Dynamical Friction. I. General Considerations: the Coefficient of Dynamical Friction.

In this paper it is shown that a star must experience dynamicaifriclion, i.e., it must sufier from a sys- tematic tendency to be decelerated in the direction of its motion. `This dynamical friction which stars ex- perience is one of the direct consequences of the fluctuating force acting on a star due to the varying com- plexion of the near neighbors. From considerations of a very general nature it is concluded that the coefficient of dynamical friction, ~, must be of the order of the reciprocal of the time of relaxation of the system. Further, an independent discussion based on the two-body approximation for stellar encounters leads to the following explicit formula for the coefficient of dynamical friction