  We calculate the axion emission rate from reactions involving thermal pions
in matter encountered in supernovae and neutron star mergers, identify unique
spectral features, and explore their implications for astrophysics and particle
physics. We find that it is about 2-5 times larger than nucleon-nucleon
bremsstrahlung, which in past studies was considered to be the dominant
process. The axion spectrum is also found be much harder. Together, the larger
rates and higher axion energies imply a stronger bound on the mass of the QCD
axion, and better prospects for direct detection in a large underground
neutrino detector from a nearby galactic supernova.
