Alnilam is the central

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

Orion's Belt Orion's Belt is an asterism in the constellation of Orion. Other names include the Belt of Orion, the Three Kings, and the Three Sisters. The belt consists of three bright and easily identifiable collinear star systems – Alnitak, Alnilam, ...

in the equatorial

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

of Orion. It has the

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

ε Orionis, which is Latinised to Epsilon Orionis and abbreviated Epsilon Ori or ε Ori. This is a massive,

blue supergiant A blue supergiant (BSG) is a hot, luminous star, often referred to as an OB supergiant. They are usually considered to be those with luminosity class I and spectral class B9 or earlier, although sometimes A-class supergiants are also deemed blu ...

star some 1,200

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s distant. It is estimated to be 419,600 times as luminous as the Sun, and 40 times as massive.

Observation

It is the 29th-brightest star in the sky (the fourth brightest in Orion) and is a

. Together with

Mintaka Mintaka , designation Delta Orionis (δ Orionis, abbreviated Delta Ori, δ Ori) and 34 Orionis (34 Ori), is a quintuple star system some 1,200 light-years from the Sun in the constellation of Orion. Together with Alnitak (Zeta ...

and Alnitak, the three stars make up

, known by many names across many ancient cultures. Alnilam is the middle star. Since 1943, the

spectrum A spectrum (: spectra or spectrums) is a set of related ideas, objects, or properties whose features overlap such that they blend to form a continuum. The word ''spectrum'' was first used scientifically in optics to describe the rainbow of co ...

of this star has served as one of the stable anchor points by which other stars are classified, for the spectral class B0Ia. Although the spectrum shows variations, particular in the

H-alpha Hydrogen-alpha, typically shortened to H-alpha or Hα, is a deep-red visible spectral line of the hydrogen atom with a wavelength of 656.28 nm in air and 656.46 nm in vacuum. It is the first spectral line in the Balmer series and is em ...

absorption line Absorption spectroscopy is spectroscopy that involves techniques that measure the absorption (electromagnetic radiation), absorption of electromagnetic radiation, as a function of frequency or wavelength, due to its interaction with a sample. Th ...

s, this is considered typical for this type of luminous hot supergiant. It is also one of the 58 stars used in

celestial navigation Celestial navigation, also known as astronavigation, is the practice of position fixing using stars and other celestial bodies that enables a navigator to accurately determine their actual current physical position in space or on the surface ...

. 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 Alpha Cygni variables are variable stars which exhibit non-radial pulsations, meaning that some portions of the stellar surface are contracting at the same time other parts expand. They are supergiant stars of Stellar classification, spectral types ...

. Its spectrum also varies, possibly due to unpredictable changes in mass loss from the surface.

Physical characteristics

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 (), an effective temperature of and a radius 24 times that of the Sun (). Searle and colleagues, using CMFGEN code to analyse the spectrum in 2008, calculated a luminosity of , an effective temperature of 27,500 ± 100 K and a radius of . Analysis of the spectra and age of the members of the Orion OB1 association yields a mass 34.6 times that of the Sun ( on the

main sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...

) and an age of 5.7 million years. A more recent detailed analysis of Alnilam across multiple wavelength bands produced very high luminosity, radius, and mass estimates, assuming the distance of 606

parsec The parsec (symbol: pc) is a unit of length used to measure the large distances to astronomical objects outside the Solar System, approximately equal to or (AU), i.e. . The parsec unit is obtained by the use of parallax and trigonometry, and ...

s suggested by the ''Hipparcos'' new reduction. Adopting the larger parallax from the original ''Hipparcos'' reduction gives a distance of 412 parsecs and physical parameters more consistent with earlier publications. The luminosity of and the mass of at 606 parsecs is the highest ever derived for this star. Using precalculated models, a 2020 study found smaller values for luminosity (), radius (), and mass (). Another spectroscopic

distance modulus The distance modulus is a way of expressing distances that is often used in astronomy. It describes distances on a logarithmic scale based on the astronomical magnitude system. Definition The distance modulus \mu=m-M is the difference between th ...

of 7.79 imply a distance of 361 parsecs. Alnilam's relatively simple spectrum has made it useful for studying the

interstellar medium The interstellar medium (ISM) is the matter and radiation that exists in the outer space, space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as cosmic dust, dust and cosmic rays. It f ...

