In astronomy, an iron star is a hypothetical type of compact star that could occur in the universe in the extremely far future, after perhaps 101500 years.
The premise behind iron stars states that cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into iron-56 nuclei. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting stellar-mass objects to cold spheres of iron. The formation of these stars is only a possibility if the proton does not decay. Though the surface of a neutron star may be iron, according to some predictions, it is distinct from an iron star.
Unrelatedly, the term is also used for blue supergiants which have a forest of forbidden FeII lines in their spectra. They are potentially quiescent hot luminous blue variables. Eta Carinae has been described as a prototypical example.