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Alnitak (ζ Orionis) Worksheets

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Key Facts & Information

  • Alnitak is one of the three main stars that form Orion’s Belt. It is also known as Zeta Orionis or 50 Orionis, which is a triple star system.
  • It is the fifth-brightest star in the constellation of Orion and the 31st  brightest star in the night sky.
  • Alnitak can be found at around 1,260 light-years / 387 parsecs away from the Sun.
  • It is the easternmost star that forms the Orion’s Belt asterism in the night sky This asterism has been known for thousands of years and was associated with many myths and religious beliefs.
  • The word, Alnitak, is an Arabic word which translates into “the girdle”. Its traditional name was alternatively spelled as Al Nitak or Alnitah.
  • Orion’s Belt is collectively known by many names in different cultures throughout the ages. The other stars forming the belt are Alnilam and Mintaka.
  • The star system of Alnitak is composed of Alnitak Aa, a blue supergiant of spectral type O9.5lab, Alnitak Ab, a blue subgiant of spectral type B1IV, and Alnitak B, a B-class blue giant star.


  • The Orion Constellation is home to many blue stars, clusters, nebulae, and clouds of dust and gas. There is also a pretty active star formation in this constellation. All three stars in this region have many other companion stars around them except for Alnilam.
  • Alnitak is a triple star system. The stars are estimated to have formed at different time periods. The primary star, Alnitak Aa, is the youngest star in the system being around 6.4 million years old.
  • The secondary star, Alnitak Ab is the oldest star around 7.2 million years, while Alnitak B, the third star, is around 7 million years old and located in the middle. All of the stars are younger than the Sun.
  • It is a part of the Orion Molecular Cloud Complex. In this complex, stars cannot exceed 12 million years old. There was a time when gas and dust from the molecular cloud were pulled together by gravity, which resulted in many of the stars that we can see in the constellation.

Distance, Size & Mass

  • The Alnitak star system is located at around 1,260 light-years / 387 parsecs away from the Sun.
  • Alnitak Aa is the biggest star in the system with around 33 solar masses and a radius of around 20 times that of the Sun.
  • Alnitak Ab is the second-biggest star with 14 solar masses and a radius of around 7.3 times that of the Sun. Some estimates say it has 23 solar masses.
  • Alnitak B is the smallest star with an estimated mass below or also around 14 solar masses.

Other Characteristics

  • Alnitak Aa is a blue supergiant of spectral type O9.5lab. It is the brightest star of Class O. It has an absolute magnitude of -6.0 and an apparent magnitude of 2.0. It is around 250,000 times brighter than the Sun. It is also the hottest star in the system with a temperature of 29,500 K, which makes it 5 times hotter than the Sun. It is a fast spinner with a rotational velocity of around 110 km / 68 mi per second.
  • Alnitak Ab is a blue subgiant of spectral type B1IV. It has an absolute magnitude of -3.9 and an apparent magnitude of 4.3. It is only 32,000 times brighter than our Sun and has a surface temperature of 29,000 K. It is also 5 times hotter than our Sun.
  • Alnitak B is a B2 III-class blue-white giant star. It has a visual magnitude of 4.01. It is around 1,100 times brighter than our Sun and it may be hotter. It appears to have the fastest rotational velocity in the star system, which is 350 km / 217 mi per second.

Stellar System

  • The binary system is composed of Alnitak Aa and Alnitak Ab and collectively called Alnitak A. These two giants complete one orbit around each other once every 2.687 days.
  • The third companion, Alnitak B, is around 3 arc-seconds away from Alnitak A. It completes one orbit around the primary system once every 1,500 years.
  • There is a fourth companion designated as Alnitak C, which is speculated to be part of a system. Studies indicate it is more of a visual companion and not physically related to the other stars.


  • Alnitak is located in the Orion Constellation, which is known as the celestial hunter. It is the part of the three stars forming the hunter’s belt. It is also the westernmost star. Orion’s Belt is often used to search for the constellation itself.
  • The Orion Constellation is also home to two first-magnitude stars, Rigel and Betelgeuse. It also hosts the nearest star formation to Earth.
  • This constellation is prominent in the sky every winter for observers in the Northern Hemisphere, and for observers in the Southern Hemisphere, during summer, where it appears upside down.


  • Alnitak Aa is so massive that it has already stopped fusing hydrogen in its core. Even if it is young, it will eventually evolve into a red supergiant similar to Betelgeuse. It will explode as a supernova and Alnitak Ab will possibly share the same fate.

Did You Know?

  • Before 1819, Alnitak was considered as a single star. German astronomer, G.K. Hunowsky, reported it to be a double star. This discovery was acknowledged by further investigation much later in 1998.
  • IC 434, a bright emission nebula, has surrounded Alnitak. Due to the star’s powerful ultraviolet light, it illuminated a nearby nebula designated as NGC 2024.
  • The best time to see Alnitak or Orion’s Belt as a whole is in mid-December and in January at around 9 PM.
  • Orion’s Belt was known as Weighing Beam for the Chinese. The stars in the belt and a couple more from the constellation are included in this asterism. Alnilam is known as the Third Star of Three Stars.
  • Orion and Sirius were associated with the divine beings Isis and Osiris who were believed to have come from Orion’s Belt and Sirius to create mankind.   
  • The three pyramids of the Giza Plateau simulate the alignment of the three belt stars. The air shafts of the pyramids are pointed directly towards the constellation.
  • Two large pyramids and a temple in Mexico were discovered to point directly to Orion. These were built in the 2nd century BC, and one of them is exactly half as tall as the Great Pyramid of Giza.
  • In Puerto Rico and Philippines, the three stars represented the three biblical Magi and they named it Tres Reyes Magos.