  Starting from first principles, we study radiative transfer by new
feebly-interacting bosons (FIBs) such as axions, axion-like particles (ALPs),
dark photons, and others. Our key simplification is to include only boson
emission or absorption (including decay), but not scattering between different
modes of the radiation field. Based on a given distribution of temperature and
FIB absorption rate in a star, we derive explicit volume-integral expressions
for the boson luminosity, reaching from the free-streaming to the
strong-trapping limit. The latter is seen explicitly to correspond to
quasi-thermal emission from a "FIB sphere" according to the Stefan-Boltzmann
law. Our results supersede expressions and approximations found in the recent
literature on FIB emission from a supernova core and, for radiatively unstable
FIBs, provide explicit expressions for the nonlocal ("ballistic") transfer of
energy recently discussed in horizontal-branch stars.
