STRUCTURE OF THE GALAXY PLASKETT 



195 



photographs of the Milky Way, how very inadequate were these 

 early conceptions of the galaxy, the distribution being assumed as 

 regular to enable the problem to be treated mathematically. While 

 they probably represent the arrangement of the stars in the neigh- 

 borhood of the sun, in the local cluster as it is now called — and it 

 should be remembered that it was only these stars whose distances 

 and distribution were then available for analysis — they took no 

 account of the complex and irregular distribution around the Milky 

 Way into the great star clouds, so obvious a feature of its structure. 



"lie LocajL Clu-stt- 



Mo 8 Side 1T00 llgU ■yea.7-5 



Ho7 



FiGUUB 4. — The local cluster. 



Even while Kapteyn was preparing his magnum opus and before 

 it was published, a research was under way by Shapley at the 

 Mount Wilson Observatory, whicli was to supersede Kapteyn's ideal 

 structure and give us a much enlarged conception of the magnitude 

 and complexity of the galaxy. Shapley developed an entirely new 

 method of determining the distances of the stars, depending upon a 

 knowledge of the intrinsic brightness of certain variable stars, the 

 Cepheids. Whereas, by the ordinary trigonometric method, stellar 

 distances beyond a hundred light-years or so are only very un- 

 certainly determined, this luminosity method gives accurate distances 

 to several thousands of light-years. Shapley thus determined the 



72774—35 14 



