Extraterrestrial Civilizations - Isaac Asimov [57]
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* 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