168 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



calculations indicate, and inversely. That our determinations may 

 be more precise, it will be next necessary to summarize briefly the 

 principal basic facts of stellar spectra. 



SPECTRUM CLASSIFICATIONS AND STELLAR TEMPERATURES. 



The classification now universally adopted is that of Harvard. We 

 will pass in review the principal classes. The classes have been ar- 

 ranged in the order of decreasing 4 temperatures. We can assure 

 ourselves of this in verifying that, from one class to the next, the 

 maximum intensity retreats toward the red (at the right) and that 

 the blue end progressively disappears. 



The first spectrum shows the type of class B, or helium stars. 5 Their 

 temperatures are very high. Their light is very white or blue. Their 

 absorption spectrum contains helium lines. Hydrogen lines present 

 are usually dark but at times bright. The metallic lines are absent. 



We pass next to the classes A and F characterized by the intensity 

 of the absorption spectrum of hydrogen. The lines of this element 

 belonging to the B aimer series are very noticeable (types A and F 

 and intermediate type F 5 ) huddling together in the violet. The 

 metallic lines make their debut in class A, but they do not become 

 very abundant until in the spectra of type F. The latter class is 

 especially noteworthy for the extraordinary development of the 

 H and K violet lines of calcium which are faint in the preceding 

 classes. 



The temperature becomes still lower in passing to class G where 

 our sun belongs. The metallic lines become definitely preponderant ; 

 those of hydrogen are yet more intense (C and F Fraunhofer lines) 

 but less so than those of calcium. The Balmer series ceases to give 

 the spectrum its characteristic aspect. The absorption lines of helium 

 have disappeared. We know that they cannot be observed in the 

 sun's ordinary spectrum. Helium was discovered in the sun because 

 of its bright line spectrum in the chromosphere. 



The stars of class K are distinctly yellow or reddish. The metallic 

 lines are so numerous and strong that the continuous spectrum is re- 

 duced to a few bright rays upon a sombre background. The scale 

 of the figure is perhaps not large enough to show this structure but 

 what can be surely seen is the weakening of the blue part of the 

 spectrum. This indicates a comparatively low temperature. 



4 The letters designating the classes are not now arranged in alphabetical order. It 

 would take too long to recount the history and evolution of- the Harvard classification. 

 Experience has arranged the classes in the order indicated. 



E We are leaving out of consideration the planetary nebulae which make up the class P, 

 the nebular stars, placed in class Q, and also the so-called Wolf-Rayet stars constituting 

 class O. The spectra of the last class contain a number of bright lines upon a feeble 

 ground. They seem related to novae or " new stars " and are apparently yet hotter 

 than the stars of type B. The study of them is not so far advanced as for the stars 

 described in the body of the text. 



