8 



The Evolution of the Universe 



concentration of stars is in the center of the galaxy, the hght from 

 these forming the Milky Way. The familiar constellations of our 

 skies such as the Great Bear, or Big Dipper, and the Southern Cross 

 are groups of individual stars situated between the sun and the 

 Milky Way or in the peripheral region of our galaxy. 



About a billion galaxies are within the range of the 200 inch 

 Hale telescope; many of them are disc-like and resemble our own 

 (Figs. 1 and 2); others are elliptic or nearly spherical in outline. 

 The amount of gas and dust between galaxies has been computed to 

 total almost as much mass as the galaxies themselves. The total 

 amount of matter in the universe is incomprehensible in terms of 

 weight measures we know. 



Fig. 1. Galaxy NGC 3031 seen nearly face on, a galaxy with moderately 

 tightly wound arms. In this and the next two pictures the individual bright 

 areas of the galaxy consist of millions of stars which cannot be resolved in- 

 dividually by present-day telescopes. (Photograph from the Mount Wilson 

 and Palomar Observatories.) 



