THE SIDEREAL UNIVERSE 165 



greatly outnumber those of superior light. In mass, in 

 surface temperature, in bulk, the sun belongs to a very 

 common class of stars; its speed of motion is near the 

 average; it shows none of the more conspicuous phe- 

 nomena such as variability which excite the attention 

 of astronomers. In the community of stars the sun 

 corresponds to a respectable middle-class citizen. It 

 happens to be quite near the centre of the local star- 

 cloud; but this apparently favoured position is dis- 

 counted by the fact that the star-cloud itself is placed 

 very eccentrically in relation to the galactic system, being 

 in fact near the confines of it. We cannot claim to be 

 at the hub of the universe. 



The contemplation of the galaxy impresses us with 

 the insignificance of our own little world; but we have 

 to go still lower in the valley of humiliation. The 

 galactic system is one among a million or more spiral 

 nebulae. There seems now to be no doubt that, as has 

 long been suspected, the spiral nebulae are "island uni- 

 verses" detached from our own. They too are great 

 systems of stars — or systems in the process of developing 

 into stars — built on the same disc-like plan. We see 

 some of them edgeways and can appreciate the flatness 

 of the disc; others are broadside on and show the ar- 

 rangement of the condensations in the form of a double 

 spiral. Many show the effects of dark nebulosity 

 breaking into the regularity -and blotting out the star- 

 light. In a few of the nearest spirals it is possible to 

 detect the brightest of the stars individually; variable 

 stars and novae (or "new stars") are observed as in our 

 own system. From the apparent magnitudes of the stars 

 of recognisable character (especially the Cepheid vari- 

 ables) it is possible to judge the distance. The nearest 

 spiral nebula is 850,000 light years away. 



