  We discuss the dark gauge boson emission from neutron stars via
nucleon-nucleon bremsstrahlung. Through the rigorous treatment of the effective
field theory prescription and the thermal effect, we derive the relevant
couplings of dark gauge bosons to hadrons in medium. As a specific example, the
$U(1)_{\rm B-L}$ gauge boson scenario is chosen to investigate dark gauge boson
emissivities during supernovae and cooling of young neutron stars. From the
stellar cooling argument, we obtain the constraints on the $\rm B-L$ gauge
coupling for given gauge boson masses in two observations: the duration of the
supernova neutrino signal of SN1987A, and the inferred x-ray luminosity of the
compact object in the remnant of SN1987A (NS1987A). In particular, the
constraint from SN1987A on the $U(1)_{\rm B-L}$ gauge boson scenario is
revisited. The excluded gauge coupling due to the emission of transverse
polarizations is an order of magnitude enhanced compared to the previous
derivation. There is also a newly excluded parameter space due to the emission
of longitudinal polarizations.
