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Zeta Orionis: Wikis


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Zeta Orionis Aa/Ab/B
Ngc2024 2mass.jpg
ζ Ori (in lower right corner) and Flame Nebula
Observation data
Epoch J2000      Equinox J2000
Constellation Orion
Right ascension 05h 40m 45.5s
Declination −01° 56′ 34″
Apparent magnitude (V) 1.70/~4/4.21
Spectral type O9 Iab/O9/B0 III
U-B color index −1.07
B-V color index 0.14 / ? / −0.01
Radial velocity (Rv) 18 km/s
Proper motion (μ) RA: 3.99 mas/yr
Dec.: 2.54 mas/yr
Parallax (π) 3.99 ± 0.79 mas
Distance approx. 800 ly
(approx. 250 pc)
Absolute magnitude (MV) −5.25/−3/−2.8
Companion Zeta Orionis B
Period (P) 1,500 yr
Semimajor axis (a) 2.728"
Eccentricity (e) 0.07
Inclination (i) 72.0°
Longitude of the node (Ω) 155.5°
Periastron epoch (T) 2070.6
Mass 28/23/? M
Radius 20/? R
Luminosity 100,000/1,300/? L
Temperature 30,000/?/24,000 K
Metallicity ?
Rotation 140 km/s / ?
Age ? years
Other designations
Alnitak, Al Nitak, Alnitah, ζ Ori, 50 Ori, HR 1948/9, BD −02°1338, HD 37742, SAO 132444, HIP 26727, TD1 5127.
Database references

Zeta Orionis (ζ Ori), traditionally known as Alnitak (Arabic: النطاق an-niṭāq‎), is a triple star some 800 light years distant in the constellation Orion. Together with Delta Orionis (Mintaka) and Epsilon Orionis (Alnilam), the three stars make up the belt of Orion, known by many names across many ancient cultures. Zeta Orionis (ζ Ori) is the left-most star.

The primary star is a hot blue supergiant with an absolute magnitude of -5.25, and is the brightest class O star in the night sky with a visual magnitude of 1.70. It has two bluish 4th magnitude companions. The stars are members of The Orion OB1 Association.


Observation history

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.[1] Much more recently, in 1998, the bright primary was found to have a close companion by a team from the Lowell Observatory after being suspected, from observations made with the Narrabri Stellar Intensity Interferometer, in the 1970s.[2] Initially thought to be around 1500 light years distant, the Alnitak system was found to be almost twice as close following measurement of its stellar parallax by the Hipparcos satellite published in 1996. Alnitak has a magnitude of +2.04.


Zeta Orionis compared to the Sun (to scale)

Alnitak is a triple star system at the eastern end of Orion's belt lying approximately 800 light years from the Solar System. The primary, now known as Alnitak Aa as it itself is a close binary, is a blue supergiant of spectral and luminosity type O9.7 Ibe, with an absolute magnitude of -5.25. It is estimated as being up to 28 times as massive as the sun, and to have a diameter 20 times greater.[3] It is the brightest star of class O in the night sky. Alnitak B is a 4th magnitude B-type star which orbits Alnitak A every 1500 years. The third star, Alnitak Ab, was only discovered in 1998.[2] It is a 4th magnitude O-type star.

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

Etymology and cultural significance

The traditional name Alnitak, alternately spelled Al Nitak or Alnitah, the name is taken from the Arabic النطاق an-nitaq, "the girdle".[1]


Orion's belt

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 H•akk 'the Accurate Scale Beam'. In Chinese mythology they were also known as The Weighing Beam.[1] 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.

In pre-Christian Scandinavia, the belt was known as Frigg's Distaff (Friggerock) or Freyja's distaff.[4] Similarly Jacob's Staff and Peter's Staff were European biblical derived terms, as were the Three Magi, or the Three Kings. Väinämöinen's Scythe (Kalevala) and Kalevan Sword are terms from Finnish mythology.[1]

The Seri people of northwestern Mexico call the three belt stars Hapj (a name denoting a hunter) which consists of three stars: Hap (mule deer), Haamoja (pronghorn), and Mojet (bighorn sheep). Hap is in the middle and has been shot by the hunter; its blood has dripped onto Tiburón Island.[5]

See also


  1. ^ a b c d Richard Hinckley Allen, Star-names and their meanings (1936), p. 314-15.
  2. ^ 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. doi:10.1086/312882.  
  3. ^ Remie H, Lamers HJGLM (1982). "Effective temperatures, and radii of luminous O and B stars - A test for the accuracy of the model atmospheres". Astronomy and Astrophysics 105 (1): 85–97. Retrieved 2007-12-13.  
  4. ^ Schön, Ebbe. (2004). Asa-Tors hammare, Gudar och jättar i tro och tradition. Fält & Hässler, Värnamo. p. 228.
  5. ^ Moser, Mary B.; Stephen A. Marlett (2005) (in Spanish and English) (PDF). Comcáac quih yaza quih hant ihíip hac: Diccionario seri-español-inglés. Hermosillo, Sonora and Mexico City: Universidad de Sonora and Plaza y Valdés Editores.  

External links

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


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