In astronomy, the term iron star has been used for two unrelated types of star:

• a blue supergiant with a forest of forbidden Fe_II lines in its spectrum.
• a hypothetical type of compact star.

An iron star is a type of blue supergiant which has a forest of forbidden Fe_II lines in its spectrum. They are potentially quiescent hot luminous blue variables. Eta Carinae has been described as a prototypical example.^[1][2]

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 the formation of iron stars states that cold fusion occurring via quantum tunnelling would cause the light nuclei in ordinary matter to fuse into iron-56 nuclei, due to its nature as the atomic nucleus with the highest binding energy. Fission and alpha-particle emission would then make heavy nuclei decay into iron, converting stellar-mass objects to cold spheres of iron.^[3] The formation of these stars is only a possibility if protons do not decay. Though the surface of a neutron star may be iron according to some predictions, it is distinct from an iron star.

By the end of 10¹⁰²⁶ to 10¹⁰⁷⁶ years, iron stars would have collapsed into neutron stars and black holes.^[3]

• The Soviet film The Andromeda Nebula is about a starship low on fuel caught by an iron star's gravity, with the star itself being so dim that it can only be seen in the infrared. It is based on the novel Andromeda Nebula by Ivan Yefremov written when steady state theory was dominant and iron stars were expected to exist in the Milky Way.

• Future of an expanding universe
• Hypothetical star
• Heat death of the universe