  The excursions of star clusters and galaxies around statistical equilibria
are studied. For an ergodic model with monotone decreasing DF Antonov's
Hermitian operator on six-dimensional phase space has the normal modes as its
eigenfunctions. The excitation energy of the system is just the sum of the
(positive) energies associated with each normal mode. The positivity of modal
energies opens the way to modelling the thermal properties of clusters in close
analogy with those of crystals. Formulae are given for the DFs of modes, which
are of the type first described by van Kampen rather than Landau. Each mode
comprises the response of non-resonant stars to driving by the gravitational
field of stars on a group of resonant tori. The structure of each mode is
sensitive to the degree of self gravity. The emergence of global distortions in
N-body models when particles are started from an analytical equilibrium is
explained in terms of the interplay of normal modes.
