As we have seen (§ 13), our sun is a member of a huge system of stars whose number must be counted in thousands of millions. In general shape this system may be compared to an oblate spheroid with very unequal axes, or, less mathematically, to a coin or round biscuit. The stars are not uniformly distributed throughout this system, being much more thickly scattered in its central parts than in its outer regions. Probably there is no clearly defined boundary, the star-density diminishing indefinitely as we recede from the centre, but never becoming quite zero. The sun lies almost exactly in the central plane of the system, although not precisely at the centre. Those stars which lie near the edge of the coin or biscuit are so remote as to appear very faint to us and constitute the Milky Way. The system of stars bounded by the Milky Way is commonly called the Galactic System.

The stars shew so little motion that for a long time astronomers failed to detect any motion at all, and they became known as “fixed stars” to distinguish them from the planets or “wandering stars” whose motion was obvious to everyone. But modern astronomy finds it possible to measure the motions of a great number of stars.