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 1,200 light-years distant. It is estimated to be 419,600 times as luminous as the Sun, and 40 times as massive.
ObservationEdit
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.<ref name=mkk/> Although the spectrum shows variations, particular in the H-alpha absorption lines, this is considered typical for this type of luminous hot supergiant.<ref name=puebla/> 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.<ref name=GCVSQF/> Its spectrum also varies, possibly due to unpredictable changes in mass loss from the surface.<ref name=puebla/>
Physical characteristicsEdit
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 (Template:Solar luminosity), an effective temperature of Template:Val and a radius 24 times that of the Sun (Template:Solar radius).<ref name="crowther"/> 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.<ref name="Searle 2008"/> 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.<ref name="Voss 2010"/>
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.<ref name=hipparcos/> Adopting the larger parallax from the original Hipparcos reduction gives a distance of 412 parsecs<ref name=perryman/> 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.<ref name="puebla"/> Using precalculated models, a 2020 study found smaller values for luminosity (Template:Solar luminosity), radius (Template:Solar radius), and mass (Template:Solar mass).<ref name="zsargo2020"/> Another spectroscopic distance modulus of 7.79 imply a distance of 361 parsecs.<ref name=oplistilova2023/>
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,<ref>Template:Cite journal</ref> 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.<ref name="crowther"/>
Nomenclature and historyEdit
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:<ref name=allen/> all three variants are evidently mistakes in transliteration or copy errors, the first perhaps due to confusion with النيلم al-nilam 'the sapphire'.<ref name=Knobel/> In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)<ref name="WGSN"/> to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016<ref name="WGSN1"/> 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.<ref name="IAU-CSN"/>
Orion's BeltEdit
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.<ref name=allen/>
In Chinese, {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}), 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.<ref name=zh/> It is one of the western mansions of the White Tiger. Consequently, the Chinese name for Alnilam is {{#invoke:Lang|lang}} ({{#invoke:Lang|lang}}, Template:Langx).<ref name=AEEA/>