  If axions exist, they are copiously produced in hot and dense plasmas,
carrying away energy directly from the interior of stars. Various astronomical
observables constrain the operation of such anomalous stellar energy-loss
channels and thus provide restrictive limits on the axion interactions with
photons, nucleons, and electrons. In typical axion models a limit m_a < 0.01 eV
is implied. The main arguments leading to this result are explained, including
more recent work on the important supernova 1987A constraint.
