Last modified on April 2, 2018, at 16:09

Zeta Orionis

Zeta Orionis
Observational Data
Designation HIP 26727
Right ascension 05h 40m 45.52666s[1]
Declination -01° 56′ 33.2649″[1]
Constellation Orion
Type of object Star
Magnitude Apparent Mag: +2.0[2]
Absolute Mag: -6.0[2]
Distance from Earth 1,260±180 ly[3]
Radial velocity 18.5±1.3[1]
Proper motion RA: 3.19 mas/yr[1]
Dec.: 2.03 mas/yr[1]
Parallax 4.43±0.64 mas[1]

Zeta Orionis, also known by the more traditional name, Alnitak, is a triple star system in the constellation of Orion. One of the brightest stars in the night sky, it has an apparent magnitude of 1.70. Zeta Orionis, along with Delta Orionis (Mintaka) and Epsilon Orionis (Alnilam), make up the belt of Orion in the constellation. Zeta Orionis is the star that is leftmost in the belt.

History of Observation

As Zeta Orionis is one of the brightest stars in the night sky, and part of Orion's Belt, the star and its two companions have been known since ancient times, and share major cultural significance throughout history.

The traditional name of Alnitak is derived from the Arabic النجاد Al Nijād, meaning "The Belt" or "The Girdle". Other similar traditional Arabic names for the star are Al Nasak النسك, meaning "the Line" and العلقات Al Alkāt, meaning "the Golden Grains or Nuts".[4]

In pre-Christian Scandinavia, the belt Zeta Orionis is part of, was seen as the staff of the Scandinavian goddess Frigg. In addition, in Finnish lore, the three stars were seen as Väinämöinen's Scythe from the Finnish poem Kalevan, based on Finnish folklore.[5] In Christian Europe the stars were often together referred to as Jacob's and Peter's Staff.

In Chinese, the three stars of the belt were collectively known as 参宿 or Shēn Xiù, seen as the western mansion of the White Tiger.[4]

Star System

Zeta Orionis itself is a triple star system once thought to be some 1,500 or so light years away, more recent measurements using the Hipparcos probe show the star system to be actually only some 820 light years distant, though there is still quite a large uncertainty. The system was first resolved as a binary star system in 1819 by German astronomer George K. Kunowsky. In 1998 the primary star itself was discovered to have a close companion after observations by the Lowell Observatory making Zeta Orionis a triple star system.[6]

Zeta Orionis Aa

Illustration of the size of Zeta Orionis Aa to the Sun.

The primary star Zeta Orionis Aa is a blue supergiant star of spectral class O9.7 Ibe, making the star one of the rare class O stars, among the brightest and hottest in the galaxy. It is, in fact, the brightest class O star seen from Earth in the night sky. The star is estimated to have 20 times the mass of our Sun, and 28 times our Sun's diameter.[6] Visual luminosity of the star is 10,500 times that of the Sun. However, because most of the radiation emitted is in the ultraviolet, the total luminosity of Zeta Orionis is some 100,000 times of the Sun.[7]

Class O stars burn through their core hydrogen fuel faster then any other type on the main sequence. It is likely from the molecular clouds in the Orion nebula.

Zeta Orionis Ab

Zeta Orionis Ab is a very close companion star of the primary, orbiting at a distance of only 11 AU. The star is a 4th magnitude class O star, and would be visible in the night sky on its own, if not obscured by its brighter primary. Zeta Orionis Ab is some 23 times as massive as our Sun, and 1,300 times as bright visually.[6]

For an Earth-type planet to be "comfortable" with liquid water, it would need to be at least 300 AUs from the Zeta Orionis Ab pair. An equivalent to over seven times the distance of Pluto from our Sun. Because of the stars' age, such a world would not exist as we understand planet formation; given there hasn't been enough time. In addition, recent observations by the Spitzer Space Telescope indicate that planetary formation would not happen around an O class star due to the photoevaporation effect.[8]

Zeta Orionis B

Zeta Orionis B is a blue-white giant of spectral class B2 III. It has an apparent magnitude of 4.2, making it normally visible in dark skies, but it is obscured by the much brighter Zeta Orionis Aa. It is estimated to have 14 times the mass of our Sun, and some 1,100 times the visual luminosity, even though much of the radiation is in the ultraviolet.[6]

Zeta Orionis C

Zeta Orionis C is a star that is some 58 arcseconds away from the primary and has an apparent magnitude of 9, making it normally invisible to the unaided eye. The star is thought to be only an optical companion, (only appearing visually along the same line of sight with the other stars), and may not be gravitationally bound to the Zeta Orionis system. Therefore, usually it is not counted among the stars that comprise Zeta Orionis.[6]


  1. 1.0 1.1 1.2 1.3 1.4 1.5 Zeta Orionis from the SIMBAD Astronomical Database
  2. 2.0 2.1 Flame nebula from Magnitude taken as that of brightest star.
  3. 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 arXiv:1306.0330
  4. 4.0 4.1 Richard Hinckley Allen, Star-names and their meanings (1936), p. 314-15
  5. Schön, Ebbe. (2004). Asa-Tors hammare, Gudar och jättar i tro och tradition. Fält & Hässler, Värnamo. p. 228
  6. 6.0 6.1 6.2 6.3 6.4 Hummel, C., White, N., Elias II, N., Hajian, A. and Nordgren, T. (2000). ζ Orionis A is a Double Star. The Astrophysical Journal, 540(2), pp.L91-L93.Online
  7. Alnitak from