We address the dynamical evolution of an isolated self–gravitating system with two stellar mass groups .
We vary the individual ratio of the heavy to light bodies , \mu from 1.25 to 50 and alter also the fraction of the total heavy mass { \cal M } _ { h } from 5 % to 40 % of the whole cluster mass .
Clean-cut properties of the cluster dynamics are examined , like core collapse , the evolution of the central potential , as well as escapers .
We present in this work collisional N -body simulations , using the high–order integrator NBODY6++ with up to { \cal N } _ { \star } = 2 \cdot 10 ^ { 4 } particles improving the statistical significancy of the lower– { \cal N } _ { \star } simulations by ensemble averages .
Equipartition slows down the gravothermal contraction of the core slightly .
Beyond a critical value of \mu \approx 2 , no equipartition can be achieved between the different masses ; the heavy component decouples and collapses .
For the first time the critical boundary between Spitzer–stable and –unstable systems is demonstrated in direct N -body models .
We also present measurements of the Coulomb logarithm and discuss the relative importance of evaporation and ejection of escapers .