Half a century ago S. Chandrasekhar wrote these words in the preface to his l celebrated and
successful book: In this monograph an attempt has been made to present the theory of stellar dy
namics as a branch of classical dynamics - a discipline in the same general category as
celestial mechanics. [ ... J Indeed several of the problems of modern stellar dy namical
theory are so severely classical that it is difficult to believe that they are not already
discussed for example in Jacobi's Vorlesungen. Since then stellar dynamics has developed in
several directions and at var ious levels basically three viewpoints remaining from which to
look at the problems encountered in the interpretation of the phenomenology. Roughly speaking
we can say that a stellar system (cluster galaxy etc.) can be con sidered from the point of
view of celestial mechanics (the N-body problem with N 1) fluid mechanics (the system is
represented by a material con tinuum) or statistical mechanics (one defines a distribution
function for the positions and the states of motion of the components of the system).
