Alnitak (Zeta Orionis)


Alnitak is the left-hand star in Orion's Belt. Image: NASA.

Alnitak (Zeta Orionis) is the left hand star in Orion's Belt, the fifth brightest in the whole of Orion, and (in terms of apparent magnitude) the brightest O star in the sky. Its Arabic name comes from a phrase meaning "the belt of al Jauza." A B-type companion lies about 3 arcsec away, the pair orbiting each other with a period of several thousand years.


Like all O stars, Alnitak is a source of X-rays that seem to come from a stellar wind that blows from its surface at nearly 2,000 km/s. The X-rays are produced when blobs of gas in the wind crash violently into one another. Alnitak is probably only about 6 million years old and will eventually become a red supergiant before exploding as a supernova. In its vicinity are several dusty interstellar clouds, including, to the south, the Horsehead Nebula.


visual magnitude 1.74
absolute magnitude -5.26
spectral type O9.5Ib
surface temperature 31,000 K
luminosity 100,000 Lsun
mass 20 Msun
distance 817 light-years
position RA 05h 40m 45.5s,
Dec -01° 56' 34"
other designations Al Nitak, 50 Ori, HR 1948/9,
BD -02°1338, HD 37742,
SAO 132444, HIP 26727