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By Root 1093 0

often “flare stars.” That is, flares of unusually bright and hot gas periodically burst out on its surface. This happens on all stars, even on our Sun, for instance. On the Sun, however, such a flare would only add a small and bearable fraction to the ordinary Solar output of light and heat. The same flare on a dim midget star would increase its light and heat output by up to 50 percent. A planet receiving a proper amount of energy from the midget star would receive far too much under flare conditions. The star’s role as incubator would be carried out in too irregular a fashion to be compatible with life.

Between tidal effects, shallowness of ecosphere, and periodic flares, the exclusion of midget stars from further consideration in connection with extraterrestrial intelligence is triply justified.

JUST RIGHT

If the stars with too much mass to serve as adequate incubators for life, those more massive than spectral class F2, make up a small fraction of all the stars, this is not the case for the stars that are less massive than spectral class M2 and also don’t serve as adequate incubators for life. Midget stars are very common. More than two-thirds of the stars in our Galaxy, and presumably in any galaxy, are too small to be suitable for life.

Between spectral classes F2 and M2 are the stars that range in mass from 1.4 times that of the Sun to 0.33 times that of the Sun. At the upper end of this range, the lifetime of the stars is barely enough to give intelligence a fair chance to evolve. At the lower end of this range, a planet barely escapes tidal effects of too serious a nature.

Within the range, though, are the “Sunlike stars,” which, all other things being equal, are suitable incubators for life. While these Sunlike stars do not make up a majority of the stars in the sky, they are not really few in number, either. Perhaps 25 percent of all the stars in the Galaxy are sufficiently Sunlike in character to serve as adequate incubators of life.

That gives us our third figure:

3—The number of planetary systems in our Galaxy that circle Sunlike stars = 75,000,000,000.

* A very massive star may radiate so much of its energy in the invisible ultraviolet region that it will seem less luminous (to the human eye) than one might expect it to be.

* Eco- is from the Greek for home or habitat.

* For details on all this, see my book, The Collapsing Universe.

† The Sun will gradually grow warmer as it ages and by its final billion years on the main sequence, life may not be possible on Earth. When the Sun expands to a red giant, it will engulf the orbits of Mercury and Venus, and though Earth will probably remain outside the Sun’s swollen sphere, it will at best be a red-hot ball of rock.

*The slowing of the rotation means a loss of angular momentum that by the law of conservation of angular momentum can’t really be lost. What happens is that the Moon is slowly moving farther away from the Earth and so is the center of gravity of the Earth-Moon system. What the Earth loses in the angular momentum of rotation, it gains in the angular momentum of a larger swing about a more distant center of gravity.

* This is a hypothetical case only, for if the center of the Sun were as close to the Earth as the center of the Moon is, the Earth would be far beneath the surface of the Sun

CHAPTER 8

Earthlike Planets

BINARY STARS

A star may be Sunlike and yet still not be a suitable incubator for life. It may have properties, other than its mass and luminosity, that make it impossible for an Earthlike planet to circle it.

A star may be like the Sun in every apparent respect, for instance, and yet have as a companion not a planet or a group of planets, but another star. The presence of two stars in close association may conceivably ruin the chances for an Earthlike planet to circle either one.

The possibility of multiple stars did not dawn on astronomers until about two centuries ago. After all, our Sun is a star without stellar companions and that made it seem a natural condition. When the stars were recognized