Extraterrestrial Civilizations - Isaac Asimov [64]
The total number of binaries in the Galaxy (and presumably in the Universe generally) is surprisingly large. Over the nearly two centuries since their discovery, the estimate of their frequency has steadily risen. At the moment, judging from the examples of those stars close enough to ourselves to be examined in detail, it would seem that anywhere from 50 to 70 percent of all stars are members of a binary system. In order to arrive at a particular figure, let us take an average and say that 60 percent of all stars and, therefore, of all Sunlike stars, too, are members of a binary system.
If we assume that any Sunlike star can form a binary with a star of any mass, then, keeping in mind the proportions of stars of various masses, we could venture a reasonable division of the 75 billion Sunlike stars in the Galaxy as follows:
30 billion (40 percent) are single
25 billion (33 percent) form a binary with a midget star
18 billion (24 percent) form binaries with each other
2 billion (3 percent) form a binary with a giant star
Ought we now to eliminate the 45 billion Sunlike stars involved in binary systems as unfit incubators for life?
Certainly, it would seem that we can omit the 2 billion Sunlike stars that form binaries with giant stars. In their case, long before the Sunlike star has reached an age where intelligence might develop on some planet circling it, the companion star would explode as a supernova. The heat and radiation of a nearby supernova is quite likely to destroy any life on the planet that already existed.
What about the remaining 43 billion Sunlike stars forming a part of binaries?
In the first place, can a binary system possess planets at all?
We might argue that if a nebula condenses into two stars, the two will be twice as effective in picking up debris as one would be. Any planetary material that might escape one would be picked up by the other. In the end, therefore, there would be two stars and no planets.
That this is not necessarily so is demonstrated by the star 61 Cygni, the first whose distance from Earth was determined, in 1838, and that is now known to be 11.1 light-years from us.
61 Cygni, as I have said earlier, is a binary star. The two component stars, 61 Cygni A and 61 Cygni B, are separated by 29 seconds of arc as viewed from Earth (a separation about 1/64 the width of the full Moon).
Each of the component stars is smaller than the Sun, but each is large enough to be Sunlike. 61 Cygni A has about 0.6 times the mass of the Sun, and 61 Cygni B about 0.5 times the mass. The former has a diameter of about 950,000 kilometers (600,000 miles) and the latter a diameter of about 900,000 kilometers (560,000 miles). They are separated by an average distance of about 12,400,000,000 kilometers (7,700,000,000 miles), or a little over twice the average distance between the Sun and Pluto, and they circle each other about their center of gravity once in 720 years.
If we imagined the planet Earth circling one of the 61 Cygni stars at the same distance it now circles the Sun, the other 61 Cygni star would appear in the night sky at various times as a bright, starlike object, showing no visible disc, delivering no significant amount of radiation, and producing no significantly interfering gravitational effect.
Indeed, we might easily imagine each 61 Cygni star as possessing a planetary system nearly as extensive as the Sun’s, each without interference from the other.* In this particular case, we need not resort entirely to speculation. The very first planetary object about another star for which some evidence was obtained involved 61 Cygni. From the manner in which the separation of the two stars changed in a wobbly manner as they circled each other, the presence of a third body, 61 Cygni C, was deduced. From the extent of the wobble, it was thought to be a large