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Alnilam lights up NGC 1990.
Photograph by Glen Youman
Observation data
Epoch J2000      Equinox J2000
Constellation Orion
Right ascension 05h 36m 12.8s[1]
Declination −01° 12′ 06.9″[1]
Apparent magnitude (V) 1.69[2] (1.64 - 1.74[3])
Spectral type B0 Ia[4]
U−B color index −1.03[2]
B−V color index −0.18[2]
Variable type α Cygni[3]
Radial velocity (Rv) 25.9[5] km/s
Proper motion (μ) RA: 1.49[1] mas/yr
Dec.: −1.06[1] mas/yr
Parallax (π) 1.65 ± 0.45[1] mas
Distance approx. 2,000 ly
(approx. 600 pc)
Absolute magnitude (MV) −6.95[6]
Mass 30-64.5[7] M
Radius 28.6-42.0[7] R
Luminosity 389,000-832,000[7] L
Surface gravity (log g) 3.0[7] cgs
Temperature 27,000[7][8] K
Rotational velocity (v sin i) 40-70[7] km/s
Age 5.7[9] Myr
Other designations
Alnilam, ε Ori, 46 Orionis, 112 G Orionis, HR 1903, BD -01°969, HD 37128, SAO 132346, FK5 210, HIP 26311, TD1 4963, 参宿二
Database references

Alnilam, designated Epsilon Orionis (ε Orionis, abbreviated Epsilon Ori, ε Ori) and 46 Orionis (46 Ori), is a large blue supergiant star some 2,000 light-years distant in the constellation of Orion. It is estimated to be 275,000 to 537,000 times as luminous as the Sun, and around 34 times as massive.


It is the 29th-brightest star in the sky (the 4th-brightest in Orion) and is a blue-white supergiant. Together with Mintaka and Alnitak, the three stars make up the belt of Orion, known by many names across many ancient cultures. Alnilam is the middle star. It is slightly variable, from magnitude 1.64 to 1.74. Since 1943, the spectrum of this star has served as one of the stable anchor points by which other stars are classified.[4] 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. 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.[citation needed] It is surrounded by a molecular cloud, NGC 1990, which it illuminates 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.[8]


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), and effective temperature of 27,000 K and a radius 24 times that of the Sun (R).[8] 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.[6] 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.[9]

A more recent detailed analysis of Alnilam across multiple wavelength bands produces very high luminosity, radius, and mass estimates, assuming the distance of 606 pc suggested by the Hipparcos new reduction.[1] Adopting the larger parallax from the original Hipparcos reduction gives a distance of 412 pc[10] and physical parameters more consistent with earlier publications. The luminosity of 863,000 L at 606 parsecs is the highest ever derived for this star.[7]

Nomenclature and history[edit]

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

The traditional name Alnilam derives from the Arabic النيلم Al-nilam, related to the word 'nilam' 'sapphire'. Related spellings are Alnihan and Alnitam:[11] all three variants are evidently mistakes in transliteration or copy errors.[12] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[13] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[14] 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.[15]

Orion's Belt[edit]

Main article: Orion's Belt
Alnilam is the middle of the three stars in the 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.[11] The belt was also the Three Stars mansion (simplified Chinese: 参宿; traditional Chinese: 參宿; pinyin: Shēn Xiù), one of the Twenty-eight mansions of the Chinese constellations. It is one of the western mansions of the White Tiger.

In pre-Christian Scandinavia, the belt was known as Frigg's Distaff (Friggerock) or Freyja's distaff.[16] Similarly Jacob's Staff and Peter's Staff were European biblical derived terms, as were the Three Magi, or the Three Kings. Väinämöinen's Scythe (Kalevala) and Kalevan Sword are terms from Finnish mythology.[11]

The Seri people of northwestern Mexico call the three belt stars Hapj (a name denoting a hunter) which consists of three stars: Hap (mule deer), Haamoja (pronghorn), and Mojet (bighorn sheep). Hap is in the middle and has been shot by the hunter; its blood has dripped onto Tiburón Island.[17]

See also[edit]


