  Heavy-ion reactions provide a unique means to investigate the equation of
state (EOS) of neutron-rich nuclear matter, especially the density dependence
of the nuclear symmetry energy $E_{sym}(\rho)$. The latter plays an important
role in understanding many key issues in both nuclear physics and astrophysics.
Recent analyses of heavy-ion reactions have already put a stringent constraint
on the $E_{sym}(\rho)$ around the saturation density. This subsequently allowed
us to constrain significantly the radii and cooling mechanisms of neutron stars
as well as the possible changing rate of the gravitational constant G.
