The luminosity function tells you what kind of stars should be in the Galaxy and the spatial function tells you where they should be. By observing stars over a large area, the spatial distribution function can be divined and the shape of the Galaxy determined. Bluer stars are to the left, fainter stars toward the bottom. This CMD is for SA184. The thin disk, which is what we are most familiar with, is a dense disk of stars about 100,000 light years across and 2,000 light years thick.
Disk stars tend to be young and metal rich.
The IPII or thick disk is 3500 light years thick, comprised mostly of intermediate age, metal-poor stars. The spherical halo has a low density and is comprised of old, metal-poor stars. Some astronomers divide the halo into two sections - a low flattened structure and a high spherical structure. In our model, the low halo and the thick disk are united into one population. The CMD allows a rough look at the populations.