during which it can have maintained the earth in a habitable condition is proportionally shortened, for we can not suppose that animal and vegetable life could be developed under the dominion of a distinctively variable star. The assumption is here made, of course, that a variable star is really a sun and not a cloud of meteorites in collision, or a partially condensed nebula, and that its planets, if it is ever to have any, have already been formed. Progress in the other direction—that is, from the white star toward the red star and variable star stage—would seem to supply longer periods of unvarying solar radiation for the evolution of planetary life, since a sun developing in that way would become a stable radiator sooner than if it had first to free itself from a sheathing of absorbing vapors created, it may be, by its own action at a certain stage of its career rather than left behind as a subsiding remnant of the original nebula.
We have remarked that, so far as the records of human history inform us, the emission of light and heat by the sun has never seriously varied. Yet it has been thought, though the evidence is not clear, that there are geological indications of considerable variations in the amount of solar radiation in past time, and the famous myth of Phaeton driving the chariot of the sun and getting so far out of his road that he endangered the earth and made it smoke with unwonted heat, has often been referred to as a possible tradition of some extraordinary outburst of solar heat within the period of man's existence. The variable character of sun-spot phenomena certainly does not contradict that supposition. The margin of existence is so narrow for many forms of life that no very great change would be required to cause a disaster. Still, notwithstanding the vagaries of sun-spots, and the apparent analogy between the sun and the variable stars, it would not do to assume that the earth is at present in any danger from a changing mood in its great governor and benefactor. So far as positive records serve as an indication of the future, there is every reason to believe that the sun will long continue in its present condition, and that astronomers a million years hence, if some cataclysm arising from ulterior causes does not intervene, may still be found studying the sun, having probably by that time ascertained whether it is getting hotter or colder. But it would not be safe to assume that any astronomers will be left upon the earth five million years hence.
