Untitled There appear to be two types of nebulA, and there is evidence suggesting that the one type is only an earlier form of the other; but this again we do not know. The more primitive nebulA would seem to be composed of gas in an extremely rarified form. It is difficult to convey an adequate idea of the rarity of nebular gases. The residual gases in a vacuum tube are dense by comparison. ± cubic inch of air at ordinary pressure would contain more matter than is contained in millions of cubic inches of the gases of nebulA. The light of even the faintest stars does not seem to be dimmed by passing through a gaseous nebula, although we cannot be sure on this point. The most remarkable physical fact about these gases is that they are luminous. Whence they derive their luminosity we do not know. It hardly seems possible to believe that extremely thin gases exposed to the terrific cold of space can be so hot as to be luminous and can retain their heat and their luminosity indefinitely. ± cold
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