Alnitak

Alnitak or Zeta Orionis (Zet Ori) is the 5th brightest naked eye star in the constellation Orion. With an apparent magnitude of 1.74c, Alnitak is the 31st brightest star in the entire sky (see: 50 Brightest Stars ). Its absolute magnitude is -5.26 and its distance is 818 light years. The Equinox J2000 equatorial coordinates are RA = 05h 40m 45.5s, Dec = -01° 56' 34".

Alnitak has a spectral type of O9.5Ib, a surface temperature of 30,000° Kelvin and a luminosity 100,000 times the Sun. It has a mass of 28 solar masses and a diameter 20 times the Sun.

The image above shows the uncropped view of Alnitak (North is up) through the Takahashi E-180 Astrograph.

Alnitak is a triple star system at the eastern end of the Belt of Orion (other Belt stars include Alnilam and Mintaka). The primary star Alnitak A is itself a close binary, comprising Alnitak Aa (a blue supergiant of spectral type O9.7 Ibe, with an absolute magnitude of -5.25 and an apparent magnitude of 2.0) and Alnitak Ab (a blue dwarf of spectral type O V, with an absolute magnitude of about -3.0 and an apparent magnitude of 4). Aa is estimated as being up to 28 times as massive as the Sun, and to have a diameter 20 times greater. It is the brightest star of class O in the night sky. Alnitak B is a 4th magnitude B-type star which orbits Alnitak A every 1500 years. 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 which is also associated with the HorseHead Nebula.

The description above is based on the Alnitak entry in Wikipedia. For more information about Alnitak, see Stars (Jim Kaler).

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