Editing Alnitak (section)

==Stellar system==
[[Image:Alnitak sun comparison.svg|thumb|left|Alnitak Aa compared to the Sun (to scale)]]
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]] of [[spectral type]] O9.5Iab with an [[absolute magnitude]] of −6.0 and an [[apparent magnitude]] of 2.0. It is estimated as being up to 33 times as massive [[Solar mass|as the Sun]] and a diameter 20 times greater. It is some 21,000 times brighter [[Solar luminosity|than the Sun]], with a [[Black body|surface brightness]] ([[luminance]]) some 500 times greater. It is the brightest star of [[Stellar classification#Class O|class O]] in the [[night sky]]. In about a million years, it will expand into a [[red supergiant]] wider than the orbit of [[Jupiter]] before ending its life in a [[supernova]] explosion, likely leaving behind a [[black hole]].

Alnitak Ab is a blue [[subgiant]] of spectral type B1IV with an absolute magnitude of −3.9 and an apparent magnitude of 4.3, discovered in 1998.<ref name="Hummel00">{{cite journal |author=Hummel CA |author2=White NM |author3=Elias NM II |author4=Hajian AR |author5=Nordgren TE|date= 2000|title=ζ Orionis A Is a Double Star |journal=The Astrophysical Journal |volume=540 |issue=2 |pages=L91–L93 |doi=10.1086/312882 |bibcode=2000ApJ...540L..91H|doi-access=free }}</ref>

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]].