Page:Popular Science Monthly Volume 87.djvu/219

Rh that is somewhat the shape of a very flat pocket-watch; more strictly, a much flattened ellipsoid or spheroid. However, it is not intended to convey the impression that the boundaries of the stellar system are sharply defined, or that the stars are uniformly distributed throughout the spheroid, and all at once, at the surface of the spheroid, cease to exist; but only that the stars are more or less irregularly distributed throughout a volume of space roughly spheroidal in form, and that the thinning out of stars near the confines of the system may be quite gradual and irregular. The equatorial plane of the spheroid is coincident with the central plane of the Milky Way. We see the Milky Way as a bright band encircling the sky, because in looking toward the Milky Way we are looking out through the greatest depth of stars. There is considerable uncertainty as to the dimensions of the system, chiefly for two reasons: first, the stars near the surface of the spheroid are everywhere too far away to let us measure their distances directly, and, in fact, so far away that we have not been able to measure their transverse motions—their proper motions—and thus to gain indirectly an idea of their distances; and secondly, the spheroid may be considerably larger than it seems because of possible, and even probable, absorption or obstruction of star-light in its passage through space. Newcomb has suggested that the shorter radius of the spheroid, at right angles to the plane of the Milky Way, may be taken as of the order of 3,000 light-years. The long radii of the spheroid, that is, the radii in the plane of the Milky Way, may be at least 10 times as great; that is, 30,000 light-years or more.

The solar system is believed to be somewhere near the center of the stellar system: the counts of stars in all parts of the sky indicate that the Milky Way structure is not much closer to us, so to speak, in one direction than in other directions; there are about as many stars on one side of a plane through the central line of the Milky Way as there are on the other. Wilhelm Struve's statistical studies of stellar distribution led him to conclude that the effective central line of the Milky Way is not a "great circle," but a "small circle," lying at a distance of 92° from the north pole of the galaxy and 88° from the south pole of the galaxy. Interpreted, this means that the solar system lies a short distance north of the central plane of the stellar system.

This conception of the stellar universe and Milky Way agrees in all important particulars with Immanuel Kant's ideas and description published in the year 1755—a remarkable contribution, based essentially on naked-eye observations, without the advantage of accurate observations laboriously made with telescopes. However, it was the star counts by the two Herschels, father and son, which put this conception of the stellar system upon the basis of confidence. Sir William Herschel, using an 18-inch reflecting telescope in the northern hemisphere, and Sir John Herschel. using the same telescope in the southern