Till', SIM-XTKA OF ST A US AND NNKUM-:. 



23 



of unknown origin, but others are definitely identified as 

 belonging to hydrogen and helium. In some eases the chief 

 line of proto-hcliiun at wave-length 4G8G is also present and 

 serves, with helium and hydrogen, as a connection between the 

 nebula' and Wolf-Rayef stars. 



Very important evidence in this connection has quite lately 

 been obtained at the Lick Observatory, where some of the most 

 characteristic lines of nebulae have been found in the atmosphere 

 round one of the Wolf-Kayet stars, while the chief lines of 

 these stars have been proved to occur in the nuclei of several 

 planetary nebulae. It can scarcely be doubted, therefore, that 

 the Wolf-Kayet stars of class 0 represent the first results of the 

 condensation of the gaseous nebulae to stellar forms, and, as we 

 have seen, they stand at the head of the stellar sequence. 



The spectral sequence from nebulae, through the Wolf-Kayet 

 stars to the various classes of white and yellow stars, to the red 

 stars, and presumably onward to a stage where luminosity 

 ceases, thus seems to be complete. It cannot be claimed that 

 every star falls into this scheme, but only that this is in all 

 probability the normal course of the evolutionary process. There 

 are a few stars which for the present we must be content to 

 regard as " peculiar." 



The widely accepted view that the ancestors of stars are 

 represented by the gaseous nebulae would seem to require that 

 bhese bodies should contain all the materials of which stars are 

 known to be composed, and it has been quite properly felt that 

 some further explanation of the simplicity of the nebular 

 spectrum is called for. The explanation given by Tait, and 

 advocated by Sir Norman Lockyer in connection with the 

 Meteoritic Hypothesis, is that nebulae are swarms of meteorites, 

 in which gases released by collisions are rendered luminous by 

 electrical discharges, thus accounting for the bright line spec- 

 trum, while the metallic elements remain in the solid form and 

 consequently exhibit no spectrum at all. 



Another view is that in the nebulae most of the chemical 

 elements do not exist in the finished state, but are gradually 

 evolved from the nebular substance as condensation proceeds. 



Kemarkable mathematical work has been done in this connec- 

 tion by Professor Nicholson, who has found close agreements 

 between the theoretical vibrations of atoms of assumed simple 

 structure and those which are represented by the nebulium and 

 other lines actually observed in nebulae. He concludes that the 

 nebulae must be largely composed of a set of chemical forms, not 

 found in the Periodic Table, which are the simplest forms in 



