n-13] Theories of Cosmogony 13 



observed to be surrounded in general by masses or wisps of nebulous matter, 

 the stars in the Orion nebula and the Pleiades being obvious instances. 

 Whether or not these processes of reasoning are sound, there is no question 

 that the theory of nebulous origin is widely and almost universally held, there 

 being some room for differences of opinion as to whether the primitive nebula 

 ought to be thought of as a mass of gas, or a dust cloud or possibly even a 

 swarm of meteoric stones. For instance Lord Kelvin suggested as the ulti- 

 mate origin of astronomical bodies, a collection of meteoric stones which were 

 vaporised by repeated collisions and so gave place in time to a gaseous nebula 

 of the Laplacean type. Sir Norman Lockyer suggested that many of the 

 observed nebulae are still in the meteoric state, a view which recent spectro- 

 scopic evidence has made untenable as a general explanation of nebular 

 structure. 



13. A theory of the order of stellar development recently put forward 

 by H. N. Russell* strikes at the root one of the principal reasons for believing 

 in the nebulous origin of stars. Before the appearance of Russell's theory, 

 the accepted order of stellar evolution, namely through the sequence of spec- 

 tral types 



Nebula, B, A, F, G, K, M, 



was almost undisputed. In this classification (, the jB-type stars are the 

 hottest and stars of M type (red stars) are the coolest. The approximate 

 temperatures of the different types, as determined by Wilsing and ScheinerJ, 

 are as follows : 



Type BO -Bo, T= 9030 Type G, T= 4450 

 58-^14, 8880 K 3970 



A5-A8, 5780 M 2960 



According to the older view of stellar evolution, the B type was supposed 

 to indicate the stage in which the star was hottest and of lowest density, and 

 so least removed from its original nebular existence ; as the star radiated 

 heat it got cooler, and so passed through the various types in succession ; a 

 spectrum of M type was supposed to characterise the oldest stars which were 

 close to extinction. 



Russell shewed that the M stars fall into two very clearly differentiated 

 classes which he called "giant" and "dwarf" stars, these names referring to 



* For an excellent statement by Prof. Eussell himself see Nature, 93, p. 227 (1914) ; see also 

 The Observatory, 37, p. 165. 



f This is the Draper Classification adopted by Harvard Observatory. A brief but excellent 

 account of spectral classifications is given in Eddington's Stellar Movements, pp. 710. 



Ast. Nach. 183 (1909), p. 87. Here and elsewhere each complete spectral class is divided 

 into ten subdivisions ; thus between a B star and an A star are supposed to be nine other stages 

 designated as B 1, B 2,... B 9. A spectrum of exact B type is called BO, and so on. 



To avoid confusion I have omitted the O or Wolf-Rayet Type from discussion altogether. 



