  We showed that compact bosonic objects can be formed through a process we
called gravitational cooling. A central issue in the subject of boson star is
whether a classical field configuration, {\it e.g.,} one described by the
Klein-Gordon equation, can collapse to form a compact star-like object, as
there is apparently no dissipation in the Klein-Gordon equation. We
demonstrated that there IS an efficient cooling mechanism to get rid of the
kinetic energy for the formation of a compact object purely through the
gravitational coupling, a mechanism universal to all self-graviting fields.
Implications of this mechanism are discussed, including the abundance of
bosonic stars in the universe, and the possibility of ruling out the axion as a
dark matter condidate.
