sun's place among the stars — ADAMS 141 



observed only in a limited volume of space around the sun, and it is 

 only by assuming that their frequency throughout the whole galaxy 

 approximates that in this small sample volume that any estimate 

 can be made of their total number. 



If we sum up our conclusions, therefore, we find that our sun is 

 one of many billions of stars forming a flattened system similar 

 to many other systems scattered throughout the observable region 

 of space. It was long thought that the size of our system was con- 

 siderably greater than that of the nearer extra-galactic nebulae which 

 can be studied in detail, but recent observations have tended both 

 to reduce the earlier estimates of the size of our galaxy and to 

 increase the size of the outer stellar systems. As a result the size 

 of our system of stars is now believed to be quite comparable with 

 that of the Andromeda nebula (pi. 1, fig. 2), one of the nearest 

 and best observable of the outer nebulae, which has a distance of 

 about 900,000 light-years. 



The total mass of our galaxy, including not only the stars but 

 all the other material contained within it, is estimated at about 

 160 billion times the mass of our sun. This value is derived from 

 the rotation of the galaxy. So large a figure gives an impression 

 of considerable average density, but the size of the galaxy is so great 

 that the actual density is extremely small. In the vicinity of the 

 sun the average separation of even the faintest dwarf stars is of 

 the order of 10 light-years. Throughout the observable universe 

 the average density is, of course, very much lower, and recent investi- 

 gations give a value corresponding to that of 15 grains of matter 

 distributed uniformly throughout a volume of space 1,000 times 

 the size of the earth. 



When we begin to consider the place of the sun among the stars 

 as a physical body and attempt to compare it with other stars we 

 naturally start with its brightness. We know that the sun is very 

 bright and that the stars are faint, but we also know that the stars 

 are very far away. A natural question to ask is how the sun would 

 compare in brightness with the average stars of the night sky if 

 it were removed to the average distance of a star. The answer is 

 comparatively simple. Among the stars, just as on the earth, the 

 so-called " inverse-square law " of light and heat holds accurately in 

 the absence of any obscuring material. This means that if we double 

 the distance we divide by four the amount of light and heat we 

 receive from an object, whether that object be a candle, the sun, or 

 a star. If the sun were 10 times as far aAvay from us as it is we 

 should receive only one hundredth part of the present amount of 

 light. Now the average distance of the sun is about 92,000,000 miles, 

 and the distance of an average nearby star is at least 33 light-years 



