138 ANNUAL REPORT SMITHSONIAN INSTITUTION, 19 3 2 



enhanced lines are produced by ionized atoms, and the general appli- 

 cation of the laws of ionization to stellar atmospheres have trans- 

 formed our whole viewpoint. We know now that the disappearance 

 of the lines of the metals from the hot stars means only that they 

 have been so highly ionized that they no longer give lines in the 

 observable region, and that the lines of the permanent gases, and the 

 nonmetals generally, are weak or absent in the cooler stars because 

 their atoms are not highly enough excited to be able to absorb the 

 observable lines. From measures of line width, and also by study 

 of multiplets, the actual number of atoms which produce a given 

 spectral line may be estimated, and an approximate quantitative 

 analysis made of the atmospheres of the sun and stars. The results 

 indicate a remarkable similarity of composition, despite the great 

 differences in the spectra of hot and cool stars. The relative abun- 

 dance of the elements is similar to that in the earth's crust or in 

 meteorites, with one noteworthy exception. Hydrogen — a minor 

 constituent here — is overwhelmingly predominant in the stars. (The 

 excess very likely escaped during the formation of our planet.) 

 Both the temperature and pressure of a star's atmosphere may be 

 found from the intensities of the spectral lines. The former agree 

 with the values deduced from the colors of starlight; the latter are 

 surprisingly small, and indicate that the atmospheres are of exceed- 

 ingly low density. The whole atmosphere of the sun, brought to 

 standard temperature and pressure, would make a layer of gas less 

 than a hundred feet thick, of which the metallic vapors form about 

 1 per cent. 



A similar conclusion was reached more than 40 years ago by Locl^- 

 yer, by the simple process of comparing the sodium lines in the solar 

 spectrum with those absorbed by tlie vapor present in a Bunsen flame. 

 The sun's atmosphere, of course, is not sharply bounded at the bot- 

 tom ; it grows hazier owing to the increasing density of the free elec- 

 trons and ions, and passes into the luminous photosphere. The prin- 

 ciples upon which this increasing opacity can be calculated are essen- 

 tially spectroscopic, and the data regarding the ionization and excita- 

 tion potentials of atoms, which it requires, have been derived spec- 

 troscopically. 



Two more applications may be mentioned — to matter in extreme 

 states of condensation and rarefaction. 



From the spectroscopic data regarding atoms it follows that, at 

 very high temperatures, inside the stars, they will be completely ion- 

 ized down to bare nuclei and electrons. Matter in this state should 

 be exceedingly compressible, but not infinitely so — the limiting fac- 

 tor being the degeneracy of the gas (in the sense of the new quan- 

 tum theory) at a density several hundred thousand times that of 



