Alnitak is a triplestar system in theconstellation ofOrion. It has the designationsζ Orionis, which isLatinised toZeta Orionis and abbreviatedZeta Ori orζ Ori, and50 Orionis, abbreviated50 Ori. The system is located at a distance of several hundredparsecs from theSun and is one of the three main stars ofOrion's Belt along withAlnilam andMintaka.
The primarystar, Alnitak Aa, is a hotblue supergiant with an absolute magnitude of −6.0 and is the brightestclass 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 theOrion OB1 association and theCollinder 70 [de] association.
Alnitak (lower right) and the Flame Nebula atinfrared wavelengths
Alnitak has been known since antiquity and, as a component ofOrion's Belt, has been of widespread cultural significance. It was reported to be a double star by amateur German astronomerGeorge K. Kunowsky in 1819.[11] Much more recently, in 1998, the bright primary was found by a team from theLowell Observatory to have a close companion; this had been suspected from observations made with theNarrabri Stellar Intensity Interferometer in the 1970s.[12]
Thestellar parallax derived from observations by theHipparcos satellite imply a distance around225 parsecs, but this does not take into account distortions caused by the multiple nature of the system. Larger distances, typically closer to400 pc, have been derived by many authors based on the orbit of the pair or the assumed properties of the components. This distance is comparable to theOrion molecular cloud complex, including the nearbyFlame andHorsehead Nebulae.[4]
Alnitak is a triplestar 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.
The traditional nameAlnitak, alternately spelled Al Nitak or Alnitah, is taken from theArabic النطاقan-niṭāq, "the girdle".[11] In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[14] included a table of the first two batches of names approved by the WGSN; which includedAlnitak for the star ζ Orionis Aa. It is now so entered in the IAU Catalog of Star Names.[15]
The three belt stars were collectively known by many names in many cultures.Arabic terms include النجادAl Nijād 'the Belt', النسكAl Nasak 'the Line', العلقاتAl Alkāt 'the Golden Grains or Nuts' and, in modern Arabic, ميزان الحقAl Mīzān al Ḥaqq 'the Scale of Justice'. InChinese mythology they were known as The Weighing Beam.[11]
^Kunitzsch, Paul; Smart, Tim (2006).A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub.ISBN978-1-931559-44-7.
^abDucati, 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.
^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:0705.0878.Bibcode:2007AN....328..889K.doi:10.1002/asna.200710776.ISSN0004-6337.S2CID119323941.
^Buysschaert, B.; Neiner, C.; Ramiaramanantsoa, T.; Richardson, N. D.; David-Uraz, A.; Moffat, A. F. J. (2016). "Understanding the photometric variability of ζ OriAa".Second Brite-Constellation Science Conference: Small Satellites – Big Science.5: 101.arXiv:1610.05625.Bibcode:2017sbcs.conf..101B.
