SCIENCE 



ILLUSTRATED SUPPLEMENT 



N.S., Vol. XLV Friday, May 25, 1917 Number 1169 



THE NEBULA 



Address of the Retiring President of the American Association for the Advancement of Science. i 

 By W. W. CAMPBELL 



Director of Lick Ohsrrciitory, University of California 



It is characteristic of most investigations in pure science that the quest is for the origin 

 and hi.story of things, and for the understanding of what now is, rather tlian for what is 

 going to occur. One does not Avisely venture to predict the future until lie has explained 

 the past and accounted for the present. Paleontologists are fruitfully studying the ex- 

 tinct animal life of our planet ; several departments of science are busy with the life of 

 to-day ; and little effort has yet been made to forecast the animal life of the future. An- 

 thropologists and ethnologists have been concerned with the men and the races of men who 

 have already lived ; they are just beginning to think scientifically of the men and the races 

 that are to come. Conditions are moderately different in the one science, astronomy, whose 

 chief domain lies far outside the earth and far beyond oiir sun. Some of the planets in our 

 solar system may be passing through stages of existence that the earth experienced long ago, 

 and others of our planets may be approximate examples of what is in store for the earth. 

 When we undertake the studj^ of the sun we have the great advantage that millions of 

 suns within onr view are representing the s Lages of stellar life through which our sun is 

 thought to have passed, and millions of others the stages through which our sun will pass 

 in the future. If we seek to know the historjr of our sun we can not avail ourselves of 

 progressive changes in the sun itself. Such changes are too slow ; we think the sun has 

 remained substantially unchanged for hundreds of thousands of years. The student of 

 stellar evolution jsroceeds by arranging the stars. in general in the supposed order of their 

 effective ages, and he endeavors to place our star and our planet at the logical points in 

 the series. In this way astronomers, not unanimously, but in the great majority, have 

 arrived at the conclusion that our own sun is in effect one of the middle-aged stars, and 

 that our earth is in effect a middle-aged planet ; and they attempt seriously to predict 

 the future histories of the two bodies. 



It is not my purpose to conduct you over the long road of stellar evolution. I shall in- 

 vite your attention chiefly to the parts which the nebulie seem to play in the development 

 of the stellar universe ; and this will lead me to touch lightly upon the birth and infancy 

 of the stars, and to neglect the periods of their youth, middle life and old age. 



1 Delivered in the American Museum of Natural History, New York City, December 26, 1916. Illus- 

 trated bv lantern slides. 



