STRUCTURE OF THE GALAXY PLASKETT 



193 



The next step (fiir. 3), 100,000,000,000,000 miles, or 17 light-years, 

 shows the relative distance and intrinsic brightness of a few of the 

 nearer fixed stars. Please remember that the width of this figure 

 corresponds to a train journey of 230,000,000 years, and yet w^e see 

 how far we are from anything approaching a galaxy of 200X10" 

 stars. If we proceed to the next step (fig. 4), however, scale 1,700 

 light-years, we are now more nearly approaching galactic dimensions 

 and begin to see what Newcomb and Seeliger were considering when 

 they estimated a diameter of 7,000 light-years with a thickness of 

 about one-fourth the diameter. This group of stars, which is now 



Distance Nearest Star .25 times Length cff Slide 

 No6 SUe 1 000 000 000 000 'miles Si::s No 5 



FiGUEB 2. — The solar neighborhood. 



called the " local cluster ", with the sun not quite central, will cer- 

 tainly extend to a diameter of 7,000 light-years or more before the 

 stars become very thinly spaced and probably represents the galaxy 

 of Newcomb and Seeliger. 



The dimensions of the galaxy were increased by Walkey in 1914 

 to a diameter of 14,000 light-years and by Eddington in 1915 to 

 about 15,000 light-years. The most complete investigation of the 

 structure and dimensions of the limited stellar system as then rec- 

 ognized is due to the famous Dutch astronomer, Kapteyn, who in 

 1920 from long-extended investigations, embracing practically the 

 aim of his life work, gave an idealized representation of the galaxy 



