enormously greater than the sun's. Planets situated as close to Sirius as the earth and the other inner planets of our system are to the sun, would be unable to endure, so far as their life-bearing functions are concerned, the gush of heat and blaze of light poured upon them—unless, indeed, the organization of living beings there were entirely different from that prevailing here. "We should then expect such stars as Sirius, if they are the centers of planetary systems at all, to be surrounded by globes revolving at comparatively great distances and in long periods of time.
Coming to the second class, or solar stars, we find that the more extensive atmospheres which surround them, and absorb no inconsiderable portion of their rays, serve as a sort of protective curtain for their planets. There can hardly be a doubt that if the envelope of metallic vapors that incloses the photosphere of the sun were suddenly removed, life, at least in many of its more complex forms, would be banished from the earth, and perhaps be rendered impossible upon any planet nearer than Jupiter.
But it is the red stars and variable stars of the third and fourth classes that present the most unfavorable features from the planetary point of view. Probably no star belonging to these varieties is free from extensive and more or less spasmodic alterations in the amount and intensity of its radiation. Take such a star as Mira, for instance, alternately dying down almost to extinction and then blazing out with more than a thousand times its former brilliancy, these tremendous changes occupying, for a complete cycle, only eleven months! Is it possible to suppose that inhabited planets exist within the domain of an orb like that? When a sun is half smothered in absorbing vapors, and subjected to paroxysms such as those which are occasionally beheld when the atmosphere of a star appears to catch fire, as it were, and the lines of hydrogen and other elements flame bright like signals of conflagration, it can no longer be the center of a system of life-bearing worlds, no matter what its past history may have been in that respect.
It is apparent, from what we have just said, that progress by our sun in either direction toward the white stars or toward the red stars—would, in the end, prove exceedingly uncomfortable if not fatal to the inhabitants of the earth. By the subsidence of the vapors of metals that now stripe the solar spectrum with their absorption we should be, in effect, removed into the presence of a Sirius whose fierce beams would smite the living world with death. On the other hand, let the sun sink into the condition of a red star, and become variable in its outpourings, and our condition would be even worse. If it be thought that a planet whose orbit is as eccentric as that of Mercury is hardly habitable because it receives twice as much solar heat at perihelion as at aphelion,
