THE COMPOSITION OF THE SUN 



By Henry Nobris Russell 

 Princeton TJu'wersitu 



[Willi 4 plates] 



Once in a long time, in the history of science, a true " royal road " 

 is found which leads to the solution of a problem which had previ- 

 ously been unassailable. There is no better example than the dis- 

 covery of the spectroscope. Seventy years ago, or a little more, 

 almost nothing was known of the composition of the heavenly bodies, 

 and there was no reasonable hope of knowing more. The high 

 densities of the moon and the inner planets, to be sure, indicated that 

 they must be solid bodies, probably similar in general constitution 

 to the earth, while the much lower densities of the sun and the major 

 planets indicated that their constitution was different, but this in- 

 formation did not go very far. Suddenly the hopeless quest became 

 a reality with Kirchhoff's discovery that a rarefied luminous gas 

 gives out light of definite wave lengths, which, analyzed by the spec- 

 troscope, produces a definite pattern of lines, fixed in position and 

 intensity for a given source, and fully characteristic of it. Gases 

 absorb the same kinds of light v\'hich they emit; hence the multitudes 

 of dark lines of the solar spectrum became immediately intelligible, 

 each carrying its own message about the sun's atmosphere. A dozen 

 or more familiar chemical elements were then immediately identified 

 in the outer parts of the sun and, shortly afterwards, in great num- 

 bers of the stars. This still remains the most impressive evidence 

 regarding the fundamental unity of nature. Atoms of the very same 

 kinds are found in our terrestrial laboratories, in the sun, the stars, 

 and in nebulae so remote that their light takes a hundred million 

 years to reach us. As the investigation of spectra on earth and in 

 the heavens progressed, the correlation of the two became continually 

 more complete. At the present time almost all the lines of any 

 strength in the spectra of the sun and the stars can be reproduced 

 at will in the laboratory; and the few exceptions are yielding year 



" First Arthur LLCturo, under the auspices of the Smithsonian Institution, Jan. 27, 1932. 



199 



