View source for Alnilam

{{short description|Star in the constellation Orion, in the center of the Orion's Belt}}
{{Starbox begin}}
{{Starbox image
 | image=
{{Location mark
|image=Orion constellation map.svg|alt=|float=center|width=260
|label=|position=right
|mark=Red circle.svg|mark_width=12|mark_link=Alnilam
|x=579|y=548
}}
|caption=Location of Alnilam (circled)
}}
{{Starbox observe
 | pronounce = {{IPAc-en|æ|l|'|n|ai|l|æ|m}}<!--not the expected stress etymologically, but seems standard--><ref name=Kunitzsch/>
 | epoch=J2000
 | ra={{RA|05|36|12.8}}<ref name=hipparcos/>
 | dec={{DEC|−01|12|06.9}}<ref name=hipparcos/>
 | appmag_v=1.69<ref name=ducati/> {{nowrap|(1.64 – 1.74<ref name=ruban/>)}}
 | constell=[[Orion (constellation)|Orion]]
}}
{{Starbox character
 | class=B0&nbsp;Ia<ref name=mkk/>
 | b-v=−0.18<ref name=ducati/>
 | u-b=−1.03<ref name=ducati/>
 | variable=[[Alpha cygni variable|α Cygni]]<ref name=ruban/>
|type=[[Blue supergiant]]}}
{{Starbox astrometry
 | radial_v=25.9<ref name=PCRV/>
 | prop_mo_ra=1.49<ref name=hipparcos/>
 | prop_mo_dec=−1.06<ref name=hipparcos/>
 | parallax=1.65
 | p_error=0.45
 | parallax_footnote=<ref name=hipparcos/>
 | dist_pc=361
 | dist_ly=1,180
 | dist_footnote=<ref name=oplistilova2023/>{{efn|Calculated, using a [[distance modulus]] of 7.79 in the following equation:<br /><math display="block">d = 10^{\frac{\mu}{5}+1} </math>. This yields a value of 361 parsecs.}}
 | absmag_v=−6.89<ref name="Searle 2008"/>
 }}
{{Starbox detail
 | source=<ref name="zsargo2020"/>
 | mass=40
 | radius=30.62
 | luminosity=419,600
 | temperature=26,540
 | gravity=3.02
 | metal=
 | rotational_velocity=80
 | age_myr=4.47
}}
{{Starbox catalog
 | names = {{odlist | name=Alnilam |B=ε&nbsp;Ori | F=46&nbsp;Orionis | Gould=Orionis | HR=1903 | BD=−01°969 | HD=37128 | SAO=132346 | FK5=210 | HIP=26311 }}, TD1&nbsp;4963, [[Three Stars (Chinese constellation)|參宿二]]
}}
{{Starbox reference
 | Simbad=HD+37128
}}
{{Starbox end}}

'''Alnilam''' is the central [[star]] of [[Orion's Belt]] in the [[celestial equator|equatorial]] [[constellation]] of [[Orion (constellation)|Orion]]. It has the [[Bayer designation]] '''ε Orionis''', which is [[Latinisation of names|Latinised]] to '''Epsilon Orionis''' and abbreviated '''Epsilon Ori''' or '''ε Ori'''. This is a massive, [[blue supergiant]] star some 2,000&nbsp;[[light-year]]s distant. It is estimated to be 419,600 times as [[Solar luminosity|luminous as the Sun]], and 40 times [[Solar mass|as massive]].

==Observation==
[[Image:Orion Belt.jpg|thumb|left|Alnilam is the middle and brightest of the three stars of Orion's Belt.]]
It is the [[List of brightest stars|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 [[stellar spectrum|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 line]]s, this is considered typical for this type of luminous hot supergiant.<ref name=puebla/> It is also one of the [[List of stars for navigation|58&nbsp;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 characteristics==
[[File:EpsilonOriLightCurve.png|thumb|left|A [[Photometric_system#Photometric_letters|blue band]] [[light curve]] for Epsilon Orionis, adapted from Krtička and Feldmeier (2018)<ref name="Krtička"/>]]
Estimates of Alnilam's properties vary. Searle and colleagues, using CMFGEN code to analyse the spectrum in 2008, calculated a luminosity of {{solar luminosity|537,000}}, an effective temperature of 27,500 ± 100 [[Kelvin|K]] and a radius of {{solar radius|32.4 ± 0.75}}.<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 ({{solar mass|40.8}} 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 [[parsec]]s 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 {{solar luminosity|832,000}} and the mass of {{solar mass|64.5}} 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 ({{solar luminosity|420,000}}), radius ({{solar radius|30.61}}), and mass ({{solar mass|40}}).<ref name="zsargo2020"/> Another [[astronomical spectroscopy|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>{{Cite journal |last1=Humphreys |first1=Roberta M. |last2=Helmel |first2=Greta |last3=Jones |first3=Terry J. |last4=Gordon |first4=Michael S. |date=2020-09-01 |title=Exploring the Mass-loss Histories of the Red Supergiants* |journal=The Astronomical Journal |volume=160 |issue=3 |pages=145 |doi=10.3847/1538-3881/abab15 |doi-access=free |arxiv=2008.01108 |bibcode=2020AJ....160..145H |issn=0004-6256}}</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 wind]]s may reach up to 2,000 [[velocity|km/s]], causing it to lose [[mass]] about 20 million times more rapidly than the Sun.<ref name="crowther"/>

