Extraterrestrial Civilizations (17 page)

The Danish-American astronomer Kaj Aage Gunnar Strand (1907–), working under Van de Kamp, detected a tiny wobble in the motion of one of the stars of the 61 Cygni two-star system, and deduced the presence of a companion body circling it, one that was much too small in mass to be a star. It was massive enough to be a large planet, however, one that was eight times as massive as Jupiter. This discovery was announced in 1943.

Since then a similar wobble was discovered in connection with Barnard’s star, a small star only 6 light-years away. In its case, the wobble may indicate the presence of two planets, one as massive as Jupiter, orbiting in 11.5 years, and one as massive as Saturn, orbiting in 20 to 25 years. Other nearby stars, such as Ross
614
and Lalande
21185
have also shown wobbles that seem to indicate the presence of large planets.

In short, we have discovered not one but half a dozen small, nearby stars that may have large planets. Under the circumstances
(and it must be admitted that the observations are so close to the limit of what can be seen that not all astronomers are ready to accept the conclusion without cautious reservations) it would seem that we must conclude that planetary systems are very common and that all the slow-rotating stars, at least, have them.

Let us be conservative and confine the planetary systems only to the slow-rotating stars, which make up 93 percent of the whole. In that case we get our second figure:

The fact that, according to our conclusions in the previous chapter, there is an enormous number of planetary systems in our Galaxy does not,
in itself
, mean that life is rampant.

Different stars may not be equally suitable as incubators of life on their planets and the next step is, therefore, to consider this possibility and to determine (if we can) which stars are suitable, and how many such suitable stars there might be.

If it turns out that the requirements for a suitable star are exceedingly numerous and complex, it may be that virtually no stars are suitable, and all those planetary systems might as well not be there, as least as far as extraterrestrial intelligence is concerned.

Such extreme pessimism is, however, unnecessary, for we begin with two statements, one of which is absolutely certain.

The certain statement is that our Sun is adequate as an incubator of life, so it is therefore possible for a star to be suitable. The second statement, somewhat less than completely certain but so near
to certainty that no astronomer doubts the fact, is that the Sun is not a particularly unusual star. If the Sun is suitable, many stars should be.

Let us begin by asking how stars might differ.

The most obvious point of difference, one that was recognized as soon as inquisitive eyes turned upward toward the night sky, is that the stars differ in brightness.

This difference, of course, may be due entirely to differences in distance. If all stars were equally bright when viewed at a given distance (if all, in other words, were of equal “luminosity”), then those that were nearer to us, in actual fact, would be brighter in appearance than those that were farther from us.

Once the distances of the stars were worked out (the first to accomplish the task, in 1838, was Bessel, who six years later discovered Sinus’s companion star) it turned out that the apparent brightnesses were not entirely due to different distances. Some stars are intrinsically more luminous than others.

Some stars are more massive than other stars, too, but mass and luminosity go hand in hand. As Eddington showed in the 1920s, a more massive star
had
to be more luminous. A more massive star had a more intense gravitational field and, in order to keep it from collapsing, the temperature at its center had to be higher. A higher central temperature produced a greater flood of energy pouring out of the star in all directions, and its surface was both hotter and more luminous.
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What is more, luminosity goes up more rapidly than mass. If Star A is two times as massive as Star B, then Star A has a greater tendency to collapse in on itself because its gravitational field is greater. To withstand the greater gravitational field of Star A, the center of that star must be much hotter; sufficiently hotter to make Star A ten times as luminous as Star B.

The most massive stars known are some 70 times the mass of the Sun, but they are 6 million times as luminous. On the other hand, a star with only 1/16 the mass of the Sun (65 times the mass of Jupiter) might be just massive enough to glow a dull red heat, and it would only be one-millionth as luminous as the Sun.

What would it be like for a planet circling a star at such extremes?

Suppose, for instance, Earth were circling a star 70 times as massive as the Sun.

Of course, if Earth were circling this giant star at the same distance at which it circles the Sun, the star would appear forty times as wide in the sky as the Sun does to us, and it would deliver 6 million times as much light and heat. The Earth would be a ball of red-hot rock.

We can easily imagine, however, that every star has a shell around it at some distance, within which a planet could circle and be heated by the star it circles to Earthlike standards of comfort. For a large star this shell, or “ecosphere,”
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would be farther away than for a small star. In the case of the 70-time-Sun giant, the ecosphere would be at a distance of hundreds of billions of kilometers from the star.

Suppose, then, that the Earth circled the giant star at a distance of 366 billion kilometers (227 billion miles). This would be a distance 2,450 times the distance of the Earth from the Sun and 62 times as far as Pluto is from the Sun. At such a distance it would take 14,500 years for the Earth to revolve about the star.

From that magnificent distance, the giant star would seem very small, so small that it would show no visible disc, but would shine merely like a star, but not like the stars we see. It would be extraordinarily bright because its temperature would be so much higher than that of the Sun (50,000° C as compared to a mere 6,000° C) that even though the giant star was so distant and so small in appearance, it would deliver as much light and heat to the distant planet as the Sun does to Earth.

