Alnilam, the Brightest Gem in the Orion’s Belt
Life and times of the blue supergiant that occupies the central position in one of the most famous and recognizable asterisms of the sky
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During the winter months, the constellation Orion, seen from the northern hemisphere of the planet, offers a magnificent spectacle, able to fascinate even those who raise their eyes towards the starry sky only distractedly, maybe once in a while. One of the most iconic elements of the constellation is the Belt, known since ancient times, formed by three very bright stars, well recognizable in a row next to each other. Their description has been handed down to us from the most diverse and distant cultural traditions.
In the West, the three stars of Orion’s Belt, as well as most of the brightest stars in the sky, have names of Arab origin. The easternmost of the three is Alnitak, a blue supergiant that is part of a triple system. The westernmost is Mintaka, which we see with the naked eye as a single star but is actually a multiple system consisting of at least six stars. The central star of the Belt, also a blue supergiant, is Alnilam, whose name derives from an Arabic word that means “row of pearls” and evidently refers to the asterism rather than to the single star.
The pearls of the Belt
These three bright gems of the night sky are most often mentioned in the scientific literature with the names attributed to them in 1603 by Johann Bayer’s Star Atlas, the Uranometria — Alnitak is Zeta Orionis (ζ Ori), Mintaka is Delta Orionis (δ Ori), Alnilam is Epsilon Orionis (ε Ori). Their apparent visual magnitudes are, respectively, 1.79, 2.41 and 1.69, which makes Alnilam the brightest star of Orion’s Belt [1] and, in general, the 29th brightest star in the sky.
The three stars of the Belt, together with many others found in their surroundings, are part of a rich star-forming region, in which there are open clusters made up of massive and bright young stars, molecular clouds that host legions of newborn stars, emission nebulae, reflection nebulae, and a large population of small- and medium-mass stars, many of which have only recently formed.
