U Orionis
{{Short description|Variable star in the constellation Orion}}
{{Starbox begin
|name =U Orionis
}}
{{Starbox image
| image=
{{Location mark
|image=Gemini constellation map.svg|alt=|float=center|width=300
|label=|position=right
|mark=Red circle.svg|mark_width=8|mark_link=U Ori
|x=916|y=420
}}
|caption=Location of U Orionis (circled)
}}
{{Starbox observe
| epoch = J2000.0
| constell = Orion
}}
{{Starbox character
| r-i =
| v-r =
| u-b =
| variable = Mira{{cite journal|bibcode=2017ARep...61...80S|title=General catalogue of variable stars: Version GCVS 5.1|journal=Astronomy Reports|volume=61|issue=1|pages=80|last1=Samus'|first1=N. N.|last2=Kazarovets|first2=E. V.|last3=Durlevich|first3=O. V.|last4=Kireeva|first4=N. N.|last5=Pastukhova|first5=E. N.|year=2017|doi=10.1134/S1063772917010085|s2cid=125853869}}
}}
{{Starbox astrometry
| parallax = 3.4924
| p_error = 0.1820
| parallax_footnote = {{cite Gaia EDR3|3399013173616323200}}
| dist_ly =
| dist_pc =
}}
{{Starbox detail
| radius = {{val|370|96}}{{cite journal | author=Van Belle| display-authors=etal| title=Angular Size Measurements of 18 Mira Variable Stars at 2.2 microns| journal=Astronomical Journal | volume=112| pages=2147 | date=1996 | bibcode=1996AJ....112.2147V | doi=10.1086/118170| doi-access=free}}
| luminosity = 7,000{{cite journal | author=Mondal | author2=Chandrasekhar | name-list-style=amp| title=Evidence of asymmetry in Mira variable U Ori| journal=Monthly Notices of the Royal Astronomical Society | volume=348| issue=4| pages=1332–1336 | date=2004 | doi=10.1111/j.1365-2966.2004.07454.x | bibcode=2004MNRAS.348.1332M| doi-access=free }}
}}
{{Starbox catalog
| names = {{odlist|V=U Ori|HIP=28041|HD=39816|HR=2063|BD=+20°1171a|SAO=77730}}{{cite simbad|title=V* U Ori|access-date=25 January 2018}}
}}
{{Starbox reference
| Simbad = U+Ori
}}
{{Starbox end}}
U Orionis (abbreviated U Ori) is a Mira-type variable star in the constellation Orion. It is a classical long period variable star that has been well observed for over 120 years.
Discovery
File:Variable star U Orionis in the constellation Orion.jpg
It was discovered on 1885 December 13 by J.E. Gore and initially it was thought to be a nova in the early stages of decline (Gore's Nova and NOVA Ori 1885 as still listed in SIMBAD), but a spectrum taken at Harvard showed features similar to that of Mira. Thus U Orionis became the first long period variable to be identified by a photograph of its spectrum.{{cite journal | author=Monck | title=Mr Gore's Nova Orionis | journal=The Observatory | volume=10| pages=69–71 | date=1887 | bibcode=1887Obs....10...69M}}
Location
U Orionis lies less than half a degree east of the small-amplitude variable star χ1 Orionis and less than an arc-minute from the much fainter eclipsing variable UW Orionis. χ1 Orionis is slightly brighter than U Orionis at its brightest maximum, while UW Orionis is more than a thousand times fainter, similar to U Orionis at minimum.
Stellar parameters
File:UOriLightCurve.png light curve of U Orionis, from AAVSO data]]
The star has a low effective temperature (variable with the pulsations, but roughly 2,700 K), a large and bloated radius of {{solar radius|370|link=y}}, and a high luminosity, 7,000 times higher than the Sun. If the Sun were replaced with U Orionis, its radius would extend beyond Mars's orbital zone (about 1.7 astronomical units).
Possible planetary system
According to Rudnitskij,{{cite journal | author=Rudnitskij | title=Molecular Masers in Variable Stars | journal=Publications of the Astronomical Society of Australia | volume=19| issue=4| pages=499–504 | date=2002| bibcode=2002PASA...19..499R | doi=10.1071/AS02018 | doi-access=free }} a 12- to 15-year "super-periodicity" has been observed. The author infers such periodicity could coincide with the revolution period of an invisible companion, probably planetary. So far no clear hint of planetary objects has been detected.
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