. Within the next million years, this star may turn into a Wolf-Rayet star and explode as a

supernova A supernova (: supernovae or supernovas) is a powerful and luminous explosion of a star. A supernova occurs during the last stellar evolution, evolutionary stages of a massive star, or when a white dwarf is triggered into runaway nuclear fusion ...

. Alnilam's high mass means that due to high mass loss, it will not become a

red supergiant Red supergiants (RSGs) are stars with a supergiant luminosity class ( Yerkes class I) and a stellar classification K or M. They are the largest stars in the universe in terms of volume, although they are not the most massive or luminous. Betelg ...

star, and will likely leave behind a

black hole A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...

instead of a

neutron star A neutron star is the gravitationally collapsed Stellar core, core of a massive supergiant star. It results from the supernova explosion of a stellar evolution#Massive star, massive star—combined with gravitational collapse—that compresses ...

. It is surrounded by a

molecular cloud A molecular cloud—sometimes called a stellar nursery if star formation is occurring within—is a type of interstellar cloud of which the density and size permit absorption nebulae, the formation of molecules (most commonly molecular hydrogen, ...

, NGC 1990, which it illuminates to make a

reflection nebula In astronomy, reflection nebulae are interstellar cloud, clouds of Cosmic dust, interstellar dust which might reflect the light of a nearby star or stars. The energy from the nearby stars is insufficient to Ionization, ionize the gas of the nebu ...

. Its

stellar wind A stellar wind is a flow of gas ejected from the stellar atmosphere, upper atmosphere of a star. It is distinguished from the bipolar outflows characteristic of young stars by being less collimated, although stellar winds are not generally spheri ...

s may reach up to 2,000 km/s, causing it to lose

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

about 20 million times more rapidly than the Sun.

Nomenclature and history

''ε Orionis'' is the star's

and ''46 Orionis'' its

Flamsteed designation A Flamsteed designation is a combination of a number and constellation name that uniquely identifies most naked eye stars in the 88 modern constellations, modern constellations visible from southern England. They are named after John Flamsteed, au ...

. The traditional name ''Alnilam'' derives from the

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

النظام ''al-niẓām'' 'arrangement/string (of pearls)'. Related spellings are ''Alnihan'' and ''Alnitam'': all three variants are evidently mistakes in transliteration or copy errors, the first perhaps due to confusion with النيلم ''al-nilam'' 'the

sapphire Sapphire is a precious gemstone, a variety of the mineral corundum, consisting of aluminium oxide () with trace amounts of elements such as iron, titanium, cobalt, lead, chromium, vanadium, magnesium, boron, and silicon. The name ''sapphire ...

'. In 2016, the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016 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.

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 Chinese mythology () is mythology that has been passed down in oral form or recorded in literature throughout the area now known as Greater China. Chinese mythology encompasses a diverse array of myths derived from regional and cultural tradit ...

, they were also known as the Weighing Beam. In Chinese, (), meaning " three stars," refers to the asterism consisting of Orion's belt (Alnilam, Alnitak, and

), with four other stars (

Betelgeuse Betelgeuse is a red supergiant star in the constellation of Orion (constellation), Orion. It is usually the List of brightest stars, tenth-brightest star in the night sky and, after Rigel, the second brightest in its constellation. It i ...

, Bellatrix, Saiph, and

Rigel Rigel is a blue supergiant star in the constellation of Orion. It has the Bayer designation β Orionis, which is Latinized to Beta Orionis and abbreviated Beta Ori or β Ori. Rigel is the brightest and most massive componentand ...

) later added. It is one of the western mansions of the

White Tiger The white tiger (ashy tiger) is a leucistic morph of the tiger, typically the Bengal tiger. It is occasionally reported in the Indian wilderness. It has the typical black stripes of a tiger, but its coat is otherwise white or near-white, and ...

. Consequently, the

Chinese name Chinese may refer to: * Something related to China * Chinese people, people identified with China, through nationality, citizenship, and/or ethnicity **Han Chinese, East Asian ethnic group native to China. **'' Zhonghua minzu'', the supra-ethni ...

for Alnilam is (, ).

Notes

References

External links

* {{DEFAULTSORT:Alnilam B-type supergiants Alpha Cygni variables Orion (constellation) Orionis, Epsilon

1903 Events January * January 1 – Edward VII is proclaimed Emperor of India. * January 10 – The Aceh Sultanate was fully annexed by the Dutch forces, deposing the last sultan, marking the end of the Aceh War that have lasted for al ...

BD-01 0969 Orionis, 46 037128 026311 Alnilam