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χ² Orionis
	Location of χ2 Orionis (circled)
Observation data; Epoch J2000.0 Equinox J2000.0 (ICRS)
Constellation	Orion
Right ascension	06h 03m 55.18437s
Declination	+20° 08′ 18.4281″
Apparent magnitude (V)	4.63
Characteristics
Evolutionary stage	Blue supergiant
Spectral type	B2Ia
U−B color index	−0.68
B−V color index	+0.28
R−I color index	+0.22
Variable type	α Cyg
Astrometry
Radial velocity (Rv)	16.8±0.9 km/s
Proper motion (μ)	RA: 2.679 mas/yr ; Dec.: −2.500 mas/yr
Parallax (π)	0.7636 ± 0.1219 mas
Distance	approx. 4,300 ly ; (approx. 1,300 pc)
Absolute magnitude (MV)	−7.6
Details
Mass	42.3±1.0 M☉
Radius	61.9 R☉
Luminosity	446,000 L☉
Surface gravity (log g)	2.35 cgs
Temperature	19,000 K
Rotational velocity (v sin i)	72 km/s
Age	5±0.6 Myr
Other designations
	χ2 Ori, Chi2 Orionis, Chi2 Ori, 62 Orionis, 62 Ori, BD+20 1233, GC 7675, HD 41117, HIP 28716, HR 2135, PPM 95316, SAO 77911
Database references
SIMBAD	data

Chi² Orionis (Chi² Ori / χ² Orionis / χ² Ori) /kaɪ.tuːˈɒraɪ/ is a B-type blue supergiant star in the constellation of Orion. It has an apparent visual magnitude of 4.63 but being quite distant, and heavily extinguished it burns with the greatest absolute visual light magnitude among stars in Orion within the near reaches of the galaxy, 0.9 of a magnitude brighter than Rigel.^{[citation needed]} Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It is considered to be a member of the Gemini OB1 association.^[2]

In apparent brightness it ranks, within Orion, admitting the higher published mean brightness of much more variable stars ranked above it, 35th.^{[citation needed]}

Chi¹ Orionis is an unrelated, yellow, main sequence star over two degrees away and far closer to the Sun.

Spectrum

χ² Orionis has a B2 bright supergiant spectrum and is one of the standard B2 Ia stars.^[7] It has been reported as having unusually narrow absorption lines and some weak emission lines and was included as one of the original Be stars.^[8] It is no longer treated as a Be star since many supergiants show some emission features at high resolution and Be stars is usually defined to exclude supergiants.^[9]

Variability

A light curve for Chi² Orionis, plotted from *Hipparcos* data^[10]

χ² Orionis was listed as having likely small amplitude variability in photometry for the Third Catalogue of Stars measured in the Geneva Observatory Photometric System, specifically varying by 22 thousandths of a magnitude.^[11] It was included in the General Catalogue of Variable Stars based on Hipparcos satellite photometry with a magnitude range (in the Hipparchos photometric system) of 4.68 - 4.72 and a period of 2.8 days.^[12] A detailed study of the Hipparcos photometry confirmed the star as an α Cyg variable and gave the amplitude of variation as 0.057 magnitudes.^[13]

References

^ ^a ^b ^c ^d ^e Vallenari, A.; et al. (Gaia collaboration) (2023). "Gaia Data Release 3. Summary of the content and survey properties". Astronomy and Astrophysics. 674: A1. arXiv:2208.00211. Bibcode:2023A&A...674A...1G. doi:10.1051/0004-6361/202243940. S2CID 244398875. Gaia DR3 record for this source at VizieR.
^ ^a ^b ^c ^d ^e ^f ^g ^h ⁱ Crowther, P. A.; Lennon, D. J.; Walborn, N. R. (2006). "Physical parameters and wind properties of galactic early B supergiants". Astronomy and Astrophysics. 446 (1): 279–293. arXiv:astro-ph/0509436. Bibcode:2006A&A...446..279C. doi:10.1051/0004-6361:20053685. S2CID 18815761.
^ ^a ^b ^c ^d HR 2135, database entry, The Bright Star Catalogue, 5th Revised Ed. (Preliminary Version), D. Hoffleit and W. H. Warren, Jr., CDS ID V/50. Accessed on line January 14, 2011.
^ khi 2 Ori, database entry, The combined table of GCVS Vols I-III and NL 67-78 with improved coordinates, General Catalogue of Variable Stars Archived 2017-06-20 at the Wayback Machine, Sternberg Astronomical Institute, Moscow, Russia. Accessed on line January 14, 2011.
^ Wilson, Ralph Elmer (1953). "General catalogue of stellar radial velocities". Carnegie Institute Washington D.C. Publication. Carnegie Institution of Washington. Bibcode:1953GCRV..C......0W.
^ ^a ^b Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (2011). "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun". Monthly Notices of the Royal Astronomical Society. 410 (1): 190–200. arXiv:1007.4883. Bibcode:2011MNRAS.410..190T. doi:10.1111/j.1365-2966.2010.17434.x. S2CID 118629873.
^ Garrison, R. F. (December 1993), "Anchor Points for the MK System of Spectral Classification", Bulletin of the American Astronomical Society, 25: 1319, Bibcode:1993AAS...183.1710G, archived from the original on 2019-06-25, retrieved 2012-02-04
^ Merrill, P. W.; Humason, M. L.; Burwell, C. G. (1925). "Discovery and Observations of Stars of Class Be". Astrophysical Journal. 61: 389. Bibcode:1925ApJ....61..389M. doi:10.1086/142899.
^ Andrillat, Y.; Jaschek, M.; Jaschek, C. (1994). "A study of Be stars in the wavelength region around Paschen 7". Astronomy and Astrophysics Supplement Series. 103: 135. Bibcode:1994A&AS..103..135A.
^ "Light Curve". Hipparcos ESA. ESA. Retrieved 11 August 2022.
^ Rufener, F.; Bartholdi, P. (1982). "List of 333 variable, microvariable or suspected variable stars detected in the Geneva photometry". Astronomy and Astrophysics Supplement Series. 48: 503. Bibcode:1982A&AS...48..503R.
^ Kazarovets, E. V.; Samus, N. N.; Durlevich, O. V.; Frolov, M. S.; Antipin, S. V.; Kireeva, N. N.; Pastukhova, E. N. (1999). "The 74th Special Name-list of Variable Stars". Information Bulletin on Variable Stars. 4659: 1. Bibcode:1999IBVS.4659....1K.
^ Lefèvre, L.; Marchenko, S. V.; Moffat, A. F. J.; Acker, A. (2009). "A systematic study of variability among OB-stars based on HIPPARCOS photometry". Astronomy and Astrophysics. 507 (2): 1141. Bibcode:2009A&A...507.1141L. doi:10.1051/0004-6361/200912304.