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Ngc2024 2mass.jpg
Alnitak (in lower right corner) and Flame Nebula
Observation data
Epoch J2000      Equinox J2000
Constellation Orion
Right ascension 05h 40m 45.52666s[1]
Declination −01° 56′ 34.2649″[1]
Apparent magnitude (V) 1.77[2] (2.08 + 4.28 + 4.01[3])
Spectral type O9.5Iab + B1IV + B0III[3]
U−B color index −1.06[2]
B−V color index −0.11 (A)
−0.20 (B)[4]
Radial velocity (Rv) 18.50[5] km/s
Proper motion (μ) RA: 3.19[1] mas/yr
Dec.: 2.03[1] mas/yr
Parallax (π) 4.43 ± 0.64[1] mas
Distance 1,260 ± 180 ly
(387 ± 54[3] pc)
Absolute magnitude (MV) −6.0 / −3.9 / −4.1[3]
Primary Aa
Companion Ab
Period (P) 2687.3 ± 7.0 d
Semi-major axis (a) 35.9 ± 0.2 m
Eccentricity (e) 0.338 ± 0.004
Inclination (i) 139.3 ± 0.6°
Longitude of the node (Ω) 83.8 ± 0.8°
Periastron epoch (T) JD 2452734.2 ± 9.0
Argument of periastron (ω)
204.2 ± 1.2°
Semi-amplitude (K1)
10.1 km/s
Semi-amplitude (K2)
19.6 km/s
Primary A
Companion B
Period (P) 1508.6 yr
Semi-major axis (a) 2.728″
Eccentricity (e) 0.07
Inclination (i) 72.0°
Longitude of the node (Ω) 155.5°
Periastron epoch (T) 2070.6
Argument of periastron (ω)
Alnitak Aa
Mass 33 ± 10[3] M
Radius 20.0 ± 3.2[3] R
Luminosity 250,000[3] L
Surface gravity (log g) 3.2 ± 0.1[7] cgs
Temperature 29,500 ± 1000[3] K
Rotation 6.67 days[8]
Rotational velocity (v sin i) 110 ± 10[3] km/s
Age 6.4[3] Myr
Alnitak Ab
Mass 14 ± 3[3] M
Radius 7.3 ± 1.0[3] R
Luminosity 32,000[3] L
Temperature 29,000[9] K
Age 7.2[3] Myr
Alnitak B
Rotational velocity (v sin i) 350[3] km/s
Age ~7[3] Myr
Other designations
ζ Orionis, 50 Orionis, 126 G. Orionis, BD−02°1338, SAO 132444, HIP 26727, TD1 5127, WDS J05407-0157
A: HR 1948, HD 37742
B: HR 1949, HD 37743
Database references

Alnitak, designated Zeta Orionis (ζ Orionis, abbreviated Zeta Ori, ζ Ori) and 50 Orionis (50 Ori), is a multiple star several hundred parsecs from the Sun in the constellation of Orion. It is part of Orion's Belt along with Alnilam and Mintaka.

The primary star is a hot blue supergiant with an absolute magnitude of -6.0 and is the brightest class O star in the night sky with a visual magnitude of +2.0. It has two bluish 4th magnitude companions, one finely resolved and one only detected interferometrically and spectroscopically, producing a combined magnitude for the trio of +1.77. The stars are members of the Orion OB1 association and the Collinder 70 association.

Observation history[edit]

Orion's Belt with Alnitak on the left

Alnitak has been known since antiquity and, as a component of Orion's belt, has been of widespread cultural significance. It was reported to be a double star by amateur German astronomer George K. Kunowsky in 1819.[10] Much more recently, in 1998, the bright primary was found by a team from the Lowell Observatory to have a close companion; this had been suspected from observations made with the Narrabri Stellar Intensity Interferometer in the 1970s.[11]

The stellar parallax derived from observations by the Hipparcos satellite imply a distance around 225 parsecs, but this does not take into account distortions caused by the multiple nature of the system and larger distances have been derived by many authors.[3]


Alnitak Aa compared to the Sun (to scale)

Alnitak is a binary star system at the eastern end of Orion's belt, the second magnitude primary having a 4th magnitude companion nearly 3 arc-seconds distant, in an orbit taking over 1,500 years.

The primary (Alnitak A) is itself a close binary, comprising Alnitak Aa (a blue supergiant of spectral type O9.5Iab with an absolute magnitude of -6.0 and an apparent magnitude of 2.0) and Alnitak Ab (a blue sub-giant of spectral type B1IV with an absolute magnitude of -3.9 and an apparent magnitude of 4.3, discovered in 1998.[11]). Aa is estimated as being up to 33 times as massive as the Sun and to have a diameter 20 times greater. It is some 21,000 times brighter than the sun, with a surface brightness (luminance) some 500 times greater. It is the brightest star of class O in the night sky.

