Characteristic Time Scales for a 
(Dissipationless) Stellar System:
_________________________________


1. Crossing time: 


2. Relaxation, or reference time: a typical 
   time scale for an average star to change 
   its energy           or the direction of 
   motion           due to the weak two-body 
   encounters only (Chandrasekhar, Spitzer):




   or:




   Note, however, that the "graininess" of 
   the collective gravitational field, or the 
   nature of orbits, can shorten substantially 
   the relaxation time as estimated from the 
   two-bodycencounters only (Pfenniger 1986, 
   A&A 165, 74; Gurzadyan & Savvidy 1986, 
   A&A 160, 203.)

3. Evolution time:  a substantial change of 
   the stellar system, due, in general, to 
   some external processes, e.g., evaporation 
   of stars, tidal shocks, mergers, etc.