ι Orionis
Orion constellation map.svg
Red circle.svg
Location of ι Ori (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Orion
Right ascension 05h 35m 25.98191s[1]
Declination –05° 54′ 35.6435″[1]
Apparent magnitude (V) 2.77[2]
ι Orionis A
Spectral type O9 III + B0.8 III/IV[3] + B2:IV:[4]
U−B color index –1.08[2]
B−V color index –0.24[2]
ι Orionis B
Spectral type B8 III[5]
Variable type Orion[6]
ι Orionis A
Radial velocity (Rv)21.5[7] km/s
Proper motion (μ) RA: +1.42[1] mas/yr
Dec.: –0.46[1] mas/yr
Parallax (π)1.40 ± 0.22 mas[1]
[4] pc
Primaryι Orionis Aa1
Companionι Orionis Aa2
Period (P)29.1338 days
Semi-major axis (a)132 R
Eccentricity (e)0.764
Inclination (i)~60°
Periastron epoch (T)2,450,072.80 HJD
ι Ori Aa1
Mass23.1[8] M
Radius8.3[8] R
Luminosity68,000[8] L
Surface gravity (log g)3.73[3] cgs
Temperature32,500[3] K
Metallicity [Fe/H]+0.10[9] dex
Rotational velocity (v sin i)122[10] km/s
Age4.0–5.5[3] Myr
ι Ori Aa2
Mass13.1[8] M
Radius5.4[8] R
Luminosity8,630[8] L
Surface gravity (log g)3.78[3] cgs
Temperature27,000[3] K
Age9.4 ± 1.5[3] Myr
ι Ori B
Mass5.12[11] M
Surface gravity (log g)4.0[9] cgs
Temperature18,000[9] K
Age~3[9] Myr
Other designations
Hatysa, ι Orionis, Na’ir al Saif, Hatsya, BD−06°1241, FK5 209, SAO 132323, ADS 4193, WDS J05354-0555
ι Ori A: 44 Orionis, HD 37043, HIP 26241, HR 1899, 2MASS J05352597-0554357
ι Ori B: V2451 Ori, 2MASS J05352645-0554445
ι Ori C: 2MASS J05352920-0554471
Database references
ι Ori B
ι Ori C

Iota Orionis (ι Orionis, abbreviated ι Ori) is a multiple star system in the equatorial constellation of Orion the hunter. It is the eighth-brightest member of Orion with an apparent visual magnitude of 2.77 and also the brightest member of the asterism known as Orion's Sword. It is a member of the NGC 1980 open cluster. From parallax measurements, it is located at a distance of roughly 1,340 light-years (412 parsecs) from the Sun.

The system has three visible components designated Iota Orionis A, B and C. Iota Orionis A has also been resolved using speckle interferometry and is also a massive spectroscopic binary, with components Iota Orionis Aa1 (officially named Hatysa /hɑːˈtsə/), Aa2, and Ab.


ι Orionis (Latinised to Iota Orionis) is the system's Bayer designation. The designations of the three constituents as Iota Orionis A, B and C, and those of A's components - Iota Orionis Aa1, Aa2, and Ab - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[12]

The system has the traditional name Nair al Saif, from the Arabic نير السيف nayyir as-sayf "the Bright One of the Sword", though this is little used.[13][14][15] Since Bečvář's 1951 Atlas Coeli, it has borne the proper name Hatysa. Kunitzsch was unable to find an older source for the latter name.[16]

In 2016, the IAU organized a Working Group on Star Names (WGSN)[17] to catalog and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[18] It approved the name Hatysa for the component Iota Orionis Aa on 5 September 2017 and it is now so included in the List of IAU-approved Star Names.[19]

Iota Orionis B is a variable star and in 2011 it was given the variable star designation V2451 Orionis.[6]


ι Orionis is the bright star to the right (south) of the Orion Nebula
ι Orionis is the bright star to the right (south) of the Orion Nebula

Iota Orionis has a parallax of 1.40±0.22 mas in the Hipparcos new reduction,[1] indicating a distance around 700 pc. The previous published Hipparcos parallax was 2.46±0.77 mas, suggesting a closer distance.[20] Gaia Data Release 2 has individual parallaxes for the two fainter components of the Iota Orionis star system of 2.3839±0.0810 mas and 2.5321±0.0484 mas,[21][22] indicating distances of 419 pc and 395 pc respectively, with margins of error of just a few parsecs. There is little doubt that all three stars are at the same distance.[3][8]

