230 AISTNUAL REPORT SMITHSONIAN INSTITUTION, 1931 



SO near that their light takes only a few years to reach us, but the 

 faintest we can see are, for the most part, at distances of about 3,000 

 light-years ; that is to say, they are so remote that their light has to 

 travel through space for about 3,000 years before it reaches us — we 

 see them by light which left them before the beginning of the Chris- 

 tion era. 



Besides this collection of individual stars, we also see a band of 

 faint pearly light encircling the whole sky ; we call it the Milky Way. 

 This also consists of stars, but of stars which are too distant to be 

 seen as individuals by our unaided eyes, although numerous enough 

 to appear as a continuous cloud. Thus, the sk}^ which our unaided 

 eyes disclose to us, consists of two distinct parts — a foreground, con- 

 sisting of separate stars, and a background, formed by a continuous 

 cloud of distant stars. No middle distance can be seen by the unaided 

 eye. 



Yet telescopic observation at once discloses that a middle distance 

 exists. Like the foreground and the background, it consists of 

 stars — in this case, of stars which are too distant to be seen individ- 

 ually without telescopic assistance, and yet are not sufficiently numer- 

 ous to form a continuous cloud; for it is only in the direction of the 

 Milky Way that the distant stars lie close enough together to affect 

 our eyes. The telescope shows that this middle distance of stars con- 

 nects the foreground of individual stars with the background which 

 we can only see as a band of light, and it becomes possible to study 

 the system of stars as a continuous whole. 



Such studies have shown that the system of stars is shaped like 

 a disk or a coin or a cartwheel. Perhaps the last of these three com- 

 parisons is the best, because it has now been found that the system 

 of stars is in a state of rotation. Early investigators, Sir William 

 Herschel in particular, imagined that the sun must be somewhere 

 near the hub of this wheel ; we now know that it is at a great distance 

 away. 



It is so far away that even the brightest stars near the hub are too 

 faint to be seen by the unaided eye. The farthest stars our unaided 

 eyes can see are only about 3,000 light-years away from us, while the 

 hub of this great wheel of stars is probably something like 40,000 

 light-years away. We still do not loiow the diameter of the wheel 

 with any approach to accuracy, but it is probably something like 

 200,000 light-years. Still less do we know the total number of stars 

 which constitute the wheel. It is almost certainly greater than a 

 hundred thousand million and may quite well be two, three, four, 

 or even five times this number. 



Thus, we shall get the best picture which modern science can give 

 us of our system of stars if we think of it as shaped like a cartwheel, 



