ASTRO-PHYSICS 303 



criterion of the colour of the star and is called 

 its colour index. 



The next method of grouping is by means 

 of stellar spectra. This was first done by Father 

 Secchi, who found that the spectra could be 

 grouped in four broad classes, agreeing closely 

 with a classification according to colour, from 

 white to dark red. 



This grouping has been superseded by a 

 great catalogue of about a quarter of a million 

 stellar spectra made at Harvard Observatory. 

 The spectra are found to fall in a continuous 

 series, the various main groups being denoted 

 by the letters O B A F G K M N R, and each 

 of these groups being subdivided. The spectra 

 range from a faint continuous background with 

 bright lines of class O, through bright spectra with 

 helium and hydrogen dark lines, and then through 

 lines of metals such as calcium to the complex 

 spectra of type G, which includes that of our sun. 

 In type K the hydrogen lines get fainter and the 

 blue end of the spectrum becomes less intense ; 

 then in groups M and N are seen absorption 

 bands due to titanium oxide and carbon 

 compounds. These latter stars are red in 

 colour. 



Now as we heat a body, it first glows with 

 a deep red light and then becomes yellow and 

 finally white hot. The spectrum shows that, 

 in accordance with this common observation, 

 for a black body which is a perfect radiator, 

 the wave-lengths which give the maximum energy 

 of radiation are shorter the higher be the 

 temperature. Hence, by measuring the distribu- 

 tion of intensity in the spectrum of a star, the 



