Chaos in ordinary stellar systems

The presence of irregular orbits in stellar systems is well established, but it is usually regarded as pertaining to special cases such as barred spiral galaxies, interacting systems, and so on. The possible existence of chaos in ordinary stellar systems is usually ignored, or even denied, and regular motion is taken for granted in current models. Spherical stellar systems offer an extreme example of how shaky the ground where those models rest actually is. While all orbits in a spherical static potential are of course regular, real spherical stellar systems are subject to perturbations that give rise to chaos: weak bar-like disturbances easily send low angular momentum stars into irregular orbits, while global oscillations cause temporal variations of the potential and provide another path to chaos. The onset of chaos is aided in the first case by the presence of anisotropic velocity distributions and, although isotropy is frequently assumed in studies of spherical stellar systems, there are both theoretical reasons and observational evidence supporting anisotropic velocity distributions for spherical stellar systems.