The significance of deep star counts for models of the Galaxy and the surface density of faint quasars

Models of the number–apparent magnitude distribution for galactic stars are constructed, and compared with recently available data. The uncertainties in both the galactic structure parameters and the star counts are such that several different parameter combinations can be fitted equally well to the data. However, highly flattened spheroids (axial ratio less than ∼ 0.5), and spheroids with a density decrease slower than about r⁻³ are difficult to reconcile with existing observations. For $$20\lesssim m_B\lesssim 24$$, the number–colour diagram shows that almost all stars are either G type or M type, independent of apparent magnitude. These may be identified with disc M and spheroid G stars. Their distribution in the colour–apparent magnitude diagram is as expected from the forms of the disc and spheroid colour–absolute magnitude relations, and the differing stellar number density variations between the disc and spheroid components of the Galaxy. An upper limit of 200 ± 50 quasars per square degree with $$B-V\lesssim 0.35$$ and $$20\leqslant m_B\leqslant 23$$ is derived.