Alnilam shines at magnitude 1.70, ranking fourth in the constellation Orion. It is the brightest of the three stars in Orion's Belt, an asterism known by many names across many cultures. Bayer lettered them in alphabetic order, from west to east, as δ Ori, ε Ori, and ζ Ori; Alnilam is the middle star. The name Alnilam derives from the Arabic "an-nizam", meaning "string of pearls". Related spellings are Alnihan and Alnitam; all three variants are evidently mistakes in transliteration or copy errors.
Properties
Though all three stars have similar colors, classes, and temperatures, Alnilam is the most luminous. A hot class B0 Iab bright supergiant, it is brightest even though farthest away. From Alnilam's measured (though rather uncertain) distance of 1340 light years - half again farther than the other two - it radiates 375,000 solar luminosities from a 25,000 degree K surface. Though seemingly single, which disallows direct measure of its mass by means of a double-star orbit, Alnilam's luminosity implies a mass of some 40 suns.
Like most supergiants, Alnilam is losing mass - about 20 million times more rapidly than the Sun, through stellar winds that may reach up to 2000 km/sec. Alnilam is surrounded by its own (faint) molecular cloud, which it illuminates to make the reflection nebula NGC 1990. Although only four million years old or so, Alnilam is already running out of hydrogen, if it has not done so already. It will evolve into a magnificent red supergiant far more luminous than nearby Betelgeuse, and then explode as a supernova.
Alnilam has served for many years as a "standard star" against which to compare others. Its brilliant blue and relatively simple spectrum has also made it useful for studying the interstellar medium.
[Adapted from STARS by Jim Kaler, Professor Emeritus of Astronomy, University of Illinois]