The shape and size of our Galaxy

When we look around at the night sky, we find that the stars are not distributed very uniformly. There is a faint band of light – the Milky Way – going around the celestial sphere in a great circle. Even a moderate telescope reveals that the Milky Way is a collection of innumerable faint stars. Herschel (1785) offered an explanation of the Milky Way by suggesting that we are near the centre of a flat disk-like stellar system. When we look in the plane of the disk, we see many more stars than what we see in the other directions, thus producing the band of the Milky Way. After the development of photography, it became much easier to record distributions of stars in different directions. In the beginning of the twentieth century, Kapteyn attempted to put Herschel's view on a firm footing, by undertaking a huge programme of counting stars in different directions and measuring their proper motions with a view of estimating distances. From a painstaking statistical analysis of these data, it was inferred that we are at the centre of an oblate stellar disk with a thickness of a few hundreds of pc and a disk radius of about a few kpc (Kapteyn and van Rhijn, 1920; Kapteyn, 1922). This model is usually referred to as the Kapteyn Universe, since it was believed at that time that this was the whole Universe!