Alnilam lights up NGC 1990. Photograph by Glen Youman.
It is also one of the 57 stars used in celestial navigation. For middle latitudes, it is at its highest point in the sky around 10 PM on December 10.
Alnilam's relatively simple spectrum has made it useful for studying the interstellar medium.
Other names and history
The name Alnilam derives from the ArabicAl Nitham or Al Nathm, which translates as "the String of Pearls". Related names also used to describe the star are Alnihan and Alnitam.
Various other names have been used to refer to Orion's belt, the line of three stars formed by Alnitak, Alnilam, and Mintaka. They include:
Alnilam will eventually turn into red supergiant. Astronomers say that it then explode as a supernova, and this is expected to take place within the next million years. It is surrounded by a molecular cloud, NGC 1990, which it brightens to make a reflection nebula. Its stellar winds may reach up to 2000 km/s, causing it to lose mass about 20 million times more rapidly than the Sun.
Alnilam is a blue-white supergiant that for many years has served as a standard star against which to compare others.
Like most supergiants, Alnilam is rapidly shedding mass: a powerful stellar wind blowing from its surface at speeds up to 2,000 km/s and carrying way about two millionths of a solar mass per year (20 million times the rate lost by the Sun).
Though only about four million years old, Alnilam is already fusing heavy elements in its core and doomed to explode, in the next million years or so, as a supernova.
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