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 2,000 light-years distant. It is estimated to be 419,600 times as luminous as the Sun, and 40 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. Searle and colleagues, using CMFGEN code to analyse the spectrum in 2008, calculated a luminosity of Template:Solar luminosity, an effective temperature of 27,500 ± 100 K and a radius of Template:Solar radius.[5] Analysis of the spectra and age of the members of the Orion OB1 association yields a mass 34.6 times that of the Sun (Template:Solar mass on the main sequence) and an age of 5.7 million years.[6] 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.[7] Adopting the larger parallax from the original Hipparcos reduction gives a distance of 412 parsecs[8] and physical parameters more consistent with earlier publications. The luminosity of Template:Solar luminosity and the mass of Template:Solar mass at 606 parsecs is the highest ever derived for this star.[2] Using precalculated models, a 2020 study found smaller values for luminosity (Template:Solar luminosity), radius (Template:Solar radius), and mass (Template:Solar mass).[9] Another spectroscopic distance modulus of 7.79 imply a distance of 361 parsecs.[10]

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,[11] 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.[12]

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

Template:Main article 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, Template:Lang (Template:Lang), 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.[18] Consequently, the Chinese name for Alnilam is Template:Lang (Template:Lang, Template:Langx).[19] It is one of the western mansions of the White Tiger.

See also

Notes

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References

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  1. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named mkk
  2. ↑ 2.0 2.1 2.2 Cite error: Invalid <ref> tag; no text was provided for refs named puebla
  3. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named GCVSQF
  4. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Krtička
  5. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Searle 2008
  6. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Voss 2010
  7. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named hipparcos
  8. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named perryman
  9. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named zsargo2020
  10. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named oplistilova2023
  11. ↑ Template:Cite journal
  12. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named crowther
  13. ↑ 13.0 13.1 Cite error: Invalid <ref> tag; no text was provided for refs named allen
  14. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named Knobel
  15. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named WGSN
  16. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named WGSN1
  17. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named IAU-CSN
  18. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named zh
  19. ↑ Cite error: Invalid <ref> tag; no text was provided for refs named AEEA
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