CAN WE ALWAYS COUNT UPON THE SUN? 661 



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 plane- 

 tary systems at all, to be surrounded by globes revolving at com- 

 paratively 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 altera- 

 tions in the amount and intensity of its radiation. Take such a 

 star as Mira, for instance, alternately dying down almost to ex- 

 tinction 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 be- 

 held 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, 



