Extraterrestrial Civilizations - Isaac Asimov [66]
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