Colloquium: The physics of axion stars

The particle that makes up the dark matter of the Universe could be an axion or axionlike particle. A collection of axions can condense into a bound Bose-Einstein condensate called an axion star. It is possible that a significant fraction of the axion dark matter is in the form of axion stars. This would make some efforts to identify the axion as the dark matter particle more challenging, but it would also open up new possibilities. The basic properties of axion stars, which can be gravitationally bound or bound by self-interactions, are summarized. Axions are naturally described by a relativistic field theory with a real scalar field, but low-energy axions can be described more simply by a classical nonrelativistic effective field theory with a complex scalar field.