In terms of averaged physical quantities, small scale fluctuations yield an additional momentum and energy transfer between the stellar and gaseous component. The nonlinearities lead to dynamical friction (ρ
s1
gg
1) and energy loss of the stellar system [1],[2]. Here is the gravitational force and the momentum density, subscripts s and g refer to the stellar and gaseous component, respectively. Since the density fluctuations in the stellar and gaseous component are correlated due to gravitational forces, these transfer functions are in general nonzero. We treat the stellar system collisionless and the ambient gas hydrodynamically including dissipative processes. We computed the effect of fluctuations induced by the pointlike structure of the stars in quasilinear approximation taking stable modes into account only. Thus the resulting transfer rates seem to be lower bounds, since the effect of growing modes, nonlinearities in the wave structure, and fluctuations induced by different mechanisms are not included. For the timescales τ
b
and τ
E
of momentum and energy loss, respectively, we find the relation
where I
1 corresponds to the ‘Coulomb logarithm’ in the usual scattering theory and I
2 is an additional asymmetry measure of the power spectrum [1]. We discussed two very different astrophysical enviroments, where this effect of density fluctuations is of considerable magnitude. First, the systematic motion of a young stellar system, i.e. with low velocity dispersion, in a molecular cloud is slowed down in a timescale τ
b
as short as 106
yr [see also Deiss et al., this Vol]. The timescale τ
E
of energy dissipation is of the same order of magnitude. This estimate shows that friction and dissipation terms may not be neglected in the phase of early evolution of stellar systems as done in [4] e.g. — Secondly, the fluctuation theory can be applied to clusters of galaxies as well [3]. In this context the heating of the intracluster medium and a possible mass segregation of the galaxies are the dominating effects. The main uncertainty arises from the badly known mass-to-light ratio of the galaxies.