Star Facts: Alnitak

Altinak and the Flame and Horse Head Nebulae
Image Credit: Claustonberry Observatory

Alnitak (Zeta Orionis) is a blue supergiant star in the Orion constellation, and forms part of the Hunter’s Belt along with the stars Alnilam and Mintaka. It is also the brightest O-type star in the entire night sky, with an apparent visual magnitude of +2.0. Alnitak is actually a binay system, and together with its companion are members of the Collinder 70 open cluster, as well as the Orion OB1 stellar association of hot giant stars that form part of the Orion Molecular Cloud Complex. Contained in the latter is the Flame Nebula (NGC 2024), which is located east of Alnitak and is illuminated by the ultraviolet light radiating from the star, with the famous Horsehead Nebula also seen further south west, as seen in the above image.

Visibility

It is very difficult not to see Orion from most of the northern hemisphere in the south-western sky during the winter months, or from the southern hemisphere during the summer time. This is because the constellation itself is situated on the celestial equator, meaning half of it is located in the northern sky and the other half in the south sky, making Orion visible from all places on Earth except near the poles. The asterism that most people first notice when viewing Orion is the sloping diagonal line of three bright stars which represent the Hunter’s belt. For northern observers, Alnitak is the star located furthest east (left) in the Hunter’s Belt, and the one that is nearest the horizon when the constellation is upright.

Quick Facts

Alnitak Aa

• Constellation: Orion
• Coordinates: RA 05h 40m 45.52666s |Dec. -01° 56′ 34.2649″
• Distance: 1,260 light years
• Star Type: O9.5Iab
• Mass: 33 sol
• Radius: 20 sol
• Apparent Magnitude: +2.08
• Luminosity: 250,000 sol
• Surface Temperature: 29,500K
• Rotational Velocity: 110 km/sec
• Age: ± 6.4 million years
• Other Designations: Zeta Orionis, 50 Orionis, 126 G. Orionis, BD-02°1338, SAO 132444, HIP 26727, TD1 5127, WDS J05407-0157

Below are some more quick facts about Alnitak’s’ companion:

Alnitak Ab:

• Star Type: B1IV
• Mass: 14 sol
• Radius: 7.3 sol
• Apparent Magnitude: +4.28
• Luminosity: 32,000 sol
• Surface Temperature: 29,000K
• Age: 7.2 million years

Physical Properties

The star Alnitak is the primary component of a binary system at the eastern extremity of Orion’s Belt. The companion star is a 4th magnitude B-type star, which orbits Alnitak once every 1 508 years or so at a distance of about 3 seconds of arc. Designated Alnitak B, the companion is a blue sub-giant, and is itself a close binary star, although not much is known about the companion of Alnitak B. A fourth star, designated Alnitak C, has also been reported, but it is uncertain whether it is actually physically related to the Aa-Ab-B system, or if it is merely a coincidental line-of-sight member of the group.

Alnitak is estimated to be about 33 times more massive than the Sun, and at least 20 times bigger, meaning that at only around 6 million years old it has already exhausted its supply of hydrogen fuel, and it will evolve into the red giant phase over the next few million years, before exploding as a supernova. Like all O-type stars, Alnitak is a source of X-ray radiation, which in the case of Alnitak, appears to be caused by collisions between globules of gas in the stars’ solar wind that leaves the star at a speed of about 2,000 km/sec.

History

The traditional name, Alnitak, derives from the Arabic phrase “an-nitaq” meaning “the girdle”, with Arabic variations also including “Al Nijad”(“the Belt”), “Al Nasak” (“the Line”), “Al Alkat” (“the Golden Grains or Nuts”), and in modern Arabic, Al Mizan al Haqq” (“the Accurate Scale Beam”). However, the latter versions refers to the stars that collectively form Orion’s Belt.

As a component of Orion’s Belt, Alnitak has been in the human consciousness since antiquity, and has been of significant cultural and religious significance for almost all known cultures across the globe. For instance, in some Christian traditions, the three Belt stars are known as “Las Tres Marías” or as “Três Marias”, meaning the “Three Mary’s” in both the Spanish and Portuguese languages. In other traditions, the Belt stars were known as either Jacob’s Staff or Peter’s Staff, or even as the Three Magi, which refers to the three Wise Men, or sometimes to Three Kings.

In pre-Christian Scandinavia, the Belt stars were collectively known as “Friggerock”, meaning either Frigg’s Distaff, or Freyja’s distaff, while in specifically Finnish mythology, the Belt stars were collectively known as Väinämöinen’s Scythe, and also as Kalevan’s Sword.

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