Page:Popular Science Monthly Volume 86.djvu/442

438 class emit a far greater amount of total radiation than do the blue stars; that they have a higher emissivity, or, in other words, that they are cooling faster than the blue stars. (In parenthesis it may be added that it is doubtful whether the interior of a red star is cooler than a blue star. The whole mass is no doubt shrinking and the temperature may be actually rising. But the idea to be conveyed is that in the various stages of stellar evolution, the "red star stage" seems to be the one in which a star is "burning out" the fastest.) The first method, viz., measuring the total radiation from red and blue stars having the same photometric brightness, as already mentioned, is somewhat uncertain, because the radiation received from a star is a function of its size, distance, temperature and especially its emissive power. Some of the largest stars are no doubt the farthest from us. The second method of observation consisted in roughly separating the star's rays into a spectrum, by means of an absorption cell of water which absorbs most of the infra-red rays and transmits the visible rays. Hence, by measuring the total radiation from a star, and also the part which is transmitted by the absorption cell of water we obtain an estimate of the relative amounts of energy in these two parts of the spectrum. This measurement is a ratio of two quantities of energy; and hence is independent of the size, the distance and the temperature of the star. It gives us direct information of the emissivity of the different parts of the star's spectrum. It is true that it gives us information of only two parts of the spectrum; but, from our knowledge of the solar spectrum, and of the spectra of terrestrial substances, this information enables us to make important deductions as regards the distribution of energy in the spectra of stars.

Some of the data obtained by measuring the total radiation from blue and from red stars, having the same brightness, will now be discussed. It was possible to make quantitative measurements on stars