THE 



POPULAR SCIENCE 



MONTHLY 



SEPTEMBER, 1915 



THE EVOLUTION OF THE STARS AXD THE FORMATION 



OF THE EARTH 1 



Bt WILLIAM WALLACE CAMPBELL 



DIRECTOR OF THE LICK OBSERVATORY, UNIVERSITY OF CALIFORNIA 



Introduction 



I TWER Y serious student of nature asks, sooner or later: What was 

 -J the origin of the stars? What has been their history? And 

 what does the future hold in store for them ? 



In harmony with our experience is the belief that all matter in the 

 universe is endowed with the property of obe}dng certain fundamental 

 laws, such as : every particle of matter attracts every other particle ; a 

 hotter body radiates its heat energy to a cooler body; gases expand in- 

 definitely unless resisted by gravitation or other effective force. Again, 

 everything in nature is growing older and changing in condition; 

 slowly or rapidly, depending npon circumstances; the meteorological 

 elements and gravitation are tearing down the high places of the Earth ; 

 the eroded materials are transported to the bottoms of valleys, lakes and 

 seas; and these results beget further consequences. In general, the 

 changes in small bodies proceed rapidly and in great bodies slowly. 



Astronomers believe there has been an orderly development of the 

 stars, in obedience to precisely the same simple laws that govern our 

 every-day affairs. Starting with the materials as already existing, our 

 problem is to trace in outline the probable course of the evolutionary 

 processes which have given us the stellar universe. 



The effort to find a solution brings us against two superlative diffi- 

 culties : 



First, save only the Earth and an occasional meteorite, all the bodies 

 that concern us are at tremendous distances. We must study them at 

 long range, through the reading and interpretation of the messages which 

 their own rays of light and heat carry from them to us. We bring 



i Second course of lectures on the William Ellery Hale Foundation, National 

 Academy of Sciences, delivered at the meeting of the Academy in the University 

 of Chicago, on December 7 and S, 1914. 



VOL. LXXXUI. — 15. 



