  The self-gravitating gas in thermal equilibrium is studied using a Newtonian
potential regularized at short distances. This short distance cutoff permits us
to obtain a complete description of the gas including its collapsed phase. We
give a field theory description of the N-body regularized self-gravitating gas
in the canonical ensemble.The corresponding functional integral is dominated in
the N -> infty limit by saddle points which provide a mean field description.
The well-known dilute solutions (isothermal spheres) are recovered. We find new
solutions which are regular in the regularized theory but become singular in
the zero cutoff limit. They describe collapsed configurations where the
particles are densely concentrated in a region of the size of the cutoff. These
collapsed solutions provide the absolute minimum of the free energy. We find
further new solutions which interpolate between the collapsed and the dilute
configurations and describe tunneling processes where the gas collapses. The
transition proba- bility for such collapse processes turns out to be extremely
small for large N. That is, the dilute solutions are in practice stable in the
regime where they are locally stable.
