The proper motion of stars is of another kind. It consists in a displacement in various direc tions of the individual stars, so that the configu ration of constellations is slowly changing. The annual proper motions yet observed vary from nothing to 8.7". The proper motion of the binary star 61 Cygni amounts to 5.2", so that in 360 years it would pass over a space equal to the moon's diameter. It must thus take thousands of years to alter sensibly the aspect of the heav ens; although, taking into account the enormous distances, the actual velocities must be great. But the observed proper motions of the stars do not give us very accurate information as to their real motions and velocities. In the first place, it is only the angular change of the star's position that we observe; and we cannot tell the corre sponding linear shift unless we know• the distance of the star from the earth. And even where this distance (parallax) is known we can obtain only the transverse motion, as projected on the sky. There may be also a component of motion directly toward us or away from us. This radial component remained entirely unknown until recently, when it became possible to meas ure it with the spectroscope.
It was first observed by Sir W. Herschel that there is a perceptible tendency in the proper motions, as observed, to make the stars generally diverge or open up in one quarter of the heavens, and draw together in the opposite quarter; and this he attributed to a proper motion of our sun with his planets in the direction of the former point. The apparent motion thus caused is com plicated with the real independent motions of individual stars. The point toward which the motion is directed, which is called the 'solar apex.' was fixed by Herschel in the constellation Hercules; and the result of subsequent and inde pendent researches aives a nearly coincident point. The velocity has been calculated at about 11 miles per second, a figure still very uncertain.
Coining now from a consideration of stellar motions and velocities to their physical charac teristics, we must again have recourse to infor mation derived from a study of star spectra. Fraunhofer was the first to study them, and his researches have been followed by Rutherford, iluggins. Sccchi, Vogel, Pickering, and others. It has thus been possible to identify in the stars many of the terrestrial chemical elements. Many stars exhibit well-marked periodic alterations of magnitude, and are hence called rariable stars. A considerable number have been observed, of which perhaps the most remarkable are Mira (the wonderful) in Cetus, and Algol in Perseus. The first attains its greatest lustre every 334 days, and sometimes appears for 14 days as a star of the second magnitude; it then decreases for two or three months, till it becomes of the sixth and even tenth magnitude. so as to he for half a year in visible to the naked eye. After this it begins to increase, but more rapidly than it decreased. It is visible to the naked eye for three or four months of its period. Of all the variable stars yet observed Algol has the shortest period, being days 20 hours 48 minutes 55.4 seconds. It appears for about 60 hours a star of the second magnitude,' then decreases for four hours, and appears for a quarter of an hour of the fourth magnitude, after which it increases again for four hours. Various explanations have been offered of these mysterious appearances; that generally accepted postulates a large dark body revolving about the luminous one, so as to in tercept more or less of its light in different positions and making of Algol-type variables simple eclipse phenomena.
Allied to the variable stars are the new or temporary stars that appear suddenly in great splendor, and then disappear without leaving a trace. Fourteen instances are on record. It is not improbable that these also may be periodic.