Inorganic evolution. 26 



two portions keep on moving further and further apart., under 

 the influence of tidal friction. All stages of this process are 

 represented among the double stars. 



Then we have star-clusters, which are collections, often on 

 a very grand scale, of stars and star-like bodies, large and small, 

 united in one system and probably of common origin. One of 

 the best known examples is the Pleiades, which contains a great 

 number of stars, mostly very small, though at least 50 or 60 of 

 them are much larger than our Sun. These clusters often 

 contain many thousands of stars, and the stars in them are often 

 comparatively very close together. In one cluster the stars have 

 been estimated to be on an average distant from each other about 

 4 -Jo part of tlie distance of the Sun from his nearest neighbour. 



Our Sun shines with a fairly constant light, but many of the 

 stars are very variable in lustre either in short or in compara- 

 tively long periods. The period of varialnlity may range from 

 a few days or even hours to several years. Sometimes the 

 variations are due to a bright star being partially eclipsed by 

 a comparatively dark companion, but in other cases there seems 

 to be a real variation in tlie quantity or quality of their light. 

 Variable stars appear mostly to be in a comparatively early stage 

 of their existence. They are all at enormous distances from us, 

 and there is reason to believe that the conditions which favour 

 instability of light do not exist amongst what we may call the 

 Sun's next-door neighbours. 



New or Temporary stars have made their appearance at various 

 times. Having been before so small as to escape observation, 

 they suddenly blaze out with great brilliancy, and after a period 

 of brightness lasting some weeks or months gradually fade away 

 to insignificance. One has been described which rivalled Venus 

 in lustre. They seem to be comparatively small bodies, which 

 from some unknown cause suddenly develop greatly increased 

 heat and light, and become surrounded by incandescent gases. 

 Some of those most recently observed appear after the outburst 

 to have settled down to the condition of nebulae. They mostly 

 occur in the Milky Way at immeasurable distances from us. 

 It has been suggested that the collision of two dark bodies may 

 be the cause of the sudden outburst of light. 



The Mdky Way encircles the Sidereal Heavens like a great 

 zone or irregular belt. It sends out streamers or branches in 

 various directions, and is intersected by rifts or gaps. It consists 

 of an innumerable multitude of stars in much closer proximity 

 to each other than in other parts of the heavens. It is considered 



