the stars in such clusters are also Population II in nature.
The point to remember about Population II stars is that they were formed out of a gas that was largely hydrogen, with a little bit of helium, and virtually nothing else. The planetary systems that formed about such stars must be made up of planets that are also of that chemical structure. What planets do form about Population II stars would rather resemble Jupiter and Saturn in composition, but would lack the admixture of ices—water, ammonia, methane, and so on—that those planets possess.
There would be no small objects in the planetary systems, since small objects would not have enough gravitational pull to retain the hydrogen and helium which were alone available.
Nor would there be life, for to have life (as we know it) we need