Page:Popular Science Monthly Volume 60.djvu/319

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STELLAR EVOLUTION.

there are two well-defined classes of these older stars, which until recently were not known to have any points in common except their red color. These are the stars of Secchi's third and fourth types. In general appearance their spectra are wholly unlike, particularly on account of the absence from third class spectra of the broad dark bands due to the absorption of carbon vapor, the most characteristic feature of the fourth type. But in spite of this apparent dissimilarity, photographs recently taken with the 40-inch Yerkes telescope show that in certain regions of the spectrum stars of the two types are practically identical and are thus probably more closely related than formerly appeared to be the

PSM V60 D319 Spectra of four red stars showing cooling.png
Fig. 12.
Spectra of four red stars (Hale and Ellerman). Showing how the dark band due to carbon increases in intensity as the star cools.

case. The measurements and reductions of a long series of photographs of fourth type spectra now in progress at the Yerkes Observatory should soon permit us to form an opinion of the nature of these interesting stars.

In both the third and fourth types it is easy to trace the successive stages of development. In stars of the fourth type the signs of increasing age are particularly striking. The carbon vapor which produces the broad dark bands becomes denser and denser, until it is not difficult to imagine that through the further increase of such absorption the light of the star might be completely extinguished (Fig. 12).