baran, Mr. Huggins detected sodium, magnesium, calcium, iron, bismuth, antimony, tellurium, mercury, and hydrogen.
The third class, in which are some stars of a red color, is comparatively small in numbers. Alpha Orionis or Betelgeux, Alpha Herculis, Beta Pegasi, Mira beti, and Antares, are good examples of this type. Their spectra, as a rule, resemble the spectrum of a solar spot, and sometimes contain bright lines. Hydrogen is still present, but so difficult to detect that, at first, it was supposed to be wanting in the spectrum of Betelgeux. But, in a state of combination, as aqueous vapor, it has been found in the stars of this order, and, most notably, in Antares. In the spectrum of Betelgeux, Mr. Huggins observed lines belonging to magnesium, sodium, iron, calcium, and bismuth.
The stars of the fourth type are very inconspicuous, but give quite peculiar spectra, consisting chiefly of three bright bands, separated by dark spaces. Such a spectrum suggests that of carbon, but really tells us nothing, as yet, of the constitution of these stars. We must, therefore, leave them out of account in our speculations. It would be easy to theorize about them, only the theories would find no place in our argument.
Now, taking the spectra of stars of the first, second, and third classes as a basis for our speculations, we have quite decent evidence of a gradual increase in chemical complexity. And, if we bring the nebulæ into line, we can devise a very neat progressive series of development up to the solid planet. Beginning with a nebula consisting mainly of nitrogen and hydrogen at low temperature and pressure, we can easily conceive of several ways by which it might gain great accessions of heat, and give a bright, continuous spectrum. A collision with meteoric or cometary matter would account for such an increase of temperature. But, given a nebula which is sufficiently hot, and from which a sun might be evolved by cooling, what shape will our speculations assume? This intensely-heated body undergoes a certain condensation, rings are thrown off from it, and a nucleus appears, which soon becomes a star or sun of the first type. Hydrogen still predominates in its constitution, but metals begin to show themselves, though very faintly. But the cooling continues, and gradually the hydrogen lines become fainter, the metallic lines stronger, a larger number of substances are detected, and we have a sun of the second class. By another slow transition, chemical action, as we recognize it, begins to set in. The hydrogen lines disappear; aqueous vapor is formed; spots, like those of the sun, which are probably centres of chemical activity, become more and more abundant, and the star enters the third order. As the spots accumulate, the star becomes more decidedly a "variable," and, after violent and prolonged convulsions of its surface, solidity is reached, the emission of light ceases, and a planet is formed. Some volcanic heat, however, yet remains; but this slowly dies away, the volcanoes become extinct, and, at the end of the