To be sure, the giant star’s temperature alters the nature of its radiation. At the distance we have imagined for Earth, the star would deliver the same total amount of energy that the Sun delivers now, but a much larger fraction of the giant star’s energy would be in the form of ultraviolet light and x-rays, and a much smaller fraction would be visible light.

Human eyes are adapted to respond to visible light so that the light of the giant star would seem dimmer than that of the Sun. On
the other hand, the flood of ultraviolet and x-rays would be deadly to Earth life.

Yet perhaps this is not a fatal objection. The Earth’s atmosphere protects us against the energetic radiation of our Sun and we can imagine Earth moved still farther from the giant star. The decline in total radiation and the amount stopped by a possibly thicker atmosphere might then be suitable for the development of life at the price of somewhat lower planetary temperatures than we are used to.

There is, however, a more vital objection to the giant star, one that can’t be countered by adjusting the planetary place within the ecosphere or by fiddling with the planetary atmosphere.

A star is not an adequate incubator for life throughout its existence. It cannot supply the energy necessary for life, for instance, while it is condensing and forming out of the primal nebula. It must first condense to the point where the nuclear fires start at the center and it begins to radiate light. Eventually, the condensation reaches a stable stage and the radiation, having reached some maximum figure, remains there.

The star is then said to have entered the “main sequence.” (It is called the main sequence because about 98 percent of the stars we can see are in that state, forming a sequence from the most massive to the least massive.)

While on the main sequence, a star’s radiation is steady and reliable and, like our Sun, it could conceivably serve as an incubator for life.

The star’s radiation depends, however, on the energy that develops as the hydrogen at its core is converted through processes of nuclear fusion into helium. At some critical point, when a large part of the hydrogen has been used up, the process begins to falter. The helium, accumulating in the core, renders the core more and more massive. It shrinks and condenses, and its temperature goes up to the point where helium fuses to form still more complicated nuclei.

At this point, the star develops enough heat to cause itself to expand against the pull of its own gravity, whereas till then, while it was on the main sequence, the inward pull of gravity and the outward push of temperature had remained in balance.

As the star now expands it leaves the main sequence and becomes relatively enormous in extent. Because of the expansion, the
surface of the star cools and becomes merely red hot, though the total radiation from its now-vast surface is much greater than it had been before. The star is a red giant.

Once a star leaves the main sequence, what follows is hectic. It remains a red giant for several hundred million years (only a short time on the astronomical scale), while what is left of the hydrogen is consumed and while the core grows hotter and hotter. Finally there is a collapse, when the energy developed by nuclear fusion at the center fails as all possible nuclear fuels are used up and the star can no longer be kept distended against its own gravity.

If the star is massive enough, the collapse is preceded by a cataclysmic explosion—a supernova. The more massive the star, the more drastic the explosion. What is left of the star then shrinks into a relatively tiny and very dense ball
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As far as life is concerned, though, the details of what happens after the star leaves the main sequence are irrelevant. As the star begins to expand toward the red giant stage, its total radiation increases dramatically. Any planet that till then had been in a position to receive radiation in quantities consistent with the formation and maintenance of life would now receive far too much. Any life present would be baked to death. (In extreme cases, the planet itself would melt and evaporate.)

We can state, therefore, that as a general, and possibly inviolable, rule, a star can serve as an incubator of life only while it is on the main sequence.

Fortunately, a star can remain on the main sequence for a long time. Our Sun, for instance, may remain on the main sequence for a total period equal to 12 or 13 billion years. Although it has been shining now, in much its present fashion, for some 5 billion years, its life as a main sequence star is not yet half over.
^†

A star that is more massive than the Sun and therefore must counter the in-pulling effect of a stronger gravitational field, must
develop higher temperatures at the center to counter gravitational contraction and, to do that, must fuse hydrogen at a greater rate. To be sure, a star more massive than the Sun possesses more hydrogen to begin with, but the increase in the rate of fusion is greater than the increase in the hydrogen supply.

The more massive the star, then, the more rapidly it consumes its admittedly greater hydrogen supply, and the shorter its stay on the main sequence.

A monster star that is 70 times as massive as the Sun must consume its hydrogen at so fearsome a rate to remain expanded under the pull of its monster gravity that its life on the main sequence may be only 500,000 years or less. Indeed, that is why we observe no stars with really large masses. Even if gigantic stars formed, the temperatures they would develop would blow them up virtually at once.

Of course, even 500,000 years is a long time as far as human experience is concerned. Human written history has, at best, existed for only one-hundredth that period.

Intelligent life, however, did not come upon the Earth at its very beginning, but only as the result of a long course of evolution. If our Sun had only shone as it does now for 500,000 years after the formation of the Earth, and had then left the main sequence, it is highly doubtful if there would have been time for even the simplest protolife to form in Earth’s oceans.

In fact, judging from the experience of Earth, it takes some 5 billion years of planetary existence for life to develop to the point of complexity where a civilization can be established.

We can’t, of course, be sure how typical Earth’s case is of the Universe as a whole. It may be that evolution has, for some trivial reason or other, been extraordinarily slow on Earth, and that on other planets much less time has been required for the evolution of intelligence. It may, on the other hand, be that evolution on Earth has, for some trivial reason or other, been extraordinarily rapid, and that on other planets much more time is required for the evolution of intelligence.