A physicist’s view of stellar dynamics: dynamical instability of stellar systems

I argue that the widely adopted framework of stellar dynamics survived since 1940s, is not fitting the current knowledge on non-linear systems. Borrowed from plasma physics when several fundamental features of perturbed non-linear systems were unknown, that framework ignores the difference in the role of perturbations in two different classes of systems, in plasma with Debye screening and gravitating systems with no screening. Now, when the revolutionary role of chaotic effects is revealed even in planetary dynamics i.e. for nearly integrable systems, one would expect that for stellar systems, i.e. non-integrable systems, their role have to be far more crucial. Indeed, ergodic theory tools already enabled to prove that spherical stellar systems are exponentially instable due to N-body interactions, while the two-body encounters, contrary to existing belief, are not the dominating mechanism of their relaxation. Chaotic effects distinguish morphological and other properties of galaxies. Using the Ricci curvature criterion, one can also show that a central massive object (nucleus) makes the N-body gravitating system more instable (chaotic), while systems with double nuclei are even more instable than those with a single one. 1. On the current framework of stellar dynamics Since this is a Joint Discussion at IAU General Assembly, I allow myself to start from some general but also provocative remarks; for detailed refs see (Allahverdyan, Gurzadyan 2002). The current framework of stellar dynamics is the one summarized in Chandrasekhar’s book of 1942. That framework was borrowed earlier from the plasma physics when many features of perturbed nonlinear systems were unknown. This resulted in the ignorance of the drastic difference in the role of perturbations for two different classes of systems, plasma and gravitating systems: with Debye screening and justified cutoff of perturbations for the former, and long range interaction and no screening for the latter. Correspondingly, the two-body (Rutherford) scatterings, i.e. neglecting the perturbations of other particles of the system, were a priori assumed as the universal mechanism of relaxation of stellar systems,1 even though it failed to explain even The two-body relaxation is postulated also in kinetic (diffusion coefficients) and other approaches to stellar dynamics.