  | |
| Observation data Epoch J2000 Equinox J2000 | |
|---|---|
| Constellation | Orion |
| Pronunciation | /ælˈnaɪlæm/[1] |
| Right ascension | 05h 36m 12.81s[2] |
| Declination | −01° 12′ 06.9″[2] |
| Apparent magnitude (V) | 1.69[3](1.64 – 1.74[4]) |
| Characteristics | |
| Evolutionary stage | Blue supergiant |
| Spectral type | B0 Ia[5] |
| U−Bcolor index | −1.03[3] |
| B−Vcolor index | −0.18[3] |
| Variable type | α Cygni[4] |
| Astrometry | |
| Radial velocity (Rv) | 25.9[6] km/s |
| Proper motion (μ) | RA: 1.49[2]mas/yr Dec.: −1.06[2]mas/yr |
| Parallax (π) | 1.65±0.45 mas[2] |
| Distance | 1,250±26 ly (384±8 pc)[7] |
| Absolute magnitude (MV) | −6.89[8] |
| Details[7] | |
| Mass | 28.4±2.0 M☉ |
| Radius | 33.6±1.8 (equatorial)R☉ 22.3±1.2 (polar) R☉ |
| Luminosity | 271,000±38,000 L☉ |
| Surface gravity (log g) | 3.01±0.10 cgs |
| Temperature | 25,000±1,000 K |
| Rotation | 4.27+1.00 −0.00 days |
| Rotational velocity (v sin i) | 220+40 −100 km/s |
| Age | 4.47[9] Myr |
| Other designations | |
| Alnilam, TD1 4963,ε Ori,46 Orionis,Orionis,BD−01°969,FK5 210,HD 37128,HIP 26311,HR 1903,SAO 132346,[10]參宿二[11] | |
| Database references | |
| SIMBAD | data |
Alnilam is the centralstar ofOrion's Belt in theequatorialconstellation ofOrion. It has theBayer designationε Orionis, which isLatinised toEpsilon Orionis and abbreviatedEpsilon Ori orε Ori. This is a massive,blue supergiant star some 1,250 light-years distant. It is estimated to be 270,000 times asluminous as the Sun, and 28 timesas massive.
It is the29th-brightest star in the sky (the fourth brightest in Orion) and is ablue supergiant. Together withMintaka andAlnitak, the three stars make upOrion's Belt, known by many names across many ancient cultures. Alnilam is the middle star.
Since 1943, thespectrum of this star has served as one of the stable anchor points by which other stars are classified, for the spectral class B0Ia.[5] Although the spectrum shows variations, particular in theH-alphaabsorption lines, this is considered typical for this type of luminous hot supergiant.[12] It is also one of the58 stars used incelestial navigation. It is at its highest point in the sky around midnight on December 15.
It is slightly variable from magnitude 1.64 to 1.74, with no clear period, and it is classified as anα Cygni variable.[13] Its spectrum also varies, possibly due to unpredictable changes in mass loss from the surface.[12]
Estimates of Alnilam's properties vary. Crowther and colleagues, using stellar wind and atmospheric modelling in 2006, came up with a luminosity 275,000 times that of the Sun (L☉), an effective temperature of27,000 K and a radius 24 times that of the Sun (R☉).[15] Searle and colleagues, using CMFGEN code to analyse the spectrum in 2008, calculated a luminosity of 537,000 L☉, an effective temperature of 27,500 ± 100K and a radius of 32.4 ± 0.75 R☉.[8] Analysis of the spectra and age of the members of the Orion OB1 association yields a current mass 34.6 times that of the Sun, initial mass of 40.8 M☉, and an age of 5.7 million years.[16]
A more detailed analysis from 2015 across multiple wavelength bands produced very high luminosity, radius, and mass estimates, assuming the distance of 606parsecs suggested by theHipparcos new reduction.[2] Adopting the larger parallax from the originalHipparcos reduction gives a distance of 412 parsecs[17] and physical parameters more consistent with earlier publications. The luminosity of 832,000 L☉ and the mass of 64.5 M☉ at 606 parsecs is the highest ever derived for this star.[12] However, the Hipparcos distance later turned out to be incorrect.[7] Using precalculated models, a 2020 study found smaller values for luminosity (420,000 L☉), radius (30.61 R☉), and mass (40 M☉).[9] Anotherspectroscopicdistance modulus of 7.79 imply a distance of 361 parsecs.[18]
Interferometric observations in 2025 with theVery Large Telescope have uncovered that Alnilam is a fast rotator being viewed at low inclination. As of such, it has an oblate shape, with a pronounced equatorial circumference that is 51% larger than the polar circumference, which at a distance of384±8 pc results in polar and equatorial radii of 22.3 and 33.6 R☉. Rotating close to itsbreakup velocity, Alnilam's fast rotation suggests it formed after themerger of two stars in abinary system.[7]
Alnilam's relatively simple spectrum has made it useful for studying theinterstellar medium. It is surrounded by amolecular cloud,NGC 1990, which it illuminates to make areflection nebula. Itsstellar winds may reach up to 2,000km/s, causing it to losemass about 20 million times more rapidly than the Sun.[15]
ε Orionis is the star'sBayer designation and46 Orionis itsFlamsteed designation.
The traditional nameAlnilam derives from theArabic النظامal-niẓām 'arrangement/string (of pearls)'. Related spellings areAlnihan andAlnitam:[19] all three variants are evidently mistakes in transliteration or copy errors, the first perhaps due to confusion with النيلمal-nilam 'thesapphire'.[20] In 2016, theInternational Astronomical Union organized aWorking Group on Star Names (WGSN)[21] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[22] included a table of the first two batches of names approved by the WGSN; which includedAlnilam for this star. It is now so entered in the IAU Catalog of Star Names.[23]
The three belt stars were collectively known by many names in many cultures. Arabic terms includeAl Nijād ('the Belt'),Al Nasak ('the Line'),Al Alkāt ('the Golden Grains or Nuts') and, in modern Arabic,Al Mīzān al H•akk ('the Accurate Scale Beam'). InChinese mythology, they were also known as the Weighing Beam.[19]
InChinese,參宿 (Shēn Sù), meaning "three stars," refers to theasterism consisting of Orion's belt (Alnilam,Alnitak, andMintaka), with four other stars (Betelgeuse,Bellatrix,Saiph, andRigel) later added.[24] It is one of the western mansions of theWhite Tiger. Consequently, theChinese name for Alnilam is參宿二 (Shēn Sù èr, English:the Second Star of the Three Stars).[11]
