31 Orionis
Orion constellation map.svg
Red circle.svg
Location of 31 Orionis (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Orion
Right ascension 05h 29m 43.98147s[1]
Declination −01° 05′ 32.0582″[1]
Apparent magnitude (V) 4.71[2] + 10.2[3]
Spectral type K5III[4] + F7V[3]
U−B color index +1.91[5]
B−V color index +1.58[5]
Variable type SR?[6][7]
Radial velocity (Rv)+6.02±0.15[1] km/s
Proper motion (μ) RA: +1.242[1] mas/yr
Dec.: -25.476[1] mas/yr
Parallax (π)6.6135 ± 0.2293 mas[1]
Distance490 ± 20 ly
(151 ± 5 pc)
Absolute magnitude (MV)−1.23[2]
31 Ori A
[1] R
Luminosity763.4±30.1[1] L
Surface gravity (log g)1.62[8] cgs
[1] K
Metallicity [Fe/H]−0.21[2] dex
Rotational velocity (v sin i)2.7[9] km/s
Other designations
31 Ori, CI Ori, BD−01°913, GC 6792, HD 36167, HIP 25737, HR 1834, SAO 132176, CCDM J05297-0106A, WDS J05297-0106A[10]
Database references

31 Orionis is a binary star[3] system in the equatorial constellation of Orion, located near the bright star Mintaka. It is visible to the naked eye as a faint, orange-hued point of light with a baseline apparent visual magnitude of 4.71.[2] The distance to this system is approximately 490 light years away based on parallax,[1] and it is drifting further away with a mean radial velocity of +6 km/s.[1]

As of 2008, the pair had an angular separation of 12.7.[3] The brighter member, designated component A, is an aging giant star with a stellar classification of K5III.[4] It is reported as a semi-regular variable with magnitude ranging from 4.68 to 4.72 over 141 days,[6] although the General Catalogue of Variable Stars describes this as unconfirmed by subsequent observations.[7] It has the variable star designation CI Orionis, while 31 Orionis is the Flamsteed designation. The magnitude 10.2 companion star, component B, is an F-type main-sequence star with a class of F7V.[3]


  1. ^ a b c d e f g h i j k Brown, A. G. A.; et al. (Gaia collaboration) (August 2018). "Gaia Data Release 2: Summary of the contents and survey properties". Astronomy & Astrophysics. 616. A1. arXiv:1804.09365. Bibcode:2018A&A...616A...1G. doi:10.1051/0004-6361/201833051. Gaia DR2 record for this source at VizieR.
  2. ^ a b c d Anderson, E.; Francis, Ch. (2012). "XHIP: An extended hipparcos compilation". Astronomy Letters. 38 (5): 331. arXiv:1108.4971. Bibcode:2012AstL...38..331A. doi:10.1134/S1063773712050015. S2CID 119257644. Vizier catalog entry
  3. ^ a b c d e Eggleton, P. P.; Tokovinin, A. A. (2008). "A catalogue of multiplicity among bright stellar systems". Monthly Notices of the Royal Astronomical Society. 389 (2): 869. arXiv:0806.2878. Bibcode:2008MNRAS.389..869E. doi:10.1111/j.1365-2966.2008.13596.x. S2CID 14878976. Vizier catalog entry
  4. ^ a b Hoffleit, D.; Warren, W. H. (1995). "VizieR Online Data Catalog: Bright Star Catalogue, 5th Revised Ed. (Hoffleit+, 1991)". VizieR On-line Data Catalog: V/50. Originally Published in: 1964BS....C......0H. 5050. Bibcode:1995yCat.5050....0H.
  5. ^ a b Mermilliod, J. C. (2006). "Homogeneous Means in the UBV System (Mermilliod 1991)". VizieR On-line Data Catalog: II/168. Originally Published in: Institut d'Astronomie. 2168. Bibcode:2006yCat.2168....0M.Vizier catalog entry
  6. ^ a b "CI Ori". The International Variable Star. AAVSO – American Association of Variable Star Observers. Retrieved 6 August 2016.
  7. ^ a b Samus, N. N.; Durlevich, O. V.; et al. (2009). "General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/gcvs. 1. Bibcode:2009yCat....1.2025S.
  8. ^ McWilliam, Andrew (December 1990). "High-resolution spectroscopic survey of 671 GK giants. I - Stellar atmosphere parameters and abundances". Astrophysical Journal Supplement Series. 74: 1075–1128. Bibcode:1990ApJS...74.1075M. doi:10.1086/191527.
  9. ^ De Medeiros, J. R.; Alves, S.; Udry, S.; Andersen, J.; Nordström, B.; Mayor, M. (2014). "A catalog of rotational and radial velocities for evolved stars". Astronomy & Astrophysics. 561: A126. arXiv:1312.3474. Bibcode:2014A&A...561A.126D. doi:10.1051/0004-6361/201220762. S2CID 54046583. Vizier catalog entry
  10. ^ "31 Ori". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2019-06-27.