OUR REVOLVING "ISLAND UNIVERSE" — SHILLING 



131 



And the extreme high speed of globular clusters of stars, as much 

 as 170 miles a second, simply means that these clusters are not part 

 of our galaxy at all, or at least do not rotate with it, and the observed 

 velocity is really our own speed of travel with the galaxy as we go 

 rapidly past the stationary clusters. 



Soon after Lindblad's explanation of "high-velocity" stars and other 

 phenomena of stellar motion, a Dutch astronomer, Oort, developed 

 a direct method of studying the rotation of the galaxy by using the 

 velocities of many stars at known distances from us. He worked out 

 a comparatively simple means of using such measurements to learn 



Center of Galaxy 

 Figxjbe 1. — Intragalactic velocities, showing relative velocities of sun (S) and 

 stars around the galactic center. Short arrows represent components of 

 motion of stars toward or from the sun, as shown by the stars' spectra. 



a great deal about the stellar system and our position in it. His plan 

 of procedure has been used since then by a number of astronomers, 

 who have applied it to various classes of stars. Notwithstanding the 

 great difficulty of making the required measurements the different 

 workers have arrived at very similar results. 



Oort's plan is based on the fact that revolving bodies controlled 

 by a central attractive force move at unequal speeds depending on 

 differences in distances from the center (fig. 1). The different veloci- 

 ties of planets revolving around the sun illustrate this. Pluto, the 

 outermost planet known, travels at 3 miles a second ; Jupiter, nearer 