Iota Orionis is generally assumed to be associated with the open cluster NGC 1980, which is at a distance of around 400 pc. However, they may not lie at exactly the same distance and Iota Orionis may have a complex history involving stellar encounters and runaway stars.[23] NGC 1980 contains few bright stars other than Iota Orionis. Only eighteen other stars are considered members in a survey down to 14th magnitude, most of them around 9th magnitude but including the 5th magnitude stars HR 1886 and 1887.[24]


Iota Orionis is dominated by the multiple star Iota Orionis A. It is clearly identified as a double-lined spectroscopic binary whose components are a stellar class O9 III star (blue giant) and a class B0.8 III/IV star about 2 magnitudes fainter.[3] The combined spectral type has long been accepted as O9 III and it was listed as a standard star for that type.[25] The collision of the stellar winds from this pair makes the system a strong X-ray source. Oddly, the two objects of this system appear to have different ages, with the secondary being about double the age of the primary. In combination with the high eccentricity (e=0.764) of their 29-day orbit, this suggests that the binary system was created through a capture, rather than by being formed together and undergoing a mass transfer. This capture may have occurred, for example, through an encounter between two binary systems, with one star being donated from each binary and two runaway stars being ejected.[3][26] A third component 155 mas away has been identified using speckle interferometry and is probably a B2 subgiant.[4][11]

The primary component of Iota Orionis A is a class O giant star with a mass of about 23 M. It has a surface temperature of 32,500 K and radius of 8.3 R, resulting in a bolometric luminosity of 68,000 L. It is calculated to be around nine million years old. The secondary star of the spectroscopic binary pair is a class B giant or subgiant with a mass of about 13 M. It has a temperature of 27,000 K and radius of 5.4 R, resulting in it radiating over 8,000 times as much energy as the sun.

Iota Orionis B is a B8 giant at 11" (approximately 5,000 AU[9]) which has been shown to be variable, and likely to be a young stellar object.[5] It is also a helium-weak chemically peculiar star.[27] The fainter Iota Orionis C is an A0 star at 49".[28]


