  The exchange of massless neutrinos between heavy fermions (e.g. $e,p,n$)
gives rise to a long-range 2-body force. It is shown that the analogous
many-body force can lead to an unphysically large energy density in white
dwarfs and neutron stars. To reduce the energy density to a physically
acceptable value, the neutrino must have a {\it minimum mass}, which is
approximately $0.4\;eV/c^2$. Some recent questions relating to the derivation
of this bound are also discussed.
