Alnitak (ζ Orionis)
The left hand or easternmost star in → Orion's Belt, which is the fifth brightest in the whole of → Orion with a → visual magnitude of 2.05. Alnitak is a → close binary system comprising Alnitak Aa and Alnitak Ab. Aa is a hot → blue supergiant of → spectral type O9.5 Iab with an → absolute magnitude of -6.0 and an → apparent magnitude of 2.0. Its mass is estimated as being up to 33 times as massive as the Sun and to have a diameter 20 times greater. It is some 250,000 times more luminous than the Sun, with a surface temperature of about 30,000 K. It is the brightest star of class O in the night sky. Alnitak Ab is a blue → subgiant of spectral type B1 IV with an absolute magnitude of -3.9 and an apparent magnitude of 4.3. Ab revolves around Ab with a period of 2,687 days. The system has a 4th magnitude companion, Alnitak B, nearly 3 arc-seconds distant. It is a B0 III type star which orbits Alnitak A every 1,500 years. Alnitak is associated with the → emission nebula → IC 434 containing the → Horsehead Nebula (C. A. Hummel et al., 2013, A&A 554, A52, arXiv:1306.0330).
Alnitak, from Ar. An-Nitaq al-Jawza'