Star Facts: Alnilam

Alnilam Star
Image Credit: F.Espenak

Alnilam (Epsilon Orionis) is a massive B-type blue supergiant star found in the Belt of Orion. With an average apparent visual magnitude of +1.69, it is the 4th brightest star in the constellation, the 29th brightest star in the entire night sky, and one of the 58 stars used in celestial navigation and identifying one’s relative positions. Alnilam is so hot and bright that it is illuminating the molecular cloud (NGC 1990) that surrounds it to produce a reflection nebula.

Like most other supergiant stars, Alnilam is losing mass at a very high rate. Studies have shown that the star’s 2,000 km/sec solar wind is blasting material away from it at the rate of two millionths of a solar mass every year, which in absolute terms, translates into a mass loss of about 20 million times that at which the Sun is losing its mass. Eventually, Alnilam is expected to become a red supergiant even more luminous than Betelgeuse in the constellation’s top left hand corner, before one day exploding as a supernova.

Quick Facts

• Constellation: Orion
• Coordinates: RA 05h 36m 12.8s |Dec. -01° 12′ 06.9″
• Distance: 2,000 light years
• Star Type: B0 Ia
• Mass: 30-64.5 sol
• Radius: 42 sol
• Apparent Magnitude: Variable between +1.64 and +1.74
• Luminosity: 275,000 – 832,000 sol
• Surface Temperature: 27,000K
• Rotational Velocity: 40-70 km/sec
• Age: 5.7 million years
• Other Designations: e Ori, 46 Orionis, 112 G Orionis, HR 1903, BD -01°969, HD 37128, SAO 132346, FK5 210, HIP 26311, TD1 4963


Northern hemisphere observers can view the constellation of Orion in the night sky from November to May, and also during the morning hours from late July to November. The best time to see the constellation, however, is around midnight during the middle of December, when the constellation reaches its highest point above the south-western horizon. Look for the star Alnilam in the middle of the Hunter’s Belt, with the stars Mintaka and Alnitak positioned either side of it. From Southern hemispheres locations, Orion can be observed in the summer time with the Hunter appearing upside down in the sky.

Physical Properties

As with the star Mintaka, another of the Belt stars, much about the physical properties of Alnilam remain uncertain. For instance, Crowther et al. used modelling based on the velocity of Alnilam’ solar wind and atmospheric properties in an attempt to constrain basic properties of the star. Their methods yielded a luminosity of 275,000 times that of the Sun, a radius 24 times that of the Sun, and an effective temperature of 27,000K.

However, using CMFGEN code to calculate the same properties for this star, Searle et al. arrived at a luminosity of 537,000 times that of the Sun, a radius of 32.4 times that of the Sun, and an effective temperature of 27,500K. Still another method that involves analyzing the spectra of other members of the OB1 association yields a mass of 34.6 times that of the Sun, and an age of only 5.7 million years.

More recent attempts to constrain basic properties for Alnilam using detailed analyses across multiple wavelengths have yielded extremely high luminosity, mass, and radius values, but these values assume that the most recent Hipparcos distance estimate of 606 parsecs (1,975 light years) is correct. If this distance value is correct, it means that Alnilam is at least 836,000 times as luminous as the Sun, which is the highest luminosity value ever derived for this star. Nonetheless, and despite the confusion, Alnilam’s relatively simple spectrum has served as a standard measure against which to classify other stars since 1943. Moreover, the prodigious quantity of light emitted by the star also serves as a means with which to study the interstellar medium.


Alnilam’s traditional name derives from the Arabic “Al-nilam”, which is related to the word “nilam”, meaning “sapphire”, presumably because of the stars’ distinctly blue color. However, related Arabic renderings of the stars’ name include” Alnihan” and “Alnitam”, and it is thought by most historians that all three variants are either the result of misspellings, mistranslations, or mistakes made during the copying and/or translations of catalogues that predate the first Arabic star lists in which the star is mentioned.

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