  1. ^ a b c d e f van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy & Astrophysics. 474 (2): 653–664. arXiv:0708.1752Freely accessible. Bibcode:2007A&A...474..653V. doi:10.1051/0004-6361:20078357. 
  2. ^ a b c Ducati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system". CDS/ADC Collection of Electronic Catalogues. 2237. Bibcode:2002yCat.2237....0D. 
  3. ^ a b Ruban, E. V.; Alekseeva, G. A.; Arkharov, A. A.; Hagen-Thorn, E. I.; Galkin, V. D.; Nikanorova, I. N.; Novikov, V. V.; Pakhomov, V. P.; Puzakova, T. Yu. (September 2006). "Spectrophotometric observations of variable stars". Astronomy Letters. 32 (9): 604–607. Bibcode:2006AstL...32..604R. doi:10.1134/S1063773706090052. 
  4. ^ a b Morgan, W. W.; Keenan, Philip C.; Kellman, Edith (1943). "AN ATLAS OF STELLAR SPECTRA". Astrophysics Monographs. 
  5. ^ Gontcharov, G. A. (November 2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system". Astronomy Letters. 32 (11): 759–771. Bibcode:2006AstL...32..759G. doi:10.1134/S1063773706110065. 
  6. ^ a b Searle, S. C.; Prinja, R. K.; Massa, D.; Ryans, R. (2008). "Quantitative studies of the optical and UV spectra of Galactic early B supergiants. I. Fundamental parameters". Astronomy and Astrophysics. 481 (3): 777–97. arXiv:0801.4289Freely accessible. Bibcode:2008A&A...481..777S. doi:10.1051/0004-6361:20077125. 
  7. ^ a b c d e f g Raul E. Puebla; D. John Hillier; Janos Zsargó; David H. Cohen; Maurice A. Leutenegger (2015). "X-ray, UV and optical analysis of supergiants: ε Ori". Monthly Notices of the Royal Astronomical Society. 456 (3): 2907. arXiv:1511.09365Freely accessible. Bibcode:2016MNRAS.456.2907P. doi:10.1093/mnras/stv2783. 
  8. ^ a b c Crowther, P. A.; Lennon, D. J.; Walborn, N. R. (January 2006). "Physical parameters and wind properties of galactic early B supergiants". Astronomy & Astrophysics. 446 (1): 279–293. arXiv:astro-ph/0509436Freely accessible. Bibcode:2006A&A...446..279C. doi:10.1051/0004-6361:20053685. 
  9. ^ a b Voss, R.; Diehl, R.; Vink, J. S.; Hartmann, D. H. (2010). "Probing the evolving massive star population in Orion with kinematic and radioactive tracers". Astronomy and Astrophysics. 520: 10. arXiv:1005.3827Freely accessible. Bibcode:2010A&A...520A..51V. doi:10.1051/0004-6361/201014408. A51. 
  10. ^ Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; Hoeg, E.; Bastian, U.; Bernacca, P. L.; Crézé, M.; Donati, F.; Grenon, M.; Grewing, M.; Van Leeuwen, F.; Van Der Marel, H.; Mignard, F.; Murray, C. A.; Le Poole, R. S.; Schrijver, H.; Turon, C.; Arenou, F.; Froeschlé, M.; Petersen, C. S. (1997). "The HIPPARCOS Catalogue". Astronomy and Astrophysics. 323: L49. Bibcode:1997A&A...323L..49P. 
  11. ^ a b c Allen, Richard Hinckley (1936). Star-names and their meanings. pp. 314–315. 
  12. ^ Knobel, E. B. (September 1909). "The name of epsilon Orionis". The Observatory. 32: 357. Bibcode:1909Obs....32..357K. 
  13. ^ "IAU Working Group on Star Names (WGSN)". Retrieved 22 May 2016. 
  14. ^ "Bulletin of the IAU Working Group on Star Names, No. 1" (PDF). Retrieved 28 July 2016. 
  15. ^ "IAU Catalog of Star Names". Retrieved 28 July 2016. 
  16. ^ Schön, Ebbe (2004). Asa-Tors hammare, Gudar och jättar i tro och tradition. Stockholm: Hjalmarson & Högberg. p. 228. ISBN 9189660412. 
  17. ^ Moser, Mary B.; Stephen A. Marlett (2005). Comcáac quih yaza quih hant ihíip hac: Diccionario seri-español-inglés (PDF) (in Spanish and English). Hermosillo, Sonora and Mexico City: Universidad de Sonora and Plaza y Valdés Editores. 

External links[edit]

Coordinates: Sky map 05h 36m 12.8s, −01° 12′ 06.9″