The general solution for the birth function B(m,t) of stars, derived by Casuso et al. (1989) was used to obtain a first-order analytical approximation to this function as a function of metallicity Z. This approximation was used to obtain a fit to the observational curve compiled by Tinsley (1980) for the cumulative function of stars with metallicity lower than a given value in the solar neighborhood. The expression was also used to obtain a good fit to the differential distributions of stars at low metallicity, given by Pagel (1987). The analysis give an infall of gas towards the solar neighborhood up to the epoch of metallicity Z = 0.0067, with a correspondingly increased star formation rate, which subsequently stabilizes, and another similar inflow up to Z = 0.0012, followed again by a steady star formation rate for larger Z.