THE 



POPULAR SCIENCE 



MONTHLY. 



FEBRUARY, 1905. 



AN" ADDEESS ON" ASTEOPHYSICS.* 



By Professor W. W. CAMPBELL, 



DIRECTOR OF THE LICK OBSERVATORY, UNIVERSITY OF CALIFORNIA. 



HHHE investigator in any field of knowledge must, as the price of 

 -■- success, both comprehend the general principles underlying his 

 special problem, and give constant care to its details. Yet it is well, 

 now and then, to leave details behind and consider the bearing of his 

 work upon the science as a whole. Whether our subject is that of 

 determining the accurate positions of the stars, or their radial velocities, 

 the orbits of the planets, or the constitution of the sun, we are making 

 but minor contributions to the solution of the two great problems 

 which at present compose the science of astronomy. These problems, 

 perhaps the most profound in the realm of matter, may be stated thus : 



1. A determination of the structure of the sidereal universe; of 

 the form of that portion of limitless space occupied by the universe; 

 of the general arrangement of the sidereal units in space ; and of their 

 motions in accordance with the law of gravitation. 



2. A determination of the constitution of the nebulae, stars, planets 

 and other celestial objects; of their physical conditions and relations 

 to each other; of the history of their development, in accordance with 

 the principles of sidereal evolution ; and of what the future has in store 

 for them. 



The first problem has for its purpose to determine where the stars 

 are and whither they are going. It has been ably treated under the 

 head of astrometry. 



The second seeks to determine the nature of the heavenly bodies 

 — what the stars really are. This field of inquiry is well named, 

 astrophysics. 



* Delivered at the St. Louis International Congress of Arts and Science. 

 vol. lxvi. — 20. 