A fourth star, 9th magnitude Alnitak C, has not been confirmed to be part of the Aa-Ab-B group, and may simply lie along the line of sight.

The Alnitak system is bathed in the nebulosity of IC 434.

Etymology and cultural significance[edit]

ζ Orionis (Latinised as Zeta Orionis) is the star's Bayer designation and 50 Orionis its Flamsteed designation.

The traditional name Alnitak, alternately spelled Al Nitak or Alnitah, is taken from the Arabic النطاق an-niṭāq, "the girdle".[10] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[12] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[13] included a table of the first two batches of names approved by the WGSN; which included Alnitak for this star. It is now so entered in the IAU Catalog of Star Names.[14]

Orion's belt[edit]

The three belt stars were collectively known by many names in many cultures. Arabic terms include النجاد Al Nijād 'the Belt', النسك Al Nasak 'the Line', العلقات Al Alkāt 'the Golden Grains or Nuts' and, in modern Arabic, ميزان الحق Al Mīzān al Ḥaqq 'the Accurate Scale Beam'. In Chinese mythology they were known as The Weighing Beam.[10]

The belt was also the Three Stars mansion (simplified Chinese: 参宿; traditional Chinese: 參宿; pinyin: Shēn Xiù), one of the Twenty-eight mansions of the Chinese constellations. It is one of the western mansions of the White Tiger.


The USS Alnitah was a United States Navy Crater-class cargo ship named after the star.

See also[edit]


  1. ^ a b c d e Van Leeuwen, F. (2007). "Validation of the new Hipparcos reduction". Astronomy and Astrophysics. 474 (2): 653. arXiv:0708.1752Freely accessible. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  2. ^ a b Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237: 0. Bibcode:2002yCat.2237....0D. 
  3. ^ a b c d e f g h i j k l m n o p q r Hummel, C. A.; Rivinius, T.; Nieva, M. -F.; Stahl, O.; Van Belle, G.; Zavala, R. T. (2013). "Dynamical mass of the O-type supergiant in ζ Orionis A". Astronomy & Astrophysics. 554: A52. arXiv:1306.0330Freely accessible. Bibcode:2013A&A...554A..52H. doi:10.1051/0004-6361/201321434. 
  4. ^ Fabricius, C.; Høg, E.; Makarov, V. V.; Mason, B. D.; Wycoff, G. L.; Urban, S. E. (2002). "The Tycho double star catalogue". Astronomy and Astrophysics. 384 (1): 180–189. Bibcode:2002A&A...384..180F. doi:10.1051/0004-6361:20011822. ISSN 0004-6361. 
  5. ^ Kharchenko, N.V.; Scholz, R.-D.; Piskunov, A.E.; Röser, S.; Schilbach, E. (2007). "Astrophysical supplements to the ASCC-2.5: Ia. Radial velocities of ∼55000 stars and mean radial velocities of 516 Galactic open clusters and associations". Astronomische Nachrichten. 328 (9): 889–896. arXiv:0705.0878Freely accessible. Bibcode:2007AN....328..889K. doi:10.1002/asna.200710776. ISSN 0004-6337. 
  6. ^ "Sixth Catalog of Orbits of Visual Binary Stars". United States Naval Observatory. Retrieved 15 May 2017. 
  7. ^ Raassen, A. J. J.; Van Der Hucht, K. A.; Miller, N. A.; Cassinelli, J. P. (2008). "XMM-Newton observations of ζ Orionis (O9.7 Ib): A collisional ionization equilibrium model". Astronomy and Astrophysics. 478 (2): 513. arXiv:0803.0873Freely accessible. Bibcode:2008A&A...478..513R. doi:10.1051/0004-6361:20077891. 
  8. ^ Buysschaert, B.; Neiner, C.; Ramiaramanantsoa, T.; Richardson, N. D.; David-Uraz, A.; Moffat, A. F. J. (2016). "Understanding the photometric variability of ζ OriAa". arXiv:1610.05625Freely accessible [astro-ph.SR]. 
  9. ^ Blazère, A.; Neiner, C.; Tkachenko, A.; Bouret, J.-C.; Rivinius, Th. (2015). "The magnetic field of ζ Orionis A". Astronomy & Astrophysics. 582: A110. arXiv:1509.02773Freely accessible. Bibcode:2015A&A...582A.110B. doi:10.1051/0004-6361/201526855. 
  10. ^ a b c Richard Hinckley Allen, Star-names and their meanings (1936), p. 314-15.
  11. ^ a b Hummel CA; White NM; Elias NM II; Hajian AR; Nordgren TE (2000). "ζ Orionis A Is a Double Star". The Astrophysical Journal. 540 (2): L91–L93. Bibcode:2000ApJ...540L..91H. doi:10.1086/312882. 
  12. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016. 
  13. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016. 
  14. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016. 

External links[edit]

Coordinates: Sky map 05h 40m 45.5s, −01° 56′ 34″