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Alnitak or Zeta Orionis is a system of three great blue stars. The biggest, Alnitak Aa, with a surface temperature of 40 000 kelvin is the brightest O-star in the sky.
A dim fourth star (with 9th visual magnitude) is next to the triple. It is not sure whether it belongs to the system too.

The left belt star of Orion lies visually next to the famous Horsehead Nebula.

Constellation: Orion
Distance: 817 light-years
Age: 6 million years
Space between Alnitak Aa and Ab: 11 AU
Space between Alnitak A and B: 680 AU
Orbit period of Alnitak A and B: 1508.6 years

Alnitak Aa

Spectral class: O9.5
Visual magnitude: 1.74
Luminosity: 100 000 * Sun
Mass: 28 * Sun
Diameter: 20 * Sun

Alnitak Ab

Spectral class: O
Visual magnitude: 4
Luminosity: circa 20 000 * Sun
Mass: 23 * Sun

Alnitak B

Spectral class: B0

Luminosity: circa 10 000 * Sun
Mass: 14 * Sun

Back: List of big and giant stars
Orion. Left in the row of three is Alnitak, the central is Alnilam and the right one Mintaka. In the upper left is Betelgeuse, up right is Bellatrix, down right Rigel and down left Saiph.
Photo: Nasa

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