Alnilam or Epsilon Orionis (Eps Ori) is the 4th brightest naked eye star in the constellation Orion. With an apparent magnitude of 1.69v, Alnilam is the 29th brightest star in the entire sky (see: 50 Brightest Stars ). Its absolute magnitude is -6.38 and its distance is 1342 light years. The Equinox J2000 equatorial coordinates are RA = 05h 36m 12.8s, Dec = -01° 12' 07".

Alnilam has a spectral type of B0Ia, a surface temperature of 25,000° Kelvin and a luminosity 375,000 times the Sun. It has a mass of 40 solar masses and a diameter 26 times the Sun.

The image above shows the uncropped view of Alnilam (North is up) through the Takahashi E-180 Astrograph.

Alnilam is a large blue-white supergiant (spectral type B0 Iab) and is the most powerful bright star in the sky. Together with Alnitak, and Mintaka, the three stars make up the Belt of Orion, known by many names across many ancient cultures. Alnilam is the middle star in the belt.

It is also one of the 57 stars used in celestial navigation. Alnilam 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.

The description above is based on the Alnilam entry in Wikipedia. For more information about Alnilam, see Stars (Jim Kaler).

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