Extraterrestrial Civilizations - Isaac Asimov [55]
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:
2—The number of planetary systems in our Galaxy = 280,000,000,000.
*There is tenuous and indirect evidence that they exist. This is something we will discuss later in the chapter.
*Any naturalistic explanation of the formation of the Solar system could not greatly precede this. The strength of the belief in creationism (that is, the formation of the Universe in accordance with the description in Genesis 1) was so strong up to that time that to deviate from it would have put the deviator into serious jeopardy.
*A very similar theory was advanced simultaneously and independently by the Soviet astronomer Otto Yulyevich Schmidt (1891-1956), whose birthplace, as it happens, is only 130 kilometers (80 miles) from my own.
† Such a far-out belt of comets was first postulated by the American astronomer Lawrence Whipple (1906-) in 1963, long after Weizsacker had first advanced his theory. Still later, Oort added detail and placed the belt very far from the Sun, a light-year or two away.
*These massive, but small and very dense stars, and others even more massive, smaller, and denser, are of no matter to us in this book and they will never be more than alluded to. If you are curious about them, you will find a complete discussion in my book The Collapsing Universe (Walker, 1977).
CHAPTER 7
Sunlike Stars
GIANT STARS
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