PROBLEMS OF MODERN SCIENCE 



of the A type. In spectra of this class the 

 hydrogen lines attain their greatest intensity. The 

 F spectrum is characteristic of the yellowish-white 

 stars. Hydrogen is less prominent, and metallic 

 lines make their appearance. 



Our sun is a typical yellow star with a G 

 spectrum. Many metallic lines are now shown, 



and these are still more prominent in the orange 

 stars, whose spectrum is K. Lastly we come 

 to the red stars typified by Betelgeuse. The 

 spectrum shows numerous bands, and closely 

 resembles that of sun-spots. To this class of stars 

 belong nearly all the long-period variables. The 

 order in which the spectra have been arranged is 

 by no means fortuitous ; it corresponds to the 

 order of decreasing temperature. The last 

 column of the table gives the effective tempera- 

 tures of the different types of stars that is, the 



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