Page:Popular Science Monthly Volume 15.djvu/527

Rh Argelander's gauges show the same concentration in telescopic stars brighter than the tenth magnitude, and it is even more plainly to be made out from Mr. Proctor's chart of his great catalogue. If parallel to the great circle of the Milky Way two small circles be passed, each at a distance from it of 30°, having between them a broad belt about the celestial sphere somewhat like the torrid zone on the earth's surface, we shall leave two spherical caps whose united area will exactly equal that of the belt—just one hemisphere. From Argelander's gauges it may be calculated that the number of stars inside these 30 circles is to that outside nearly as 2 to 1, for stars of the ninth magnitude, and about as 2 to 1 for the eighth, diminishing with brighter stars. This condensation increases without interruption, to the Milky Way itself. The law holds also with stars visible to the naked eye, though not so conspicuously; for these, Mr. Peirce found the same ratio to be only as 4 to 3. He was also surprised to see that the stars were very little more numerous in the track of the Milky Way than at a distance of 20° from it, the decrease in density appearing almost suddenly about 30°. But as we approach the sun, the rate of condensation becomes greater again. Of the twenty stars classed as first magnitude by the best observers, fifteen are within the 30° circles; and of the five outside, but two, Arcturus and one far southern star, are equal in brightness to the average of the twenty. We have no right, however, unless we are dealing generally with a very great number of stars, to take light as a reliable indication of distance. Of our twelve nearest neighbors yet recognized, being all that have a parallax greater than one sixth of a second, and distant from us less than twenty years' journey of light, four are telescopic stars, to which attention was attracted by their large proper motion. Ten stars out of these twelve, it should therefore be added, are either in the Milky Way or within 15 of it. The exceptions are two minute stars in Ursa Major.

Will these facts enable us to decide what is the actual form of the immense cluster of stars in which our sun holds so humble a rank? We may conclude from them, with safety, that the strongly marked and surprising concentration of brightest and nearest stars in the galactic plane is irreconcilable with a generally prevailing uniform distribution, and agrees hardly better with Struve's theory of condensation in parallel planes. For this theory, it will be seen, requires a more decided concentration with a greater distance, the planes of equal density appearing to approach the galactic circle and each other as do the parallel lines of a perspective drawing. We do see some tendency of this kind in telescopic magnitudes, so that we might suppose that Struve's theory began to express the facts at the distance of the faintest visible stars—unless it could be shown that the density of aggregation in the central plane also varies at different distances. In Mr. Peirce's opinion, photometric observations have proved that this density increases from the seventh to the ninth magnitude, and that