AN EVOLVING UNIVERSE JEANS 231 



with the Sim perhaps a third or a halfway along one of the spokes, 

 and rotating like a cart wheel. The Milky Way is formed of all the 

 stars which are at great distances from the sun, including of course 

 the great number which are near the rim of the wheel. 



The wheel is held together by the gravitational attractions of the 

 different stars of which it is composed. As a con£ec|uence, the outer- 

 most stars move with the slowest speeds and take longest to perform 

 a complete revolution — just as in the solar system the outermost 

 planets move most slowly and take the longest time to describe their 

 orbits round the sun. So far as is at present known, the sun moves 

 at about 200 miles per second, and requires something over 200,000,000 

 years to perform a complete revolution. 



In the early days of astronomy our galactic system was thought to 

 be the only system of stars in the slvy, but we now^ know that it is 

 only one of innumerable systems. If you look to the north of the 

 star Beta, in the constellation of Andromeda, you will, if 3^our eye- 

 sight is good, see a faint hazy patch. This is the object known as the 

 Great Nebula in Andromeda. It looks at first like diffused star- 

 light, as though a bit of the Milky Way had broken away — the 

 astronomer Marius described it as looking like candlelight seen 

 through a horn, while Herschel described it and similar objects as 

 " shining fluid." 



When this patch of light is viewed through a powerful telescope, 

 a certain amount of detail begins to appear; we can see dark lanes 

 across the background of light and notice a certain regularity in the 

 form and structure of the object. But to study it properly we must 

 photograph it with an exposure of man}^ hours. Endless new detail 

 now appears. The Nebula is found to be far larger than can be 

 seen either by the unaided eye or by direct vision through a telescope; 

 it is found to cover about 20 times as much sky as the full moon. 

 The only part we can see with the unaided eye is a comparatively 

 bright central mass, wdiich is fuzzy in appearance and ill defined in 

 outline. Round this is a detailed structure which lies hidden until 

 it is photographed with a very long exposure. 



Just as Galileo's telescope broke up the Milky Way into separate 

 points of light which he at once identified as stars, so the modern 

 high-power telescope breaks up the outermost regions of this Nebula 

 into separate points of light. We know that these, too, are stars. 

 Many of them do not shine with a steady light, but fluctuate in a 

 very characteristic and quite unmistakable way with which we are 

 very familiar, because many stars of our own system do precisely the 

 same. Indeed stars of this type are so peculiar, so uniform in their 

 behavior, and so similar to one another that we can estimate the dis- 

 tance of the Nebula from the apparent faintness of these stars. 



