164 MAN'S PLACE IN THE UNIVERSE 



extent. It is partly broken up into subordinate con- 

 densations, which are probably coiled in spiral form like 

 the spiral nebulae which are observed in great numbers 

 in the heavens. The centre of the galactic system lies 

 somewhere in the direction of the constellation Sagit- 

 tarius; it is hidden from us not only by great distance but 

 also to some extent by tracts of obscuring matter (dark 

 nebulosity) which cuts off the light of the stars behind. 



We must distinguish then between our local star- 

 cloud and the great galactic system of which it is a part. 

 Mainly (but not exclusively) the star-counts relate to 

 the local star-cloud, and it is this which the largest 

 telescopes are beginning to exhaust. It too has a flat- 

 tened form — flattened nearly in the same plane as the 

 galactic system. If the galactic system is compared to 

 a disc, the local star-cloud may be compared to a bun, 

 its thickness being about one-third of its lateral ex- 

 tension. Its size is such that light takes at least 2000 

 years to cross from one side to the other; this measure- 

 ment is necessarily rough because it relates to a vague 

 condensation which is probably not sharply separated 

 from other contiguous condensations. The extent of 

 the whole spiral is of the order 100,000 light years. It 

 can scarcely be doubted that the flattened form of the 

 system is due to rapid rotation, and indeed there is 

 direct evidence of strong rotational velocity; but it is 

 one of the unexplained mysteries of evolution that 

 nearly all celestial bodies have come to be endowed with 

 fast rotation. 



Amid this great population the sun is a humble unit. 

 It is a very ordinary star about midway in the scale of 

 brilliancy. We know of stars which give at least 10,000 

 times the light of the sun; we know also of stars which 

 give 1/10,000 of its light. But those of inferior light 