  1. ^ a b c d e f van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653–664. arXiv:0708.1752. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. S2CID 18759600.
  2. ^ a b c Nicolet, B. (1978). "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System". Astronomy and Astrophysics Supplement Series. 34: 1–49. Bibcode:1978A&AS...34....1N.
  3. ^ a b c d e f g h i j k Bagnuolo, William G., Jr.; et al. (June 2001). "ι Orionis-Evidence for a Capture Origin Binary". The Astrophysical Journal. 554 (1): 362–367. Bibcode:2001ApJ...554..362B. doi:10.1086/321367.
  4. ^ a b c Maíz Apellániz, J.; Barbá, R. H. (2020). "Spatially resolved spectroscopy of close massive visual binaries with HST/STIS". Astronomy & Astrophysics. 636: A28. arXiv:2002.12149. doi:10.1051/0004-6361/202037730. S2CID 211532687.
  5. ^ a b Abt, Helmut A. (2008). "Visual Multiples. IX. MK Spectral Types". The Astrophysical Journal Supplement Series. 176 (1): 216–217. Bibcode:2008ApJS..176..216A. doi:10.1086/525529.
  6. ^ a b Kazarovets, E. V.; Samus, N. N.; Durlevich, O. V.; Kireeva, N. N.; Pastukhova, E. N. (2011). "The 80th Name-List of Variable Stars. Part I - RA 0h to 6h". Information Bulletin on Variable Stars. 5969: 1. Bibcode:2011IBVS.5969....1K.
  7. ^ Evans, D. S. (June 20–24, 1966). Batten, Alan Henry; Heard, John Frederick (eds.). "The Revision of the General Catalogue of Radial Velocities". Determination of Radial Velocities and Their Applications, Proceedings from IAU Symposium No. 30. University of Toronto: International Astronomical Union. 30: 57. Bibcode:1967IAUS...30...57E.
  8. ^ a b c d e f g h Marchenko, Sergey V.; Rauw, Gregor; Antokhina, Eleonora A.; Antokhin, Igor I.; Ballereau, Dominique; Chauville, Jacques; Corcoran, Michael F.; Costero, Rafael; Echevarria, Juan; Eversberg, Thomas; Gayley, Ken G.; Koenigsberger, Gloria; Miroshnichenko, Anatoly S.; Moffat, Anthony F. J.; Morrell, Nidia I.; Morrison, Nancy D.; Mulliss, Christopher L.; Pittard, Julian M.; Stevens, Ian R.; Vreux, Jean-Marie; Zorec, Jean (2000). "Coordinated monitoring of the eccentric O-star binary Iota Orionis: Optical spectroscopy and photometry". Monthly Notices of the Royal Astronomical Society. 317 (2): 333. Bibcode:2000MNRAS.317..333M. doi:10.1046/j.1365-8711.2000.03542.x.
  9. ^ a b c d e Conti, P. S.; Loonen, J. P. (1970). "Coarse analysis of the helium weak B star Iota Ori B". Astronomy and Astrophysics. 8: 197. Bibcode:1970A&A.....8..197C.
  10. ^ Uesugi, Akira; Fukuda, Ichiro (1970). "Catalogue of rotational velocities of the stars". Contributions from the Institute of Astrophysics and Kwasan Observatory. University of Kyoto. Bibcode:1970crvs.book.....U.
  11. ^ a b "Multiple Star Catalog". Retrieved 2021-01-17.
  12. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707 [astro-ph.SR].
  13. ^ Allen, Richard Hinckley (1899). Star-names and their meanings. G. E. Stechert. p. 317.
  14. ^ Hoffleit, D. "Bright Star Catalogue, 5th Revised Ed. (note)". VizieR. Centre de Données astronomiques de Strasbourg. Retrieved 28 October 2018.
  15. ^ Bakich, Michael E. (1995). The Cambridge Guide to the Constellations. Cambridge University Press. p. 120. ISBN 0521449219.
  16. ^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern Star Names. Sky Publishing. p. 62. ISBN 1931559449.
  17. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016.
  18. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  19. ^ "Naming Stars". IAU.org. Retrieved 16 December 2017.
  20. ^ Perryman, M.A.C.; et al. (1997). "The Hipparcos Catalogue". Astronomy & Astrophysics. 323: L49–L52. Bibcode:1997A&A...323L..49P.
  21. ^ 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.
  22. ^ 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.
  23. ^ Kounkel, Marina; Hartmann, Lee; Calvet, Nuria; Megeath, Tom (2017). "Characterizing the Stellar Population of NGC 1980". The Astronomical Journal. 154 (1): 29. arXiv:1705.07922. Bibcode:2017AJ....154...29K. doi:10.3847/1538-3881/aa74df. S2CID 119428410.
  24. ^ Kharchenko, N. V.; Piskunov, A. E.; Röser, S.; Schilbach, E.; Scholz, R.-D. (2004). "Astrophysical supplements to the ASCC-2.5. II. Membership probabilities in 520 Galactic open cluster sky areas". Astronomische Nachrichten. 325 (9): 740. Bibcode:2004AN....325..740K. doi:10.1002/asna.200410256.
  25. ^ Morgan, W. W.; Abt, Helmut A.; Tapscott, J. W. (1978). Revised MK Spectral Atlas for stars earlier than the sun. Bibcode:1978rmsa.book.....M.
  26. ^ Hoogerwerf, R.; De Bruijne, J. H. J.; De Zeeuw, P. T. (2001). "On the origin of the O and B-type stars with high velocities". Astronomy & Astrophysics. 365 (2): 49–77. doi:10.1051/0004-6361:20000014.
  27. ^ Renson, P.; Manfroid, J. (2009). "Catalogue of Ap, HGMN and Am stars". Astronomy and Astrophysics. 498 (3): 961. Bibcode:2009A&A...498..961R. doi:10.1051/0004-6361/200810788.
  28. ^ Parenago, P. P. (1954). "Untersuchung der Sterne im Gebiet des Orion-Nebels. Tabelle III: Katalog der genauen Positionen. (Bestimmung von photographischen Beobachtungen)". Publ. Astr. Inst. Sternberg. 25: 393. Bibcode:1954TrSht..25....1P.