{{Short description|Astronomical category of stars}} {{about|a category of stars|iron cores of white dwarf stars|white dwarf#Iron core}} In astronomy, the term '''iron star''' has been used for two observed types of star: * Binary stars with highly variable emission and stellar spectral type Fe * a blue supergiant with a forest of forbidden Fe<sub>II</sub> lines in its spectrum.

== XX Oph and AS 352 == The term iron star was applied to a star called "XX Oph" or "Merrill's Iron Star" with a complex, highly variable spectrum with a forest of Fe II emission lines. First discovered in 1908 by Williamina Fleming, it was studied over 30 years by Paul W. Merrill. The star is believed to be a non-eclipsing binary star.<ref>{{Cite journal |last1=Howell |first1=Steve B. |last2=Kalari |first2=Venu M. |last3=Adamson |first3=Andy |last4=Everett |first4=Mark |date=July 1, 2025 |title=Inside the Iron Curtain: A Long Term Look at the Iron Stars XX Oph and AS 325 |journal=Publications of the Astronomical Society of the Pacific |volume=137 |issue=7 |pages=074205 |doi=10.1088/1538-3873/adeec6 |issn=0004-6280|doi-access=free }}</ref>

A second binary star, AS 325, has a similar spectrum and complex spectral variability and has been characterized as the second iron star.<ref>{{Cite journal |last1=Bopp |first1=Bernard W. |last2=Howell |first2=Steve B. |date=November 1989 |title=AS 325 - Another 'iron star'? |journal=Publications of the Astronomical Society of the Pacific |language=en |volume=101 |pages=981 |doi=10.1086/132562 |issn=0004-6280}}</ref>

==Blue supergiant== Other sources characterize an iron star as a type of blue supergiant which has a forest of forbidden Fe<sub>II</sub> lines in its spectrum. They are potentially quiescent hot luminous blue variables. Eta Carinae has been described as a prototypical example.<ref name=walborn>{{cite journal|bibcode=2000PASP..112...50W|title=The OB Zoo: A Digital Atlas of Peculiar Spectra|journal=The Publications of the Astronomical Society of the Pacific|volume=112|issue=767|pages=50|last1=Walborn|first1=Nolan R.|author1-link=Nolan R. Walborn|last2=Fitzpatrick|first2=Edward L.|year=2000|doi=10.1086/316490|doi-access=free}}</ref><ref name="clark">{{cite journal|doi=10.1051/0004-6361/201118440|title=On the nature of candidate luminous blue variables in M 33|journal=Astronomy & Astrophysics|volume=541|pages=A146|year=2012|last1=Clark|first1=J. S.|last2=Castro|first2=N.|last3=Garcia|first3=M.|last4=Herrero|first4=A.|last5=Najarro|first5=F.|last6=Negueruela|first6=I.|last7=Ritchie|first7=B. W.|last8=Smith|first8=K. T.|arxiv = 1202.4409 |bibcode = 2012A&A...541A.146C |s2cid=17900583}}</ref>

==In popular culture== * 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.{{cn|date=March 2026}}

* In the ''Wrath of the Gods (mod)'' add-on for the ''Calamity Mod'' (a mod for the video game ''Terraria''), the Avatar of Emptiness boss utilizes an attack where it summons and compresses an iron star to the point of it cracking, causing beams of energy to discharge from the fissures toward the player. The continued compression causes the iron star to eventually explode into damaging debris.{{cn|date=March 2026}}

==Iron black dwarf== An '''iron black dwarf''' is a hypothetical iron core black dwarf would only occur in the very distant future on the order of 10<sup>1500</sup> years. According to modeling calculations, higher mass black dwarf stars would supernova, becoming the last notable astronomical activity in the universe.<ref name=caplan2020>{{cite journal |last1=Caplan |first1=M E |title=Black dwarf supernova in the far future |journal=Monthly Notices of the Royal Astronomical Society |date=1 October 2020 |volume=497 |issue=4 |pages=4357–4362 |doi=10.1093/mnras/staa2262 |doi-access=free |arxiv=2008.02296 }}</ref>

==See also== * Future of an expanding universe * Hypothetical star * Heat death of the universe

== References ==

{{reflist | 30em}}

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