ALNILAM (Epsilon Orionis). Three brilliant stars mark the belt of Orion the Hunter, from right to left (west to east) Mintaka, Alnilam, and Alnitak. The names of all three refer to the whole set. The outer two are named after the "belt" of the Arabs "Central One (a mysterious feminine figure), while Alnilam comes from an Arabic word that aptly means "the String of Pearls," which the trio so well represents. The brightest of the Belt stars (which Bayer lettered in alphabetic order, again from right to left, Delta, Epsilon, and Zeta), and ranking fourth in the whole constellation, Alnilam shines at bright second magnitude (1.70). Though all three stars have similar colors, classes, and temperatures, Alnilam, a hot B (B0) bright supergiant, is notably the most luminous: it is brightest even though farthest away, half again farther than the other two (which lie at nearly the same distance, about 870 light years). From Alnilam's measured (though rather uncertain) distance of 1340 light years, it spectacularly radiates (after correction for its great amount of ultraviolet light) 375,000 solar luminosities from its 25,000 Kelvin surface, and is so hot that it illuminates its own (faint) nebulous cloud from the surrounding interstellar gases. Alnilam has served for many years as a "standard star" against which to compare others. Its brilliant blue and relatively simple spectrum also provides a fine background against which to study the gases of the intervening interstellar space. Like most supergiants, Alnilam is losing mass. A powerful wind blows from the star's surface at speeds up to 2000 kilometers per second, the flow rate two millionths of a solar mass per year (20 million times that from the Sun). Though seemingly single, which disallows direct measure of mass by means of a double-star orbit, the luminosity tells of an evolving star with a mass some 40 times solar. Currently only 4 or so million years old, its internal hydrogen fusion is shutting down, if it has not done so already. The star will shortly turn into a magnificent red supergiant far more luminous than nearby (on the sky) Betelgeuse, its only fate someday to explode.
- - Written by Jim Kaler