Alnitak, Zeta Orionis, is a triple star with a visual magnitude of 1.70 in the constellation Orion. It is the easternmost star in the Orion's Belt, along with Delta Orionis (Mintaka) and Epsilon Orionis (Alnilam). The traditional name Alnitak, alternately spelled Al Nitak or Alnitah, the name is taken from the Arabic "an-nitaq" (the girdle).
Physical Properties
Zeta Orionis is a triple star system some 820 light years distant. The primary, now known as ζ Ori Aa (since it is itself a close binary), is the brightest class O star in the night sky. It is a blue supergiant of spectral class O9.7 Ibe, with an absolute magnitude of -5.25, and a total luminosity around 100,000 suns. This is much greater than its visual luminosity of 10,500 suns, due to the fact that its 31,000-degree-K surface radiates mostly ultraviolet wavelengths that are invisible to human eyes. It mass is estimated as being up to 28 times the Sun's, and its diameter 20 times greater. A planet like Earth would have to be 300 times farther from Alnitak than Earth is from the Sun for life like ours to survive.
Other Components
Alnitak was first discovered to be a double star in 1819 by amateur German astronomer George K. Kunowsky. Its companion, ζ Ori B, is a blue-white giant of spectral class B2 III with an apparent magnitude of 4.2. It is currently separated by about 2.3", or around 680 AU, from ζ Ori A, in a nearly circular orbit (e=0.07) with a period of 1500 years. It may have 14 times the Sun's mass, and around 1,100 times its visual luminosity.
In 1998, a team from Lowell Observatory found a third, close companion (ζ Ori Ab) only 11 AU away from the bright primary (ζ Ori Aa). It is a 4th magnitude star, and would be easily visible to the naked eye if it were farther from its bright neighbor. Given its high visual magnitude, it has a high visual luminosity (over 1,300 suns) and mass (23 suns), and presumably a youthful age of only a few million years, shared with its primary. It is probably a late class O main sequence star.
A fourth component, ζ Ori C, has an apparent magnitude of 9, and is located around 57.6" away. It is probably an optical companion, not gravitationally bound to the Alnitak system.
Alnitak lies in a region crowded with clouds of interstellar gas and dust actively forming new stars, including NGC 2024 (the Flame Nebula) and IC 434 (which contains the famous Horsehead Nebula) to the south. The system is a member of the Orion OB1 Association, where massive young stars can be found in abundance.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]