ASTRONOMY 



The researches which I have just outlined may 

 serve to give some idea of the growth and extent 

 of our knowledge regarding the motions of the 

 nearer stars that is to say, those within 500 light- 

 years of the sun. Much remains to be discovered, 

 and the remoter parts of the system are still 

 unexplored. 



Astronomy, as a system of organised know- 

 ledge regarding the stars, would be very incom- 

 plete if it confined itself to the investigation of 

 facts concerning their position and motion. A 

 complete survey must take account of their con- 

 stitution and physical condition. In pursuing in- 

 vestigations into these matters the great instrument 

 of research is the spectroscope. It takes the light 

 of the star, and broadens it out into a coloured band, 

 so that each component element in the light can 

 be separately examined. When the light is thus 

 split up, it is found that the spectra belong to a 

 few well-defined types corresponding to a rough 

 colour classification of the stars as Bluish-white, 

 White, Yellow, and Red. The typical spectra are 

 denoted by the letters B, A, F, G, K, and M. 



The bluish-white stars show a B type of 

 spectrum. They are the brightest, hottest, and 

 most massive of all the stars. Their spectra are 

 characterised by the great strength of the hydrogen 

 lines and by the presence of helium lines. 



Sirius is a typical white star, its spectrum being 



63 



