ASTRONOMY: W. S. ADAMS 
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INVESTIGATIONS IN STELLAR SPECTROSCOPY. I. A QUANTI- 
TATIVE METHOD OF CLASSIFYING STELLAR SPECTRA 
By Walter S. Adams 
MOUNT WILSON SOLAR OBSERVATORY, CARNEGIE INSTITUTION OF WASHINGTON 
Received by the Academy, February 8, 1916 
The basis of the classification of stellar spectra is at present largely 
empirical. In the absence of sufficient knowledge as to the modifications 
of spectra produced by different physical conditions it has not been pos- 
sible to establish with certainty a system of classification which will rep- 
resent the actual order of stellar development. Hence the stars have 
been classified into types simply in accordance with the characteristics 
of their spectra. The appearance of new lines and the disappearance 
of others, systematic variations in the intensities of certain lines, the 
presence of bands, the intensity of the continuous spectrum, and other 
similar criteria have been used to separate the stars into several spectral 
groups. 
To some extent the system of classification now in general use by as- 
tronomers, that devised by the Harvard Observatory, probably has a 
physical basis. Thus it is well known that the differences between the 
spectrum of the sun and that of a star like Arcturus are very similar to 
those between the spectrum of the sun and that of sun-spots. In the 
latter case investigations have shown that a reduction of temperature is 
the principal agent in producing the modifications observed. Similarly 
the presence of bands characteristic of certain compounds which are 
found in the spectra of stars like a Orionis is an indication of relatively 
low temperature. Accordingly it seems probable that the successive 
types of stellar spectra, represented by the sun, Arcturus, and a Orionis', 
are characterized by successively lower temperatures in the gases form- 
ing the atmospheres of these stars. This does not of necessity indicate, 
however, that Arcturus and a Orionis have developed from stars like our 
sun. Lockyer and some others consider that the curve of stellar devel- 
opment has both an ascending and a descending branch, and that some 
stars of low temperature will become hotter before beginning to cool 
permanently. Stars which differ greatly in size and mass must almost 
certainly differ in the rate, and quite possibly in the order, of their 
development as well. 
The principal lines used in the Harvard system of classification for 
the separation of stars into the several types are certain lines due to 
calcium, the more prominent lines of such metals as iron, and, most im- 
