  The behavior of the relative magnesium abundances in the thin-disk stars
versus their orbital radii suggests that the star formation rate in the thin
disk decreases with increasing Galactocentric distance, and there was no star
formation for some time outside the solar circle while this process was
continuous within the solar circle. The decrease in the star formation rate
with increasing Galactocentric distance is responsible for the existence of a
negative radial metallicity gradient in the thin disk. At the same time the
relative magnesium abundance exhibits no radial gradient. It is in detail
considered the influence of selective effects on the form of both age -
metallicity and age - relative magnesium abundance diagrams. It is shown that
the first several billion years of the formation of the thin disk interstellar
medium in it was on the average sufficiently rich in heavy elements (<[Fe/H]> =
-0.22), badly mixed (\sigma_[Fe/H] = 0.21), and the average relative magnesium
abundance was comparatively high (<[Mg/Fe]> = 0.10). Approximately 5 billion
years ago average metallicity began to systematically increase, and its
dispersion and the average relative magnesium abundance - to decrease. These
properties may be explained by an increase in star formation rate with the
simultaneous intensification of the processes of mixing the interstellar medium
in the thin disk, provoke possible by interaction the Galaxy with the
completely massive by satellite galaxy.
