ASTRO-PHYSICS 305 



order if only the one ten-thousandth part of that 

 of the others. 



Professor H. N. Russell, who discovered 

 these two types, calls them ** giant stars" and 

 "dwarf stars" respectively. They illustrate in 

 a marvellous way the mathematical theory of 

 stellar evolution. Beginning as a diffuse nebulous 

 mass, our new-born star, as we saw on page 302, 

 grows hotter by contraction, and passes up the 

 scale of spectral types from R through M, and 

 if it be large enough, reaches the class B or 

 even O. All through these ages, it is radiating 

 energy fiercely, and shining afar. It is a "giant" 

 star. But a maximum temperature is reached, 

 perhaps in the types A or B, and thereafter, the 

 density having already become great, the heat 

 gained by further contraction is less than that 

 lost by radiation. The temperature of the outer 

 radiating layers drops back through its old range, 

 and so the light of the star also passes back 

 along the series of spectral types from B or A 

 towards M, N, and R, though certain differ- 

 ences between ascending and descending spectra 

 have been recognised. But now the star is 

 no longer inwardly a gigantic mass of turbulent 

 vapour ever growing hotter, but a much smaller, 

 denser body, with a colder, calmer future before 

 it. The star has become a " dwarf." As it 

 declines in vigour, its light becomes redder, like 

 that of a cooling iron bar, and finally it vanishes 

 out of sight, to make its existence known to us, 

 if at all, by passing periodically as a dark body 

 round a still luminous partner. 



When any branch of learning first finds itself 



X 



