Till: Sl'KCTRA OF STARS AND X K 111' L.V.. 



17 



It was not long before this classification of stellar spectra 

 came to be regarded as representing something more than a 

 mere convenience of description. If we look at a few stars at 

 random wemight see no obvious relation between their spectra, 

 bul when there is a sufficient number to choose from, it is found 

 that the spectra can be arranged in a continuous series in which 

 the successive types merge into each other. That is to say, the 

 different types of stellar spectra are not abruptly divided, but 

 are connected by spectra representing well-marked transition 

 stages. 



Here we get a definite indication of an evolution of the stars 

 somewhat analogous to that which Darwin enunciated for organic 

 life. The differences in the spectra of the stars are not believed 

 to be due primarily to differences in chemical composition, but to 

 their having reached different stages in an orderly development 

 from masses of similar materials. The continuity of the spectral 

 series practically compels us to believe, for example, that our 

 sun was once a star like Sirius, and that Sirius will in due course 

 become a star like the sun, the sun meanwhile having become a 

 red star with a spectrum of bands. The order in which Seech i 

 numbered his types is accordingly regarded as representing the 

 general sequence of spectra as a star passes through different 

 stages in its evolution, and we now speak quite freely of young, 

 or early-type, stars, and of old, or late-type, stars. 



This idea of a celestial evolution is the foundation of all 

 subsequent attempts to classify the stars on a rational basis.. 

 While there is nearly unanimous agreement as to the sequence 

 of the various spectral classes, different systems of naming them 

 have been suggested. Secchi's numeration is by no means 

 obsolete, but on the ground of greater adaptability for the 

 distinction of intermediate types, the Harvard classification is 

 now most widely adopted, though it is recognized that it may 

 not be final. Here the designations are alphabetical, but it is 

 rather unfortunate that the sequence of the letters is not in 

 entire accordance with the order of types. 



The relation of the Harvard to the Secchi classification is 

 shown in the appended table, which also gives the chief 

 characteristics of the spectra and indicates the names of typical 

 examples. Intermediate stars on the Harvard system are in- 

 dicated by such symbols as F 5 G, indicating that such a star is 

 five- tenths advanced from type F to type G. It will be noted 

 that a few classes of stars not adequately provided for in 

 Secchi's system are recognized in the Harvard scheme. Atten- 

 tion should especially be drawn to the group at the head of the 



c 



