Alnilam

Alnilam (Epsilon Orionis, ε Orionis) is a large blue supergiant star in the constellation of Orion. Its Flamsteed designation is 46 Orionis.

It is the 30th brightest star in the sky (the 4th brightest in Orion) and is a blue-white supergiant. Together with Mintaka and Alnitak, the three stars make up the belt of Orion, known by many names across many ancient cultures. Alnilam is the middle star.

Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified. It is also one of the 57 stars used in celestial navigation. It is at its highest point in the sky around midnight on December 15.

Alnilam's relatively simple spectrum has made it useful for studying the interstellar medium. Within the next million years, this star may turn into a red supergiant and explode as a supernova. 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.