31 Orionis

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]

• 
  A light curve for CI Orionis, plotted from Hipparcos data^[12]
• 
  Orion's Belt, with the orange 31 Orionis towards the west (right)

31 Orionis

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]
Characteristics
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]
Astrometry
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]
Distance	490 ± 20 ly (151 ± 5 pc)
Absolute magnitude (MV)	−1.23[2]
Details
31 Ori A
Mass	5.2[8] M☉
Radius	62[8] R☉
Luminosity	1,361[8] L☉
Surface gravity (log g)	0.92[8] cgs
Temperature	4,610[8] K
Metallicity [Fe/H]	−0.21[2] dex
Rotational velocity (v sin i)	2.7[9] km/s
31 Ori B
Mass	1.1[10] M☉
Radius	1.2[10] R☉
Luminosity	1.9[10] L☉
Surface gravity (log g)	4.24[10] cgs
Temperature	6,111[10] K
Metallicity [Fe/H]	−0.23[10] dex
Age	3.9[10] Gyr
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[11]
Database references
SIMBAD	data

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]