Extraterrestrial Civilizations (20 page)

Suppose we imagined Alpha Centauri B circling our Sun exactly as it, in fact, circles Alpha Centauri A. If we plotted Alpha Centauri B’s orbit relative to the Sun, it would follow an elliptical path that would carry it well beyond the orbit of Neptune at its farthest recession from the Sun, and nearly as close as the orbit of Saturn as its nearest approach.

Under such circumstances, neither star could have a very extensive planetary system of the sort the Sun has now. Planets at the distance of Jupiter or the other giants, circling either star, would be interfered with by the gravitational influence of the other star and would have unstable orbits.

On the other hand, an inner planetary system might still exist. If Alpha Centauri B were circling our Sun as it circles Alpha Centauri A, we on Earth could scarcely tell the difference with our eyes closed. Alpha Centauri B would be a bright, starlike object in the sky, which at its closest approach would be 5,000 times brighter than our full Moon and 1/100 as bright as our Sun. It would add anywhere from 0.1 percent to 1 percent to the heat we receive from the Sun, depending on what part of its orbit it was in, and we could live with that. Nor would its gravitational influence affect Earth’s orbit in any significant way.

For that matter, Alpha Centauri B could have an inner planetary system, too. A planet circling in its ecosphere (which would of course be closer to itself than the ecosphere is to either Alpha Centauri A or the Sun) would not be seriously interfered with by its somewhat larger companion.

As in the case of the 61 Cygni system, both Alpha Centauri A and Alpha Centauri B would have what we might call a “useful ecosphere,” one in which an Earthlike planet could orbit without
serious interference from the companion in terms of either radiation or gravitation.

Robert S. Harrington of the U.S. Naval Observatory in 1978 reported the results of high-speed computer studies of orbits about binary stars.

If a Sunlike star is part of a binary system, and if the separation between the two stars is at least 3.5 times the distance of the ecosphere from the Sunlike star, then it is a useful ecosphere. In the case of our own Solar system, it would mean that the Sun could have a companion at a distance equal to that of the planet Jupiter, without interfering with Earth gravitationally. If the companion were somewhat less luminous than Alpha Centauri B, it would not interfere with Earth significantly as far as radiation was concerned.

There are binary systems with stars even closer together than those of the Alpha Centauri system. The two stars of the Capella binary system are separated by a distance of only 84 million kilometers (52 million miles) or less than the distance of Venus from the Sun.

Neither star in such a binary could have a planetary system in the Sun’s sense. Planetary orbits about one of the stars would be interfered with gravitationally by the other and the orbit would not be stable.

If a planet were far enough away, however, it would not circle either one star or the other but would circle, instead, about the center of gravity of the two stars. Such a planet would treat the two stars gravitationally as a single dumbbell-shaped object.

Harrington calculates that a planet whose distance from the center of gravity of the binary system was equal to at least 3.5 times the distance of separation between the two stars would have a stable orbit. In the case of the Capella system, a planet, to have a stable orbit, would have to be at least 300 million kilometers (185 million miles) from the center of gravity.

In a close binary system, where the two stars are of the proper total luminosity, such an outer orbit might well be within the ecosphere of the two stars taken together. This is another way in which a binary might have a useful ecosphere.

There are pairs of stars that circle each other so closely that our best telescopes cannot make them out as separate stars. Their existence
as pairs is given away by the spectroscope, when the dark lines of the spectrum sometimes double, rejoin, double, rejoin, and so on, over and over.

The simplest explanation is to suppose that there are two stars very close together and circling each other, so that one is receding from us while the other is approaching us. In that case, one would produce a red shift, while the other was simultaneously producing a violet shift, and that is why the lines would appear to double. It is the same principle that causes the lines of a rotating star to broaden. The revolution of two stars is more rapid than the rotation of one star, so that in the latter case the broadening is carried on to the point of actual spreading apart into two lines.

The first such “spectroscopic binary” to be discovered was Mizar, and it was in 1889 that the American astronomer Edward Charles Pickering (1846–1919) detected the doubling of its spectral lines. Actually, the component stars of Mizar are separated by 164 million kilometers (102 million miles), which is a larger separation than that of the stars of the Capella system. The Mizar pair fail to be seen as a pair in the telescope because the system is so far away.

The component stars of some spectroscopic binaries are much closer to each other than that. They can be within a million kilometers of each other, almost touching, and making a complete circle about the center of gravity in a couple of hours.

If we could imagine the Sun replaced by two stars, each half as luminous as the Sun and separated by less than 42,700,000 kilometers (26,500,000 miles)—somewhat less than the distance between the Sun and Mercury—the Earth would remain stably in its orbit. Planets at the distance of Mercury and Venus could not, under those conditions, remain in stable orbit, but Earth could.

In such a case, of course, the sum of the mass of the two stars would be greater than that of the Sun, and Earth’s period of revolution would be considerably less than a year. In addition, with two separate stars at changing distances, Earth’s seasons would show more complicated variations, perhaps, than they now do. Neither of these two factors, however, need render Earth unsuitable for life.

Well, then, how many Sunlike stars in our Galaxy have useful ecospheres?

To begin with, we may fairly assume that all the Sunlike stars
that are single have useful ecospheres, and that means 30 billion right there.

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:

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 such elements as carbon, oxygen, nitrogen, and sulfur, which are not present in appreciable amounts in Population II planetary systems.

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 miles), only ten times the distance between the Sun and Pluto.