tended stratum of stars. To introduce a familiar object, its figure was that of a large thin grindstone, our Solar System being near the center. Considering only the general aspect of the heavens, this conclusion was plausible. Suppose a mass of a million of stars scattered through a space of this form. It is evident that an observer in the center, when he looks through the side of the stratum, would see few stars. The latter would become more and more numerous as he directed his vision toward the circumference of the stratum. In other words, assuming the universe to have this form, we should see a uniform, cloud-like arch spanning the heavens—a galaxy in fact.
This view of the figure of the universe was also adopted by Struve, who was, the writer believes, the first astronomer after Herschel to make investigations which can be regarded as constituting an important addition to thought on the subject. To a certain extent we may regard the hypothesis as incontestable. The great mass of the visible stars is undoubtedly contained within such a figure as is here supposed.
To show this let Fig. 1 represent a cross section of the heavens at
right angles to the Milky Way, the Solar System being at S. It is an observed fact that the stars are vastly more numerous in the galactic regions G G than in the regions P P. Hence, if we suppose the stars equally scattered, they must extend much farther out in G G than in P P. If they extend as far in the one direction as in the other, then they must be more crowded in the galactic belt. It will still remain true that the greater number of the stars are included in the flat region A B C D, those outside this stratum being comparatively few in number.[1]
But we cannot assume that this hypothesis of the form of the universe affords the basis for a satisfactory conception of its arrangement. Were it the whole truth, the stars would be uniformly dense along the whole length of the Milky Way. Now, it is a familiar fact that this is not the case. The Milky Way is not a uniformly illuminated belt, but a chain of irregular, cloud-like aggregations of stars. Starting from this fact as
- ↑ Regarding the galaxy as a belt spanning the heavens, the central line of which is a great circle, the poles of the galaxy are the two opposite points in the heavens everywhere 90° from this great circle. Their direction is that of the two ends of the axle of the grindstone, as seen by an observer in the center, while the galaxy would be the circumference of the stone.
