140 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1935 



The Sim is situated within one of these systems of stars, which 

 is known as the galaxy. Like most other well-developed stellar 

 systems containing great numbers of stars it is not round but lens- 

 shaped in form, with a length some 5 to 10 times its thickness 

 (fig. 1). As we look out into the galaxy from our position inside, 

 the circle of the Milky Way forms the largest dimension of our 

 lens, or the equator of our galaxy, and we see a vast number of 

 stars because we look through a great depth throughout which they 

 are scattered. At right angles to the galaxy where the thickness 

 is much less the number of stars is also much less. Our system of 

 stars is probably not definitely bounded but fades off more or less 



irregularly into empty 

 space, and included 

 within it are not only 

 stars but clouds of 

 gas, cosmic dust, star 

 clusters, star clouds. 



Figure 1. — Diagram of our stellar system. The smaller and the numeroUS 

 local system about our sun is also indicated. , , „ . i • i 



other forms m which 

 the material that builds the stars can occur. Especially in the direc- 

 tion of the center of the galaxy in the constellation of Sagittarius 

 are there great masses of dark cosmic clouds, nebulous stars, and 

 star clouds that point to a marked concentration of matter in this 

 region (pi. 2). 



Our galaxy is about 100,000 light-years along its greatest diameter, 

 and the sun is situated about halfway between the center and edge, 

 or some 25,000 or 30,000 light-years from the center, and slightly 

 north of the equatorial plane. As a result of modern investigations 

 it is certain that the entire galaxy is in rotation about its center or, 

 more accurately, that the stars are in revolution about the center of 

 ^avity of the system just as the planets are in revolution about the 

 sun. The stars nearest the center revolve most rapidly and in the 

 shortest period. The velocity of revolution of the sun about the 

 center of the galaxy is about 165 miles a second, and it would require 

 some 225 million years for the sun to complete one entire revolution. 



Scattered throughout this great volume of space are the stars that 

 constitute our system, variously estimated at from 100 to 200 billion 

 in number. The difficulty in determining the number of the stars 

 arises from the necessity for making the proper allowance for the 

 great proportion of faint small stars, which certainly outnumber 

 greatly the more luminous stars in our galaxy. These small stars, 

 the dwarfs of our sj^stem, which we shall have occasion to consider 

 later, become so faint at great distances that they are quite beyond 

 the reach of the largest telescopes. Accordingly such stars can be 



