Alnitak is a triple star system in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

of Orion. It has the designations ζ Orionis, which is Latinised to Zeta Orionis and abbreviated Zeta Ori or ζ Ori, and 50 Orionis, abbreviated 50 Ori. The system is located at a distance of several hundred

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (au), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, an ...

s from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

and is one of the three main stars of

Orion's Belt Orion's Belt or the Belt of Orion, also known as the Three Kings or Three Sisters, is an asterism in the constellation Orion. It consists of the three bright stars Alnitak, Alnilam and Mintaka. Looking for Orion's Belt is the easiest way to ...

along with

Alnilam Alnilam is the central star of Orion's Belt in the equatorial constellation of Orion. It has the Bayer designation ε Orionis, which is Latinised to Epsilon Orionis and abbreviated Epsilon Ori or ε Ori. This is a massive, blue supe ...

and

Mintaka Mintaka , designation Delta Orionis (δ Orionis, abbreviated Delta Ori, δ Ori) and 34 Orionis (34 Ori), is a multiple star system some 1,200 light-years from the Sun in the constellation of Orion. Together with Alnitak (Zeta Or ...

. The primary

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

, Alnitak Aa, is a hot

blue supergiant A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They have luminosity class I and spectral class B9 or earlier. Blue supergiants are found towards the top left of the Hertzsprung–Russell diagram, above ...

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 companions—Ab and B, the latter known for the longest time and the former discovered recently, producing a combined magnitude for the trio of +1.77. The stars are members of the

Orion OB1 association Orion OB1 (Ori OB1) is a contingent group of several dozen hot giant stars of spectral types O and B in Orion. Associated are thousands of lower-mass stars, and a (smaller but significant) number of protostars. It is part of the larger Orion mo ...

and the association.

Observational history

Alnitak has been known since antiquity and, as a component of

, has been of widespread cultural significance. It was reported to be a double star by amateur German astronomer George K. Kunowsky in 1819. 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 The Narrabri Stellar Intensity Interferometer (NSII) was the first astronomical instrument to measure the diameters of a large number of stars at visible wavelengths. It was designed by (amongst others) Robert Hanbury Brown, who received the Hughe ...

in the 1970s. The stellar parallax derived from observations by the '' Hipparcos'' satellite imply a distance around 225

s, 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.

Stellar system

Alnitak is a triple star system at the eastern end of Orion's Belt, the second-magnitude primary having a 4th-magnitude companion nearly 3 arcseconds distant, in an orbit taking over 1,500 years. The part called Alnitak A is itself a close binary, comprising the stars Alnitak Aa and Alnitak Ab. Alnitak Aa is a blue

supergiant Supergiants are among the most massive and most luminous stars. Supergiant stars occupy the top region of the Hertzsprung–Russell diagram with absolute visual magnitudes between about −3 and −8. The temperature range of supergiant stars spa ...

spectral type In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

O9.5Iab with an

absolute magnitude Absolute magnitude () is a measure of the luminosity of a celestial object on an inverse logarithmic astronomical magnitude scale. An object's absolute magnitude is defined to be equal to the apparent magnitude that the object would have if it ...

of −6.0 and an

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's ...

of 2.0. It is estimated as being up to 33 times as massive as the Sun and a diameter 20 times greater. It is some 21,000 times brighter than the Sun, with a

surface brightness In astronomy, surface brightness (SB) quantifies the apparent brightness or flux density per unit angular area of a spatially extended object such as a galaxy or nebula, or of the night sky background. An object's surface brightness depends on it ...

(

luminance Luminance is a photometric measure of the luminous intensity per unit area of light travelling in a given direction. It describes the amount of light that passes through, is emitted from, or is reflected from a particular area, and falls withi ...

) some 500 times greater. It is the brightest star of class O in the

night sky The night sky is the nighttime appearance of astronomical object, celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when the Sun is below the horizon. Natural light sources in a ...

. Alnitak Ab is a blue

subgiant A subgiant is a star that is brighter than a normal main-sequence star of the same spectral class, but not as bright as giant stars. The term subgiant is applied both to a particular spectral luminosity class and to a stage in the evolution ...

of spectral type B1IV with an absolute magnitude of −3.9 and an apparent magnitude of 4.3, discovered in 1998. 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 IC 434 is a bright emission nebula in the constellation Orion. It was discovered on February 1, 1786 by William Herschel. The Horsehead Nebula is a dark nebula silhouetted against it. See also * Orion molecular cloud complex The Orion mole ...

Etymology and cultural significance

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

Flamsteed designation A Flamsteed designation is a combination of a number and constellation name that uniquely identifies most naked eye stars in the modern constellations visible from southern England. They are named for John Flamsteed who first used them while co ...

. The traditional name ''Alnitak'', alternately spelled Al Nitak or Alnitah, is taken from the

Arabic Arabic (, ' ; , ' or ) is a Semitic languages, Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C ...

النطاق ''an-niṭāq'', "the girdle".Richard Hinckley Allen, ''Star-names and their meanings'' (1936), p. 314–15. In 2016, the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 included a table of the first two batches of names approved by the WGSN; which included ''Alnitak'' for the star ζ Orionis Aa. It is now so entered in the IAU Catalog of Star Names.

Orion's Belt

The three belt stars were collectively known by many names in many cultures.

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 Scale of Justice'. In

Chinese mythology Chinese mythology () is mythology that has been passed down in oral form or recorded in literature in the geographic area now known as Greater China. Chinese mythology includes many varied myths from regional and cultural traditions. Much of t ...

they were known as The Weighing Beam. The belt was also the Three Stars mansion (), one of the twenty-eight mansions of the Chinese constellations. It is one of the western mansions of the

White Tiger The white tiger or bleached tiger is a leucistic pigmentation variant of the Mainland tiger. It is reported in the wild from time to time in the Indian states of Madhya Pradesh, Assam, West Bengal, Bihar, Odisha, in the Sunderbans region and ...

. In

Chinese Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of ...

, (), meaning Three Stars (asterism), refers to an asterism consisting of Alnitak,

and

(Orion's Belt), with Betelgeuse, Bellatrix,

Saiph Saiph , designation Kappa Orionis (κ Orionis, abbreviated Kappa Ori, κ Ori) and 53 Orionis (53 Ori), is the sixth-brightest star in the constellation of Orion. Of the four bright stars that compose Orion's main quadrangle, it is the sta ...

and

Rigel Rigel is a blue supergiant star in the constellation of Orion. It has the Bayer designation β Orionis, which is Latinized to Beta Orionis and abbreviated Beta Ori or β Ori. Rigel is the brightest and most massive componentand ...

later added. Consequently, the

Chinese name Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are often a ...

for Alnitak is (, en, the First Star of Three Stars).AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 25 日
/ref> It is one of the western mansions of the

Namesakes

The USS ''Alnitah'' was a

U.S. Navy The United States Navy (USN) is the maritime service branch of the United States Armed Forces and one of the eight uniformed services of the United States. It is the largest and most powerful navy in the world, with the estimated tonnage o ...

''Crater''-class cargo ship named after the star.

References

External links

* * * {{DEFAULTSORT:Alnitak O-type supergiants B-type giants Triple star systems Orion (constellation) Orionis, Zeta 1948 9 BD-02 1338 Orionis, 50 037742 026727 Orionis, 126 Stars named from the Arabic language