304 PHYSICAL SCIENCE 



effective temperature may be estimated. It 

 proves to range from about 25,000° C. for the 

 hottest stars of type O, to about 2300° C. for 

 stars classed as R. These figures, of course, 

 refer to the radiating layer towards the outside 

 of the star ; within, the temperature must be 

 much higher, mounting perhaps to some millions 

 of degrees. 



The spectrum of a star must not be expected 

 to show lines corresponding to all the elements 

 which that star contains. Experiment in our 

 laboratories, as we have seen, shows that electric 

 ionization greatly increases the intensity of the 

 spectral lines of the element ionized, and it 

 will be chiefly these ** enhanced" lines that 

 mark a stellar spectrum. Ionization depends 

 on temperature as well as on the nature of 

 the elements present — another reason why a 

 classification by spectra is also a classification 

 by temperature. 



If the distance of a star be known, the 

 apparent magnitude may be used to calculate 

 the absolute magnitude, that is, the brightness 

 the star would show if removed to a standard 

 distance. 



When stars in the different spectral types 

 OBAFGKMNRare examined for absolute 

 magnitude, a remarkable result becomes apparent. 

 While the brightness of the very hot stars in 

 class B is of the same order throughout, ranging 

 only from about 40 to 1600 times the brightness 

 of our sun, the cooler stars such as those of type 

 M, fall into two well-marked groups, one with 

 luminosities approaching those of the hottest 

 stars, and the other with a brightness of the 



