The Romance of the Heavens 15 



a spiral nebula, and that the other spiral nebulae are "other 

 universes." 



Vast as is the Solar System, then, it is excessively minute in 

 comparison with the Stellar System, the universe of the Stars, 

 which is on a scale far transcending anything the human mind 

 can apprehend. 



THE SOLAR SYSTEM 



THE SUN 



But now let us turn to the Solar System, and consider .the 

 members of our own little colony. 



Within the Solar System there are a large number of 

 problems that interest us. What is the size, mass, and distance 

 of each of the planets? What satellites, like our Moon, do they 

 possess? What are their temperatures? And those other, 

 sporadic members of our system, comets and meteors, what are 

 they? What are their movements? How do they originate? 

 And the Sun itself, what is its composition, what is the source of 

 its heat, how did it originate? Is it running down? 



These last questions introduce us to a branch of astronomy 

 which is concerned with the physical constitution of the stars, 

 a study which, not so very many years ago, may well have 

 appeared inconceivable. But the spectroscope enables us to 

 answer even these questions, and the answer opens up questions 

 of yet greater interest. We find that the stars can be arranged 

 in an order of development that there are stars at all stages of 

 their life-history. The main lines of the evolution of the stellar 

 universe can be worked out. In the sun and stars we have 

 furnaces with temperatures enormously high ; it is in such condi- 

 tions that substances are resolved into their simplest forms, and 

 it is thus we are enabled to obtain a knowledge of the most 

 primitive forms of matter. It is in this direction that the spectro- 



