THE PROBLEMS OF ASTROPHYSICS 



BY WILLIAM WALLACE CAMPBELL 



[William Wallace Campbell, Astronomer and Director, Lick Observatory, Univer- 

 sity of California, b. April 11, 1862, Hancock County, Ohio. B.Sc. University 

 of Michigan, 1886; (Hon.) M.Sc. ibid. 1899; (Hon.) D.Sc. University of West- 

 ern Pennsylvania, 1900; LL.D. University of Wisconsin, 1902. Professor of 

 Mathematics, University of Colorado, 1886-88; Instructor in Astronomy, 

 University of Michigan, 1888-91; Astronomer, Lick Observatory, 1891. Mem- 

 ber of the Astronomische Gesellschaft ; Astronomical Society of the Pacific; 

 Astronomical and Astrophysical Society of America; American Association 

 for the Advancement of Science ; Italian Spectroscopic Society ; National Acad- 

 emy of Sciences; Associate of Royal Astronomical Society, etc. Organized 

 the D. O. Mills Expedition to Chile, 1903, to measure the radial velocities of 

 southern stars. In charge of Crocker Eclipse Expedition from the Lick Observa- 

 tory to India, 1898; Georgia, 1900. Author of Elements of Practical Astronomy.] 



THE 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 veloc- 

 ities, 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 nebula?, stars, 

 planets, and other celestial objects; of their physical conditions and 

 relations to each other; of the history of their development, in accord- 

 ance 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 astro- 

 physics. 



The motives of these problems are distinct and definite; but, 

 judged by the ultimate bearing of his results, nearly every astronomer 

 is working in both fields. The astrophysicist borrows the tools of the 



