Page:The Music of the Spheres.djvu/53

 The little star just above the star in the crook of the handle of the Dipper is sometimes spoken of as a 'rider.' The Arabs call these two stars a "Horse and its Rider," the English call the rider "Jack-on-the-Middle-Horse," while the Germans call him "Hans-on-the-Middle Horse." Hans chose this position in preference to any other on the face of the earth or in the kingdom of Heaven.

Astronomers have still another name for the Horse and its Rider. To them it is a "naked-eye double," the tiny star being called "Alcor," and the one on the Dipper's handle just below it, "Mizar." A 3-inch telescope discloses a still closer companion to Mizar which has a decided greenish tinge in its light. Of the two stars composing Mizar, each one is itself composed of two, which revolve around a common center of gravity in a period to be counted in thousands of years. This wonderful law of gravitation which holds not only planets in their orbits, but also stars, was discovered by Sir Isaac Newton, an English philosopher and mathematician. Two stars revolving around a common center of gravity in this manner are called a "binary"; in the case of Mizar and its companion, each of the two visual components is called a "spectroscopic binary." The brighter component was discovered to be a binary in 1889 by E. C. Pickering with the aid of a spectroscope and the fainter component was found to be a spectroscopic binary in 1908 by Frost and Lee. Alcor is also a spectroscopic binary.

The stars are such an exceedingly great distance from the earth that even though they are in constant motion, they do not seem to change their relations to other stars through long periods of time. The whole configuration of the Big Dipper will some day be changed because its stars are traveling in various directions. Through the skill of various scientists this infinitesimal difference in motion is detected and recorded,—not only that a star is moving, but which way and how fast! Thus the spectroscope exploded the