It is possible to estimate the galactic mass density in the solar neighbourhood either directly by summing up the mass densities of individual subsystems of stars and interstellar matter or indirectly from dynamical considerations.

Observational data on the number density of visible stars lead to mutually consistent results on the stellar component of the mass density. The mean of different estimates is ⍴stars=0.052±0.010 Mʘpc−3. By adding the probable contributions of intrinsically faint undetected objects and of interstellar matter the value ⍴=0.09±0.02 Mʘpc−3 has been obtained for the total mass density.