Galactic stellar orbits in the postepicyclic approximation

Relationships are found for the galactic orbits of stars in the second order of the epicyclic theory. An explicit expression is derived, describing the drift motion of stars along a circular trajectory in the rotating discs of galaxies. This small drift motion, which adds to the circular one, is analogous to the magnetic drift of plasma charged particles and is due to the differential rotation of the system. It is shown that, because of the stars' drift motion in the galactic disc's regions which are at the boundary of Jeans' stability, excitation is possible of gravitation-gradient instability of nonaxisymmetric perturbations of the gravitational potential and of the surface density. Due to its small increment, this gradient instability cannot serve as the mechanism generating global spiral density waves in the stellar disc of our Galaxy.