Alnilam (Epsilon Orionis)
Alnilam (Epsilon Orionis) is a star in the constellation Orion, magnitude 1.7.
The middle pearl of Orion’s Belt — a blue supergiant hundreds of thousands of times brighter than the Sun.
Alnilam is one of the most powerful stars you can see without a telescope. Estimates of its output range from about 275,000 to more than 500,000 times the Sun’s — its exact distance is hard to pin down. It is a blue supergiant star of roughly 30 to 40 solar masses, with a surface near 27,000 K, and only a few million years old.
In 2025 observations with the Very Large Telescope found it spinning close to its break-up speed, flattened so that it is 51% bigger around the equator than over the poles. Such a fast spin suggests that Alnilam was born when two stars merged. Its wind blows at up to 2,000 km/s, carrying off mass about 20 million times faster than the Sun loses it.
Its name comes from the Arabic al-niẓām, ‘the string of pearls’ — the Belt itself. Arab astronomers also called the Belt ‘the golden grains’; in China it was a weighing beam.
Alnilam (Epsilon Orionis) is 1,300 light-years away: the light we see from it set off around AD 730, 75 years before Charlemagne was crowned emperor.
Did you know?
- Lighting up a cloud. Alnilam is wrapped in a molecular cloud, NGC 1990, which it lights up as a faint reflection nebula.
- A standard for blue supergiants. Since 1943 Alnilam’s spectrum has been one of the fixed reference points for classifying stars — the standard for type B0Ia.
The middle and brightest of the three stars of Orion’s Belt, a hot blue-white supergiant. Compare it with Alnitak and Mintaka on either side. Binoculars show the Belt set in a loose spray of fainter stars, the Collinder 70 cluster.
When and where to see it
It is best placed in the evening sky around January, when it crosses the meridian at about 10 pm local time (around midnight in December). It can be seen from everywhere south of 89°N.
At its highest it stands 39° up from 50°N, 54° up from 35°N, 89° up from the equator, 56° up from 35°S.
Facts
- Type
- Star
- Constellation
- Orion
- Designations
- Epsilon Orionis, ε Ori, 46 Ori, HR 1903, HD 37128, HIP 26311
- Magnitude
- 1.7
- Position
- RA 5h 36.2m, Dec −1° 12′ (J2000)
- Spectral type
- B0Ia
- Colour
- blue-white (B−V -0.18)
- Distance
- 1,300 light-years
- Discovered
- Known since antiquity
Open AstroPilot to see when Alnilam (Epsilon Orionis) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view.