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Zeta Orionis, 50 Orionis

Proper NameAlnitak
Bayer DesignationZeta Orionis
Flamsteed Number50 Orionis
HR (BSC)1948
Right Ascension5h 40m 46s
Declination-1° 56' 33"
Distance736 light years
226 parsecs
MagnitudeApparent: +1.7 to +0.6
Absolute: -5.0
Spectral ClassO9Ib Blue-white Supergiant
Optimum VisibilityDecember / January
NotesThough only some twelve times the Sun's diameter, this supergiant star emits extraodinary levels of energy: it is some 9,000 times more intrinsically luminous than the Sun in visible light, and produces more than 150,000 times as much energy across the entire spectrum.

The easternmost of the three stars that make up Orion's Belt, Alnitak is a binary. The primary is an O-type supergiant, one of the most luminous of star-types, meaning that shines as brightly as magnitude +1.7 despite being more than 700 light years away.

The sky around Alnitak is densely populated with nebulae. To the immediate east is the Flaming Tree Nebula, composed of branching lanes of dark dust against a glowing background, while to the south of the star the distinctive shape of the Horsehead Nebula can be seen. Between the two nebulae, southeastward from Alnitak, is a patch of blue-white light known as the Lump Star which, though appearing star-like, is in fact another nebula within the surrounding complex. Imagery provided by Aladin sky atlas


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