It is interesting to follow our line of reasoning to the stars, which differ widely in temperature at various stages in their life-cycle.[*] A sun-spot is a solar tornado, wherein the intensely hot solar vapors are cooled by expansion, giving rise to the compounds already named. A red star, in Russell's scheme of stellar evolution, is a cooler sun, vast in volume and far more tenuous than atmospheric air when in the initial period of the "giant" stage, but compressed and denser than water in the "dwarf" stage, into which our sun has already entered as it gradually approaches the last phases of its existence. Therefore we should find, throughout the entire atmosphere of such stars, some of the same compounds that are produced within the comparatively small limits of a sun-spot. This, of course, on the correct assumption that sun and stars are made of the same substances. Fowler has already identified the bands of titanium oxide in such red stars as the giant Betelgeuse, and in others of its class. It is safe to predict that an interesting chapter in the chemistry of the future will be based upon the study of such compounds, both in the laboratory and under the progressive temperature conditions afforded by the countless stellar "giants" and "dwarfs" that precede and follow the solar state. [Footnote *: See Chapter II.] [Illustration: Fig. 35. Electric furnace in the Pasadena laboratory of the Mount Wilson Observatory. With which the chemical phenomena observed in sun-spots and red stars are experimentally imitated.] ASTROPHYSICAL LABORATORIES It is precisely in this long sequence of physical and chemical changes that the astrophysicist and the astrochemist can find the means of pushing home their attack. It is true, of course, that the laboratory investigator has a great advantage in his ability to control his experiments, and to vary their progress at will. But by judicious use of the transcendental temperatures, far out ranging those of his furnaces, and extreme conditions, which he can only partially imitate, afforded by the sun, stars, and nebulae, he may greatly widen the range of his inquiries. The sequence of phenomena seen during the growth of a sun-spot, or the observation of spots of different sizes, and the long series of successive steps that mark the rise and decay of stellar life, resemble the changes that the experimenter brings about as he increases and diminishes the current in the coils of his magnet or raises and lowers the temperature of his electric furnace, examining from time to time the spectrum of the glowing vapors, and noting the changes shown by the varying appearance of their lines. [Illustration: Fig. 36. Titanium oxide in red stars.