Page:Popular Science Monthly Volume 66.djvu/302

298 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 micrometer 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 parallaxes 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 astrometry 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 beginnings of astrophysical inquiry are somewhat older. Theories of planetary evolution by Kant and Laplace; observations of nebulæ and star clusters by the elder Herschel, and his wonderfully sagacious deductions 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 Kirchhoff'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 principal instrument. However, the spectrum is not his only field, nor the spectroscope his only tool. Radiation in all its aspects, and the instruments 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 example, 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 condition with millions of other stars, but it is the only one near enough