Alnilam

Alnilam is the central star of Orion's Belt in the equatorial constellation of Orion. It has the Bayer designation ε Orionis, which is Latinised to Epsilon Orionis and abbreviated Epsilon Ori or ε Ori. This is a massive, blue supergiant star some 1,200 light-years distant. It is estimated to be 419,600 times as luminous as the Sun, and 40 times as massive.

Alnilam

Location of Alnilam (circled)
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−B color index	−1.03[3]
B−V color 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	1,180 ly (361 pc)[7][a]
Absolute magnitude (MV)	−6.89[8]
Details[9]
Mass	40 M☉
Radius	30.62 R☉
Luminosity	419,600 L☉
Surface gravity (log g)	3.02 cgs
Temperature	26,540 K
Rotational velocity (v sin i)	80 km/s
Age	4.47 Myr
Other designations
Alnilam, ε Ori, 46 Orionis, Orionis, BD−01°969, FK5 210, HD 37128, HIP 26311, HR 1903, SAO 132346, TD1 4963, 參宿二
Database references
SIMBAD	data

Observation

Alnilam is the middle and brightest of the three stars of Orion's Belt.

It is the 29th-brightest star in the sky (the fourth brightest in Orion) and is a blue supergiant. Together with Mintaka and Alnitak, the three stars make up Orion's Belt, 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, for the spectral class B0Ia.^[5] Although the spectrum shows variations, particular in the H-alpha absorption lines, this is considered typical for this type of luminous hot supergiant.^[10] It is also one of the 58 stars used in celestial 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 an α Cygni variable.^[11] Its spectrum also varies, possibly due to unpredictable changes in mass loss from the surface.^[10]

Physical characteristics

A blue band light curve for Epsilon Orionis, adapted from Krtička and Feldmeier (2018)^[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 of 27,000 K and a radius 24 times that of the Sun (R_☉).^[13] 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 ± 100 K 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 mass 34.6 times that of the Sun (40.8 M_☉ on the main sequence) and an age of 5.7 million years.^[14]

A more recent detailed analysis of Alnilam across multiple wavelength bands produced very high luminosity, radius, and mass estimates, assuming the distance of 606 parsecs suggested by the Hipparcos new reduction.^[2] Adopting the larger parallax from the original Hipparcos reduction gives a distance of 412 parsecs^[15] 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.^[10] Using precalculated models, a 2020 study found smaller values for luminosity (420,000 L_☉), radius (30.61 R_☉), and mass (40 M_☉).^[9] Another spectroscopic distance modulus of 7.79 imply a distance of 361 parsecs.^[7]

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 Wolf-Rayet star and explode as a supernova. Alnilam's high mass means that due to high mass loss, it will not become a red supergiant star,^[16] and will likely leave behind a black hole instead of a neutron star. It is surrounded by a molecular cloud, NGC 1990, which it illuminates to make a reflection nebula. Its stellar winds may reach up to 2,000 km/s, causing it to lose mass about 20 million times more rapidly than the Sun.^[13]

Nomenclature and history

ε Orionis is the star's Bayer designation and 46 Orionis its Flamsteed designation.

The traditional name Alnilam derives from the Arabic النظام al-niẓām 'arrangement/string (of pearls)'. Related spellings are Alnihan and Alnitam:^[17] all three variants are evidently mistakes in transliteration or copy errors, the first perhaps due to confusion with النيلم al-nilam 'the sapphire'.^[18] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[19] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016^[20] included a table of the first two batches of names approved by the WGSN; which included Alnilam for this star. It is now so entered in the IAU Catalog of Star Names.^[21]

Orion's Belt

Main article: Orion's Belt

The three belt stars were collectively known by many names in many cultures. Arabic terms include Al 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'). In Chinese mythology, they were also known as the Weighing Beam.^[17]

In Chinese, 參宿 (Shēn Sù), meaning Three Stars (asterism), refers to an asterism consisting of Alnilam, Alnitak and Mintaka (Orion's Belt), with Betelgeuse, Bellatrix, Saiph and Rigel later added.^[22] Consequently, the Chinese name for Alnilam is 參宿二 (Shēn Sù èr, English: the Second Star of Three Stars).^[23] It is one of the western mansions of the White Tiger.

See also

• List of most massive stars

Notes

1. Calculated, using a distance modulus of 7.79 in the following equation:
   $${\displaystyle d=10^{{\frac {\mu }{5}}+1}}$$. This yields a value of 361 parsecs.