CHAOS IN ORDINARY STELLAR-SYSTEMS

The presence of irregular orbits in stellar systems is well establishe d, but it is usually regarded as pertaining to special cases such as b arred spiral galaxies, interacting systems, and so on. The possible ex istence of chaos in ordinary stellar systems is usually ignored, or ev en denied, and regular motion is taken for granted in current models. Spherical stellar systems offer an extreme example of holy shaky the g round where those models rest actually is. While all orbits in a spher ical static potential are of course regular, real spherical stellar sy stems are subject, to perturbations that give rise to chaos: weak bar- like disturbances easily send low angular momentum stars into irregula r orbits, while global oscillations cause temporal variations of the p otential and provide another path to chaos. The onset of chaos is aide d in the first case by the presence of anisotropic velocity distributi ons and, although isotropy is frequently assumed in studies of spheric al stellar systems, there are both theoretical reasons and observation al evidence supporting anisotropic velocity distributions for spherica l stellar systems.