U Orionis

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U Orionis
Gemini constellation map.svg
Red circle.svg

Location of U Orionis (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Orion
Right ascension 05h 55m 49.1689s
Declination +20° 10′ 30.687″
Apparent magnitude (V) 4.8 - 13.0
Spectral type M8III
B−V color index 1.26
Variable type Mira-type
Parallax (π) 2.29 ± 1.21 mas
Distance 997.56 ly
(306±61 [1] pc)
Radius 370 ± 96[2] R
Luminosity 7,000[3] L
Temperature ≈2,750[3] K
Other designations
U Ori, HIP 28041, HD 39816, HR 2063, BD+20 1171a, RAFGL 837, SAO 77730
Database references

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 from the United Kingdom for over 120 years.


U Orionis in the center of the image at approx visual mag. 12 on February 5th 2017

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.[4]


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[edit]

The star has a low effective temperature (roughly 2700 K) but a large and bloated radius of 370 solar radii[2] and a high luminosity, 7,000 times higher than the Sun.[3] If the Sun were replaced with U Orionis, its radius would extend beyond Mars's orbital zone (about 1.7 astronomical units), and, to be habitable with water at liquid state and a comfortable temperature, a planet would have to be located within 85 AU, within the Kuiper belt's orbital zone.[citation needed]

Hints of a planetary system?[edit]

According to Rudnitskij,[5] 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.