Alnilam is the central

star A star is an astronomical object comprising a luminous spheroid of plasma held together by its gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night, but their immense distances from Earth make ...

Orion's Belt Orion's Belt or the Belt of Orion, also known as the Three Kings or Three Sisters, is an asterism in the constellation Orion. It consists of the three bright stars Alnitak, Alnilam and Mintaka. Looking for Orion's Belt is the easiest way to lo ...

in the equatorial constellation of Orion. It has the

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. T ...

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 have luminosity class I and spectral class B9 or earlier. Blue supergiants are found towards the top left of the Hertzsprung–Russell diagram, above a ...

star some 2,000

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 101 ...

s distant. It is estimated to be 275,000 to 832,000 times as luminous as the Sun, and 40 to 44 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 multiple star system some 1,200 light-years from the Sun in the constellation of Orion. Together with Alnitak (Zeta Or ...

and

Alnitak Alnitak is a triple star system in the constellation of Orion. It has the designations ζ Orionis, which is Latinised to Zeta Orionis and abbreviated Zeta Ori or ζ Ori, and 50 Orionis, abbreviated 50 Ori. The system is located at a ...

, the three stars make up

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

spectrum A spectrum (plural ''spectra'' or ''spectrums'') is a condition that is not limited to a specific set of values but can vary, without gaps, across a continuum. The word was first used scientifically in optics to describe the rainbow of color ...

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 H-alpha (Hα) is a specific deep-red visible spectral line in the Balmer series with a wavelength of 656.28 nm in air and 656.46 nm in vacuum; it occurs when a hydrogen electron falls from its third to second lowest energy level. H-alph ...

absorption line A spectral line is a dark or bright line in an otherwise uniform and continuous spectrum, resulting from emission or absorption of light in a narrow frequency range, compared with the nearby frequencies. Spectral lines are often used to ident ...

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. 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 (), and 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 continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

) 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, a ...

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 at 606 parsecs is the highest ever derived for this star. Alnilam's relatively simple spectrum has made it useful for studying the

interstellar medium In astronomy, the interstellar medium is the matter and radiation that exist in the space between the star systems in a galaxy. This matter includes gas in ionic, atomic, and molecular form, as well as dust and cosmic rays. It fills interstella ...

. Within the next million years, this star may turn into a

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

and explode as a supernova. 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, the density and size of which permit absorption nebulae, the formation of molecules (most commonly molecular hydroge ...

, NGC 1990, which it illuminates to make a

reflection nebula Reflection or reflexion may refer to: Science and technology * Reflection (physics), a common wave phenomenon ** Specular reflection, reflection from a smooth surface *** Mirror image, a reflection in a mirror or in water ** Signal reflection, in s ...

. Its

stellar wind A stellar wind is a flow of gas ejected from the 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 spherically symmetric. ...

s may reach up to 2,000

km/s The metre per second is the unit of both speed (a scalar quantity) and velocity (a vector quantity, which has direction and magnitude) in the International System of Units (SI), equal to the speed of a body covering a distance of one metre in ...

, causing it to lose

mass Mass is an intrinsic property of a body. It was traditionally believed to be related to the quantity of matter in a physical body, until the discovery of the atom and particle physics. It was found that different atoms and different element ...

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 modern constellations visible from southern England. They are named for John Flamsteed who first used them while c ...

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

Arabic Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walte ...

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'' '

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, chromium, vanadium, or magnesium. The name sapphire is derived via the Latin "sap ...

'. In 2016, the

International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach ...

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 List of proper names of stars, proper names for stars for the international astronomical community. It operates under ...

(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 in the geographic area now known as Greater China. Chinese mythology includes many varied myths from regional and cultural traditions. Much of ...

, they were also known as the Weighing Beam. In

Chinese Chinese can refer to: * Something related to China * Chinese people, people of Chinese nationality, citizenship, and/or ethnicity **''Zhonghua minzu'', the supra-ethnic concept of the Chinese nation ** List of ethnic groups in China, people of v ...

, (), meaning '' Three Stars (asterism)'', refers to an asterism consisting of Alnilam,

and

(Orion's Belt), with

Betelgeuse Betelgeuse is a red supergiant of spectral type M1-2 and one of the largest stars visible to the naked eye. It is usually the tenth-brightest star in the night sky and, after Rigel, the second-brightest in the constellation of Orio ...

, Bellatrix,

Saiph Saiph , designation Kappa Orionis (κ Orionis, abbreviated Kappa Ori, κ Ori) and 53 Orionis (53 Ori), is the sixth-brightest star in the constellation of Orion. Of the four bright stars that compose Orion's main quadrangle, it is the st ...

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. Consequently, the

Chinese name Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are oft ...

for Alnilam is (, en, the Second Star of Three Stars). It is one of the western mansions of the

White Tiger The white tiger or bleached tiger is a leucistic pigmentation variant of the Mainland tiger. It is reported in the wild from time to time in the Indian states of Madhya Pradesh, Assam, West Bengal, Bihar, Odisha, in the Sunderbans region and e ...

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 19 – The first west–east transatlantic radio broadcast is made from the United States to England (the first east–west broadcast having been ...