Astronomy:Alnilam

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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 1,250 light-years distant. It is estimated to be 270,000 times as luminous as the Sun, and 28 times as massive.

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.[1] Although the spectrum shows variations, particular in the H-alpha absorption lines, this is considered typical for this type of luminous hot supergiant.[2] 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.[3] Its spectrum also varies, possibly due to unpredictable changes in mass loss from the surface.[2]

Physical characteristics

A blue band light curve for Epsilon Orionis, adapted from Krtička and Feldmeier (2018)[4]

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☉).[5] 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 current mass 34.6 times that of the Sun, initial mass of 40.8 M☉, and an age of 5.7 million years.[7]

A more detailed analysis from 2015 across multiple wavelength bands produced very high luminosity, radius, and mass estimates, assuming the distance of 606 parsecs suggested by the Hipparcos new reduction.[8] Adopting the larger parallax from the original Hipparcos reduction gives a distance of 412 parsecs[9] 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.[2] Using precalculated models, a 2020 study found smaller values for luminosity (420,000 L☉), radius (30.61 R☉), and mass (40 M☉).[10] Another spectroscopic distance modulus of 7.79 imply a distance of 361 parsecs.[11]

interferometric observations in 2025 with the Very 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 of 384±8 pc results in polar and equatorial radii of 22.3 and 33.6 R☉. Rotating close to its breakup velocity, its fast rotation suggest Alnilam formed after the merger of two stars in a binary system.[12]

Alnilam's relatively simple spectrum has made it useful for studying the interstellar medium. 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.[5]

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:[13] all three variants are evidently mistakes in transliteration or copy errors, the first perhaps due to confusion with النيلم al-nilam 'the sapphire'.[14] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[15] to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[16] 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.[17]

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.[13]

In Chinese, 參宿 (Shēn Sù), meaning "three stars," refers to the asterism consisting of Orion's belt (Alnilam, Alnitak, and Mintaka), with four other stars (Betelgeuse, Bellatrix, Saiph, and Rigel) later added.[18] It is one of the western mansions of the White Tiger. Consequently, the Chinese name for Alnilam is 參宿二 (Shēn Sù èr, English: the Second Star of the Three Stars).[19]

See also

Notes

References

  1. ↑ Morgan, W. W.; Keenan, Philip C.; Kellman, Edith (1943). "An Atlas of Stellar Spectra". Astrophysical Monographs 152 (3849): 147. doi:10.1038/152147a0. Bibcode: 1943Natur.152..147.. https://www.ucl.ac.uk/ucl-observatory/sites/ucl-observatory/files/mkkbook.pdf. Retrieved 2020-02-06. 
  2. ↑ 2.0 2.1 2.2 Puebla, R.E.; Hillier, D.J.; Zsargó, J.; Cohen, D.H.; Leutenegger, M.A. (2015). "X-ray, UV and optical analysis of supergiants: ϵ Ori". Monthly Notices of the Royal Astronomical Society 456 (3): 2907–2936. doi:10.1093/mnras/stv2783. Bibcode: 2016MNRAS.456.2907P. 
  3. ↑ "GCVS Query forms". http://www.sai.msu.su/gcvs/cgi-bin/search.cgi?search=eps+Ori. 
  4. ↑ Krtička, J.; Feldmeier, A. (September 2018). "Light variations due to the line-driven wind instability and wind blanketing in O stars". Astronomy & Astrophysics 617: A121. doi:10.1051/0004-6361/201731614. Bibcode: 2018A&A...617A.121K. https://www.aanda.org/articles/aa/pdf/2018/09/aa31614-17.pdf. Retrieved 11 August 2022. 
  5. ↑ 5.0 5.1 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. doi:10.1051/0004-6361:20053685. Bibcode: 2006A&A...446..279C. 
  6. ↑ 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. doi:10.1051/0004-6361:20077125. Bibcode: 2008A&A...481..777S. 
  7. ↑ 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. doi:10.1051/0004-6361/201014408. A51. Bibcode: 2010A&A...520A..51V. 
  8. ↑ van Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy & Astrophysics 474 (2): 653–664. doi:10.1051/0004-6361:20078357. Bibcode: 2007A&A...474..653V. 
  9. ↑ Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; Hoeg, E.; Bastian, U.; Bernacca, P. L.; Crézé, M.; Donati, F. et al. (1997). "The HIPPARCOS Catalogue". Astronomy and Astrophysics 323: L49. Bibcode: 1997A&A...323L..49P. 
  10. ↑ Zsargó, J.; Fierro-Santillán, C. R.; Klapp, J.; Arrieta, A.; Arias, L.; Valencia, J. M.; Sigalotti, L. Di G.; Hareter, M. et al. (2020-11-01). "Creating and using large grids of precalculated model atmospheres for a rapid analysis of stellar spectra". Astronomy and Astrophysics 643: A88. doi:10.1051/0004-6361/202038066. ISSN 0004-6361. Bibcode: 2020A&A...643A..88Z. 
  11. ↑ Oplištilová, A.; Mayer, P.; Harmanec, P.; Brož, M.; Pigulski, A.; Božić, H.; Zasche, P.; Šlechta, M. et al. (2023). "Spectrum of the secondary component and new orbital elements of the massive triple star δ Ori A". Astronomy and Astrophysics 672: A31. doi:10.1051/0004-6361/202245272. Bibcode: 2023A&A...672A..31O. 
  12. ↑ Oplištilová, Alžběta; Brož, Miroslav; Hummel, Christian A.; Harmanec, Petr; Barlow, Brad (2025-07-03). "VLTI observations of the Orion Belt stars: I. eps Orionis". arXiv:2507.02276 [astro-ph.SR].
  13. ↑ 13.0 13.1 Allen, Richard Hinckley (1936). Star-names and their meanings. pp. 314–315. 
  14. ↑ Knobel, E. B. (September 1909). "The name of epsilon Orionis". The Observatory 32: 357. Bibcode: 1909Obs....32..357K. 
  15. ↑ "IAU Working Group on Star Names (WGSN)". https://www.iau.org/science/scientific_bodies/working_groups/280/. 
  16. ↑ "Bulletin of the IAU Working Group on Star Names, No. 1". http://www.pas.rochester.edu/~emamajek/WGSN/WGSN_bulletin1.pdf. 
  17. ↑ "IAU Catalog of Star Names". http://www.pas.rochester.edu/~emamajek/WGSN/IAU-CSN.txt. 
  18. ↑ 陳久金 (2005) (in zh). 中國星座神話. 五南圖書出版股份有限公司. ISBN 978-986-7332-25-7. https://books.google.com/books?id=0Vex0rYzdu8C. Retrieved 2025-07-04. 
  19. ↑ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 25 日

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



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