ASTRO-PHYSICS 299 



respectively, while the outer planets, Mars and 

 Neptune, fall as low as —38" and —221°. If 

 there are, indeed, inhabitants on Mars, it seems 

 that, according to terrestrial ideas, they must 

 lead a very chilly existence. 



We may now collect the various threads of 

 thought we have followed, and weave them into 

 a picture of the physical universe and its 

 history. 



Our stellar system seems to be a flattened 

 lens-shaped galaxy of some 1500 million stars 

 and nebulae, about 300,000 light years across 

 its diameter. Though the stars vary greatly 

 to us in brightness, that variation is chiefly an 

 effect of distance or temperature, and the absolute 

 masses of most stars range from about the same 

 dimensions as those of our sun to some twenty 

 times larger. 



Interspersed with the stars, or perhaps in 

 some cases beyond our stellar system, are 

 nebulae, some irregular clouds of light, others 

 of regular lens-like form, and again others with 

 spiral arms, like an instantaneous photograph 

 of a "Catharine's wheel" firework. 



Laplace suggested that our sun and planets 

 were formed from a nebula, and, in the three 

 kinds of nebulae mentioned, modern science sees 

 the development of star-worlds in the making. 

 Mathematical analysis shows that a mass of 

 nebulous matter of the size of our puny solar 

 system would not develop as Laplace thought ; 

 its power of gravitation would not be enough, 

 and its gaseous matter would diffuse into space 

 and not condense. But the regular and spiral 



