pr]-:sij:)F.>tial a])Dress — spxtiox a. 41 



and the order of evolution was supposed to be in the direction 

 from the B-type stars to the M-type stars, although 

 Sir Norman Lockyer always maintained that there must be 

 stars of ascending- temperature as well as stars of descending- 

 temperature. The discovery of giant and dwarf stars supports 

 Sir jSiorman Lockyer's view, and gives us more explicit ideas^ 

 of stellar evolution. A large mass of gas in an extremely 

 diffuse state may be a giant M, or red, star. Obeying the 

 laws of a perfect gas, it will shrink or increase in density 

 and rise in temperature, passing through the spectral classes 

 K, Q, F, A to the helium stage B when it has its maximum 

 temperature. Throughout this part of its history its absolute 

 magnitude remains the same, its diminishing surface area 

 balancing the increased emissivity due to higher temperatures. 

 At the helium stage the star's density has become so great 

 that the perfect gas law is no longer obeyed. The loss of 

 heat by radiation is now greater than that generated within 

 the star, which nov/ commences to fall in temperatvire. The 

 star now passes down the spectral scale in the order 

 A, F, G, X, and becomes once more a M-type star, but 

 now its density is great, i.e., it is a dwarf star. Thus, when 

 the M-type stars are classified according to absolute 

 magnitude, they fall into two distinct groups, with a con- 

 siderable difference of absolute magnitude between them. 

 There will be similar groups for the K, Gr, F and A classes, 

 with decreasing differences of magnitude between the classes, 

 but there w411 be only one class of B-type star. Not all stars 

 can reach the helium class — they are not sufficiently large 

 enough to develop the necessary high temperature. Our Sun, 

 comparatively a small star, is thought not to have risen above 

 the F-, or calcium-, type. The helium stars are considered 

 to be the largest of all stars, and to be probably all of, 

 roughly, the same size, of the order of about one thousand 

 times the Sun's mass. Professor Eddington has recently 

 studied the condition of stellar existence as a consequence 

 of the laws of perfect gases, and concludes that there is aj 

 definite limit to the mass of a star, bevond which it could 

 not exist as an entity. This limit is probably represented by 

 the helium-type stars. 



There is an important relationship between spectral-type 

 and stellar radial velocity, the sig-nificance of which is not 

 yet thoroughly understood. Each spectral class has a certain 

 velocity, Avliich diminishes from the M-type to the B-type, 

 the slowly-moving helium stars. By some it is considered 

 that this represents equipartition of energy amongst the 

 stars, i.e., for all the stars the product of the mass and the 

 square of the velocity is constant, so that the massive B stars 

 have low velocity and the less massive M-stars greater 

 velocities. Considering the comparative emptiness of space 

 and the enormous distances between stars, it is very difficult 

 to see how equipartition of energy has come about. Such 

 a view would compare the distribution of stars in the 

 Universe and their movements Avith the motions of molecules 

 in a gas. 



