66o THE POPULAR SCIENCE MONTHLY. 



within this category. In these stars the absorption of their vapor- 

 ous envelopes is so pronounced that their spectra are darkened by 

 bands as well as lines. Sometimes bright lines of hydrogen appear 

 in the spectra of stars of this class, indicating that an envelope of 

 that element surrounding them has blazed out with an intensity 

 of heat exceeding that of the photosphere itself. Betelgeuse, the 

 great orange-colored star in the shoulder of Orion, is a representa- 

 tive of the third class. The wonderful variable Mira, in Cetus, 

 also belongs in the third class of stars. 



The fourth class is small in number, and its members are incon- 

 spicuous in brightness and shine with a deep red light. Their 

 spectra are also filled with bands of absorption which are peculiar 

 ' in that they shade off gradually toward the blue end of the spec- 

 trum, while the bands in the third-class spectra shade off toward 

 the red end. This peculiar spectrum appears to arise from a com- 

 pound of carbon filling the atmosphere of the star. Variable stars 

 also abound in the fourth class and bright lines are sometimes seen 

 in their spectra. Even if we grant that the progress of stellar 

 evolution is from the white through the yellow to the red stars, 

 and so on to complete extinction, it does not appear possible to say 

 with certainty that the stars of the fourth class are any closer to 

 final extinguishment than those of the third. It would be a very 

 beautiful thing if one variety of red stars could be recognized as 

 representing a class younger than Sirius, while all other red stars 

 were known to be older than the sun, but that can not be affirmed. 

 So far as our present knowledge guides us, the most that we can 

 assert is that red stars may be either the youngest or the oldest of 

 suns, or some may be young and some old ; but that, at any rate, 

 they probably stand near one or the other end of the progression, 

 since they are clearly inferior in efficiency of radiation to the other 

 stellar varieties. 



Now, as regards the existence of planets circling around the 

 various classes of suns, we can only reason from analogy ; and 

 opinions upon the subject range all the way from Dr. Whewell's 

 conclusion that the earth is probably the only inhabited world in 

 the universe, to Dr. Chalmers's delightful picture of the starry 

 heavens filled everywhere with intelligent beings worshiping their 

 Creator. Suppose we examine the probable conditions prevailing 

 around the stars of each of the four great classes. The white 

 stars, like Sirius, possess an extraordinary potency of radiation. 

 Their atmospheres are not strongly absorbent, and probably not 

 extensive, and consequently nearly the full vigor of their beams 

 is poured upon the satellites that surround them, if any such there 

 be. According to recent estimates, Sirius, while shining with per- 

 haps seventy times the light of our sun, is only between two and 

 three times as massive, so that the intensity of its radiation is 



