within this category. In these stars the absorption of their vaporous 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 representative 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 inconspicuous 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 spectrum, while the bands in the third-class spectra shade off toward the red end. This peculiar spectrum appears to arise from a compound 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 perhaps seventy times the light of our sun, is only between two and three times as massive, so that the intensity of its radiation is
