Alnilam
What is Alnilam?
Stars· Distance ~2,000 ly · size 30× Sun. The blue supergiant at the centre of Orion's Belt — the name means 'string of pearls' in Arabic. It is the most distant of the three Belt stars, yet matches them in brightness thanks to some 400,000 times the Sun's luminosity; the easiest signpost for finding Orion on winter nights.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
Where Alnilam is in your sky right now: https://nowsky.org/solar/sky?sel=alnilam — location and time set automatically, no login.
Distance and spectral type
Alnilam is , in the constellation Orion. Its apparent magnitude is 1.7, visible to the naked eye.
Stellar figures
| Distance | ~2,000 ly |
|---|---|
| Magnitude | 1.7 |
| Type | blue supergiant B0Ia |
| Constellation | Orion (the Belt) |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Behind the name
Arabic "string of pearls" — the middle belt star, the farthest (~2,000 ly) and most luminous of the three.
Frequently asked
- How far is Alnilam from Earth?
- ~2,000 ly.
- How bright is Alnilam (apparent magnitude)?
- 1.7.
- What kind of object is Alnilam?
- blue supergiant B0Ia.
- Which constellation is Alnilam in?
- Orion.
- Where is Alnilam in the sky right now?
- Open the sky view for your location and time and it shows the position in the sky right now.
The nebula around Alnilam — is it there at all?
Catalogues list a reflection nebula, NGC 1990, around Alnilam. Herschel recorded it in 1786 as "faint light about the star", but modern deep exposures show no distinct nebula there. Recent assessments hold that what Herschel saw was the bright star's light spreading in his optics and the atmosphere, and that the nebula is absent or extremely faint — one of the few catalogued objects whose existence is in doubt.
That there is no nebula around Alnilam is natural enough. With a surface at 25,000 °C, the star ionises and pushes away the gas around it with intense ultraviolet light and a stellar wind, losing about a millionth of a solar mass to space each year. Unlike Alnitak, with the Flame and Horsehead nebulae pressed against it, the middle star of the belt shines in a space it has cleared for itself.
The star in depth
Alnilam is the middle star of Orion's Belt. A blue supergiant with a surface hotter than 25,000 K, it shines with about 400,000 times the Sun's luminosity — among the most powerful stars visible to the naked eye. At roughly 2,000 light-years it lies far beyond its neighbours Alnitak and Mintaka, yet its sheer output makes the three appear almost equally bright in a row.
The Belt stars are not a chance alignment; they belong to the young Orion OB1 association. Alnilam is one of its heavyweights at close to 40 solar masses, which means a short life and a supernova within a few million years. A faint reflection nebula lit by the star surrounds it. Sitting almost exactly on the celestial equator, it can be seen from every inhabited part of Earth.
On winter evenings find the three stars in a line in the southern sky; the central one is Alnilam. Select it in the Sky view to highlight all of Orion, and look just below the Belt for the sword — the Orion Nebula lives there.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.