  We review the current status of astrophysical bounds on QCD axions, primarily
based on the observational effects of nonstandard energy losses on stars,
including black-hole superradiance. Over the past few years, many of the
traditional arguments have been reexamined both theoretically and using modern
data and new ideas have been put forth. This compact review updates similar
Lecture Notes written by one of us in 2006 [Lect. Notes Phys. 741 (2008)
51-71].
