THE SPECTRA OP STARS AND NEBULiE. 



1!) 



in erroT as absolute measurements, as the observations are 

 extremely difficult, but the important thing is that the sequence 

 of temperatures is in satisfactory agreement with that derived 

 from the spectral lines. 



There is not time to go into much detail, but it will be in- 

 teresting to see what progress has been made in the interpreta- 

 tion of the spectral phenomena. The inclusion of solar, or 

 second type, stars in the evolutionary scheme necessarily 

 implies that all the stars are similar to the sun in chemical 

 constitution, but we shall see that independent evidence of the 

 universal distribution of terrestrial kinds of matter is to be 

 found in abundance in the analysis of individual stars. 



It will be most instructive to begin at the lower end of the 

 series, where there is every reason to believe that the tempera- 

 tures of the stars involved are relatively low, so that the 

 reproduction in our laboratories of the lines and bands by 

 which they are characterized should present the minimum of 

 difficulty, provided we have the same substances to deal with. 



In the type IV, or N, stars, which are all faint, Secchi him- 

 self recognized that the principal dark bands were identical 

 with those seen in the blue base of a candle flame, and 

 attributed to carbon. This has been beautifully confirmed by 

 Professor Hale's photographic studies of these stars, which also 

 proved the presence of other dark bands, due to cyanogen, in 

 the violet region beyond the range of visual observations. 

 Many of the numerous lines which accompany the bands are 

 traceable to various metals, such lines being capable of produc- 

 tion at relatively low temperatures in our laboratories. Pro- 

 fessor Hale's photographs also show a gradual progression 

 from stars in which the carbon bands are very strong 

 to stars in which these bands are very weak, as would be 

 expected on the hypothesis of evolution. 



The bands which are characteristic of stars of the third, or M, 

 type, were not traced to their source until 1904, when they 

 were identified by the lecturer with bands of titanium oxide. 

 There is no obvious reason why this substance should be selected 

 for such striking manifestation in stars at this stage of their 

 evolution, and we have just to accept it as a fact of observation 

 and experiment ; it is a significant fact, however, that the 

 strongest of these bands occur also in the spectra of sun-spots, 

 which we have other reasons for believing to be at a lower 

 temperature than the solar surface in general. The lines which 

 occur in the M stars are generally similar to those of the X 

 stars. 



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