298 POPULAR SCIENCE MONTHLY. 



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 

 astronomer of position, the latter uses the results of the former, and 

 vice versa. Let me give two illustrations. Astrophysics desires to 

 know the relative radiating power of matter in different types of 

 stars — the Sirian and solar types, for example. The meridian circle 

 and the telescope discovered a companion to Sirius; the microm- 

 eter determined the form and position of the orbits; the heliometer 

 observed the star's distance ; and the photometer measured the quantity 

 of light received from it. Computations determine from these data 

 that Sirius is but two and one half times as massive as our sun, 

 whereas it radiates twenty-one times as much light; from which it 

 follows that a given quantity of matter in Sirius radiates many times 

 as effectively as the same quantity of solar matter — a fact of prime 

 importance in the astrophysical study of all Sirian stars. The paral- 

 laxes of the stars are needed by the student of stellar evolution as 

 well as by the student of the structure of the heavens. 



Again, the measurement of radial velocities of the stars has been 

 left almost completely to those observers who are especially interested 

 in astrophysical problems and methods, yet it is the student of as- 

 trometry who is eager to use their results. The overlapping of the two 

 departments of astronomy is but the symbol of progress. 



The term astrophysics is of the present generation, but the be- 

 ginnings of astrophysical inquiry are somewhat older. Theories of 

 planetary evolution by Kant and Laplace; observations of nebulae and 

 star clusters by the elder Herschel, and his wonderfully sagacious deduc- 

 tions concerning them; various studies of planetary markings and 

 conditions; systematic investigations of the sun spots, including 

 Schwabe's discovery of their eleven-year period — these constituted the 

 main body of the science in 1859. But the spirit of inquiry as to the 

 nature of the heavenly bodies was latent in many quarters; and Kirch- 

 hoff's immortal discovery of the fundamental principles of spectrum 

 analysis opened a gateway which many were eager to enter. The 

 spectroscope became at once, and has remained, the astrophysicist's prin- 

 cipal instrument. However, the spectrum is not his only field, nor the 

 spectroscope his only tool. Radiation in all its aspects, and the in- 

 struments for determining its quantity and quality, are the means to 

 the ends in view. And the great generalizations of scientific truth, 

 the doctrines of evolution and of the conservation of energy, for ex- 

 ample, have been no less helpful here than elsewhere. 



The study of our sun forms the principal basis of astrophysical 

 research. The sun is an ordinary star, comparable in size and condi- 

 tion with millions of other stars, but it is the only one near enough 



