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 arcseconds 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