158 ANNUAL EEPOET SMITHSONIAN INSTITUTION, 1916. 



CLASSIFICATION. 



As proposed at Harvard College Observatory, the following classi- 

 fication of stellar spectra has been generally adopted: Class O 

 ( Wolf-Eayet type) . The spectrum consists of hright lines on a faint 

 continuous background. Class B (Orion type). Dark lines of 

 helium are sparsely set on a bright ground. Class A (Sirian type). 

 Plydrogen lines are most conspicuous. Class F (calcium type). 

 Hydrogen lines are still conspicuous, but many lines of metals 

 appear faintly, and notably two great lines of calcium in the violet. 

 Class G (solar t^qoe). Numerous strong metallic lines occur as in 

 sunlight. Class K (sun-spot type). The lines are darker and sun- 

 spot flutings occur. Hydrogen lines are faint. Type M (fluted 

 type). Titanium oxide flutings are strong, and carbon flutings also 

 well marked. Several other classes are noted by specialists, but 

 those above named are the chief ones (pi. 1). 



The classification of the stellar spectra is a very important aid to 

 research. It is found that motions, position in the heavens with 

 reference to the Millvy Way, size, temperature, distance, and other 

 characteristics of stars, vary with spectral class. Director Picker- 

 ing, of Harvard College Observatory, has already done a work of 

 high value in securing the spectra and publishing the classification 

 of about G,000 stars covering both the northern and southern hemis- 

 pheres. Now a new publication is about to be made by Harvard 

 College Observatory, giving the classification of the spectra of above 

 200,000 stars, all observed by the Harvard staff, and all examined by 

 Miss A. J. Cannon within the last four years. 



TEMPERATURES. 



Cold iron does not shine in the dark, but let the smith heat it in 

 his forge and soon it glows red, then yellow, then white hot. The 

 hotter the body is the more its spectrum is enriched toward the violet 

 end as compared with the red. Exact mathematical relations are 

 known to connect temperature and distribution of light in the spec- 

 trum. Working on this basis, it is found that our sun's surface 

 appears to be at about 6,000° centigrade (10,800° Fahrenheit) above 

 the melting temperature of ice. (See pi. 2.) 



By photographic methods Wilsing and Scheiner, of the Astro- 

 physical Observatory at Potsdam, in Germany, have assigned tem- 

 peratures to about 100 of the brighter stars. The results run from 

 9,000° C. for class B down to 3,000° C. for class M, varying in fairly 

 regular progression. 



Very recently Coblentz, of the National Bureau of Standards, 

 working temporarily at Lick Observatory in California, has suc- 

 ceeded in measuring the heat caused by the rays of stars so faint that 



