Alnilam, also known as ε Orionis or 46 Orionis, is a large blue star in the constellation of Orion. It is the 30th brightest star in the sky, the 4th brightest in Orion, and a blue-white supergiant. It is the middle star of Orion's belt.

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 Arabic النظام an-niżām, related to the word نظم nażm "string of pearls". Related spellings are Alnihan and Alnitam: all three variants are evidently mistakes in transliteration or copy errors.

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:

• The Weighing Beam (Chinese mythology)
• Jacob's Staff, Peter's Staff, the Three Magi, or the Three Kings (the Bible)
• Freya's Staff (Norse mythology)

These stars also influenced the construction of the Pyramids of Giza.

Physical characteristics

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.