Colisional Evolution of Stellar Systems

Astronomy 626: Spring 1995

How do stellar-dynamical systems evolve over times longer than the relaxation time,

        t_r = ---- t_c ?
              ln N

At any instant a stellar system almost exactly obeys the CBE, but stellar encounters slowly transform the system along a sequence of near-equilibria. The discussion here focuses on the abstract problem of a Newtonian system of N point masses.

Heuristic Arguments

Escaping stars carry away mass and energy.

Negative specific heat supports an outward flow of energy.

Binary processes may take over energy production.

The Fokker-Planck Equation

Weak encounters dominate evolution (F-P approximation).

Phase-space diffusion coefficients describe effects of weak encounters.

Relaxation timescales at core and median-mass radii are derived from diffusion coefficients.

Results for Spherical Systems

Escape due to ejection (insignificant) and evaporation.

Core collapse results from outward energy flow.

Equipartition between stars of different masses accelerates core collapse.

Expansion of post-collapse cluster is driven by interactions involving binary stars.


Joshua E. Barnes (

Last modified: April 27, 1995