Dynamical Friction in Stellar Systems: an introduction

An introductory exposition of Chandrasekhar's gravitational dynamical friction, appropriate for an undergraduate class in mec hanics, is presented. This friction results when a massive particle moving through a ``sea'' of much lighter star particles experiences a retarding force du to an exchange of energy and momentum. General features of dynamical friction are presented, both in an elementary and in a more elaborate way using hyperbolic two-body interactions. The orbital decay of a massive particle in an homogeneous gravitational system is solved analytically, that leads to an underdamped harmonic oscillator type of motion. A numerical integration of the equation of motion in a more realistic c ase is done. These results are compared to those of an N-body computer simulation. Several problems and projects are suggested to students for further st udy.