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Alnilam (ε Orionis) Worksheets

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Key Facts & Information

  • Alnilam is part of the Orion’s Belt asterism. It is known as Epsilon Orionis, or 46 Orionis. It is the 4th brightest star in Orion and the 29th brightest star in the night sky. It is a bright blue supergiant located at around 1,975 light-years / 600 parsecs away from the Sun.
  • Alnilam is the middle star in the Orion’s Belt asterism, which has been known for thousands of years and has been associated with myths and religious beliefs.
  • Alnilam is an Arabic word that translates to “spring of pearls”. Its traditional name can also be spelled as Alnihan or Alnitam.
  • All three variants are evidently mistakes on transliteration or copy errors. Orion’s Belt is also known by many names in different cultures throughout the ages. The other stars forming Orion’s Belt are Alnitak and Mintaka.
  • It is a spectral type B0 Ia star with an apparent magnitude of 1.69. It is also a variable star of the Alpha Cygni class, thus, its brightness varies from 1.64 to -1.74. Its absolute magnitude has been estimated to be around -6.89.
  • Alnilam is the most massive star in the Orion’s Belt asterism. It is also the only star which appears to be a single star. The other stars have many more companion stars, which are gravitationally bound to them.


  • 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.
  • Alnilam is the loneliest out of the stars in Orion’s Belt. It is also the youngest star with an estimated age of 5.7 million years, which makes it younger than our Sun, like the other stars in the asterism.
  • 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 stars that we can see in the constellation.

Distance, Size & Mass

  • It is located at around 1,975 light-years away / 600 parsecs away from the Sun. Some estimates done made it farther or closer.
  • It is also 30 times bigger than the Sun and the biggest star in the Orion’s Belt asterism.
  • It is also the most massive star in the asterism having 40 times of the Sun’s mass. Its radius is estimated to be at around 32.4 times than that of the Sun.

Other Characteristics

  • Alnilam is about 570,000 times brighter than the Sun, which makes it the brightest star in the asterism. Its brightness illuminates the molecular cloud, NGC 1990, that appears to have surrounded the star. Some calculations put it at 832,000 times brighter than the Sun. 
  • It is quite a hot star with an average surface temperature of 27,500K, which makes it around 4.7 times hotter than the Sun.
  • It has powerful stellar winds that reach speeds up to 2,000 km / 1,242 mi per second. This makes Alnilam lose its mass around 20 million times faster than the rate of Sun’s mass loss. It is a fast spinning star with a rate of 40–70 km / 24.8–43.4 mi per second.
  • The simple spectrum of Alnilam has served since 1943 as a standard measure of which other stars are classified.


  • Alnilam 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.

The Future

  • Because Alnilam is losing its mass at such a high rate, and also due to its size and and other properties, the star is believed to be on its way to become a red supergiant in the near future. If this happens, it is expected to be brighter than Betelgeuse. Its fate will inevitably reach a point where it will explode as a supernova.

Did You Know?

  • Alnilam, together with Betelgeuse, Rigel, and Bellatrix, is part of the 58 stars selected for celestial navigation.
  • 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.
  • In pre-Christian Scandinavia, the belt is known as Frigg’s Distaff or Freya’s staff, while in northwestern Mexico, the Seri people called it Hapj which denotes a hunter.