One key to these questions depends on the “geography” of space. Astronomers are still not at all certain of the age of our galaxy, but we can pick 10 billion years as a convenient value. Ten billion years ago, there was no Milky Way galaxy, no stars, no planets, no life. Only a vast, distended cloud of tenuous gas—a nearly perfect vacuum by human standards, but so large that it contained more than 200 billion times the mass of our sun. (Where this gas came from is a cosmological question that will be carefully avoided here.) This tremendous cloud consisted of hydrogen atoms, simple protons and electrons. Nothing more. Much of this primordial gas is still present between the stars today; we see it in the brilliant swirls of nebulae, we hear its 21-centimeter-wavelength “song” on our radio telescopes.
In some unknown manner, the cloud began to rotate and contract. As it did so, tiny swirls and eddies began to appear, to break into still smaller whirls and ultimately to produce stars. (The first stars, evidently, were produced in large batches. We can see them today. They are very ancient globular clusters which may contain 100,000 or a million individual stars, packed together as closely as the planets of our own solar system.) As the original gas cloud continued to rotate and contract it produced many more stars. The nucleus of the Milky Way is so thick with stars that our own region of the galaxy, out toward the edge, must be classed as a stellar desert. Thus the central portions of our galaxy, according to astronomical theory, contain the oldest stars.
As the gas cloud condensed, its rotation became faster. Its shape became flattened, bulging at the center. Finally, to maintain stability, the cloud began to fling off great belts of gas from its middle. These belts—long, twisted filaments of star-producing gas—became the spiral arms of our galaxy, thousands of light-years in cross-section, tens of thousands of light-years in length. In one of these belts, known as the Carina-Cygnus Arm, is the sun and our solar system, some 25,000 light-years from the star-thronged center of the galaxy.
It would appear, then, that our sun is a latecomer to the galaxy. Indeed, astronomers refer to the sun as a “second generation” star. Of course, many of the stars in our region of the galaxy are much younger. Sirius, for example, can hardly be more than a few hundred million years old and Rigel is probably no older than man himself—one million years.
Before we go any further, we had better straighten out a bit of astronomical jargon. Astronomers frequently refer to two types of stars in the Milky Way. Stars in our own quarter of the galaxy—including the sun—are called Population I. Other stars, such as those nearer the galaxy’s center and in the globular clusters, are called Population II. The confusing thing is that the Population II stars are older, hence are “first generation” stars, while the younger Population I stars are “second generation.” In addition to their different locations in the galaxy, Population II stars apparently have rather different chemical compositions than our own neighbors of Population I. This difference is one of degree, and at first glimpse would seem triviaclass="underline" Population II stars are comparatively poor in heavier elements. Now, all stars of all populations are about 99 percent hydrogen and helium; the younger the star, the higher the percentage of hydrogen compared to helium. In any case, the heavier elements—such as the metals—are restricted to about one percent of the star’s mass. But, just as in a detective story, this seemingly insignificant fact is a critical clue.
The older Population II stars are metal-poor. The younger Population I stars are relatively metal-rich. If the galaxy began with nothing but hydrogen gas where did the metals come from?
The answer to that riddle was first proposed about a dozen years ago by a group of English astronomers and mathematicians, among them Thomas Gold, Fred Hoyle and Hermann Bondi. The stars are nuclear cookers, they said. We know that the sun is fusing hydrogen into helium, and in the process converting four million tons of mass into energy every second. But, said Gold, Hoyle and Bondi, this is only the beginning of a star’s career. At a certain point in its lifetime (some five billion years from now, for the sun) a star reaches a critical stage. Its hydrogen fuel is becoming depleted. At the core of the star is a large amount of helium—”ash” from the hydrogen fires—under tremendous pressure and, consequently, at very high temperatures, perhaps 100 million degrees Kelvin.
Under these conditions, the helium will begin to fuse into heavier elements: oxygen, carbon, neon. Eventually, the star goes on to produce constantly heavier elements at constantly higher internal temperatures. Finally the star runs out of energy sources, collapses and explodes. Most of its material—from hydrogen on up through the heavier elements—is hurled out into space. This is a supernova.
The theory that results is that the older Population II stars “cooked” the heavier elements within their cores and then spewed them out in supernova explosions. (Supernovas occur about once every five hundred years in the Milky Way, on the average.) The remnants flung into interstellar space mix with the primeval hydrogen and thus provide new raw material for “second generation” stars. But notice that these newer stars have a much richer raw material to build with—it contains helium, oxygen, neon, iron and many other elements. Even rare, short-lived radioactive elements, such as californium (an “artificial” element on Earth) have been observed in the spectra of old Population II stars.
Now then, what has all this stellar cookery to do with the possibilities of intelligent life throughout the galaxy? Simply this:
The oldest stars in the Milky Way were built on hydrogen alone. They could not have planetary systems like ours because the heavier elements were not yet available. There might be a few spheres of frozen hydrogen circling these stars at great distances, but they would be sterile worlds.
The sun is a Population I star, a “second generation” luminary. It possesses a relatively large amount of heavy elements; it also possesses a planetary system that harbors life and intelligence. But the sun is a rather old Population I star—age, five billion years, about half as old as the entire Milky Way galaxy. Can it be that the first five billion years of the galaxy’s existence were spent mainly in building up heavier elements so that “second generation” stars like the sun could arise and produce planets, life and intelligence? If so, then we might well be one of the first intelligent races in the Milky Way. The teeming center of the galaxy might be devoid of life and intelligence.
Although this kind of astronomical evidence might lend support to the speculation that we are among the galaxy’s elder citizens, we should be very careful about reaching conclusions from an admittedly oversimplified paste-up of assumptions and theories. The idea has a certain satisfaction to it from an egocentric point of view, and it goes a long way toward explaining why They have not visited Earth. There might not be any of Them. Or, if there are, They might not yet have attained the advanced technology necessary for interstellar flight.
But to assume that we are in first place in the galaxy’s IQ rating is rash indeed. If astronomy has taught man anything it is the painful fact that we are not special creatures in any sense of the term. Our star is an average one, and the conditions that led to the formation of our planet and ourselves are probably not very extraordinary. Even granting that we might be among the elder citizens of the Milky Way, we must assume that among the galaxy’s 100 billion stars there are some that harbor much more intelligent species.