== Nomenclature and history ==
''ε Orionis'' is the star's [[Bayer designation]] and ''46&nbsp;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 Belt===
{{Main article|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.<ref name=allen/> 

In [[Chinese astronomy|Chinese]], {{lang|zh|參宿}} ({{lang|zh-Latn|Shēn Sù}}), meaning ''[[Three Stars (Chinese constellation)|Three Stars]] (asterism)'', refers to an [[Asterism (astronomy)|asterism]] consisting of Alnilam, [[Alnitak]] and [[Mintaka]] (Orion's Belt), with [[Betelgeuse]], [[Bellatrix]], [[Saiph]] and [[Rigel]] later added.<ref name=zh/> Consequently, the [[Chinese star names|Chinese name]] for Alnilam is {{lang|zh|參宿二}} ({{lang|zh-Latn|Shēn Sù èr}}, {{langx|en|the Second Star of Three Stars}}).<ref name=AEEA/> It is one of the western mansions of the [[White Tiger (mythology)|White Tiger]].

==See also==
*[[List of most massive stars]]

==Notes==
{{Notelist}}

==References==
{{Reflist|30em|refs=

<ref name=AEEA>{{in lang|zh}}  [http://aeea.nmns.edu.tw/2006/0605/ap060525.html AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 25 日] {{Webarchive|url=https://web.archive.org/web/20110716122642/http://aeea.nmns.edu.tw/2006/0605/ap060525.html |date=2011-07-16 }}</ref>

<ref name=zh>{{in lang|zh}} ''中國星座神話'', written by 陳久金. Published by 台灣書房出版有限公司, 2005, {{ISBN|978-986-7332-25-7}}.</ref>

<ref name=oplistilova2023>{{cite journal |bibcode=2023A&A...672A..31O |title=Spectrum of the secondary component and new orbital elements of the massive triple star δ Ori A |last1=Oplištilová |first1=A. |last2=Mayer |first2=P. |last3=Harmanec |first3=P. |last4=Brož |first4=M. |last5=Pigulski |first5=A. |last6=Božić |first6=H. |last7=Zasche |first7=P. |last8=Šlechta |first8=M. |last9=Pablo |first9=H. |last10=Kołaczek-Szymański |first10=P. A. |last11=Moffat |first11=A. F. J. |last12=Lovekin |first12=C. C. |last13=Wade |first13=G. A. |last14=Zwintz |first14=K. |last15=Popowicz |first15=A. |last16=Weiss |first16=W. W. |journal=Astronomy and Astrophysics |date=2023 |volume=672 |pages=A31 |doi=10.1051/0004-6361/202245272 |arxiv=2301.10290 |s2cid=256226821 }}</ref>

<ref name="IAU-CSN">{{cite web
 | url=http://www.pas.rochester.edu/~emamajek/WGSN/IAU-CSN.txt
 | title=IAU Catalog of Star Names |access-date=28 July 2016}}</ref>

<ref name="WGSN1">{{cite web
 | url=http://www.pas.rochester.edu/~emamajek/WGSN/WGSN_bulletin1.pdf
 | title=Bulletin of the IAU Working Group on Star Names, No. 1 |access-date=28 July 2016}}</ref>

<ref name="WGSN">{{cite web
 | url=https://www.iau.org/science/scientific_bodies/working_groups/280/
 | title=IAU Working Group on Star Names (WGSN)|access-date=22 May 2016}}</ref>

<ref name=allen>{{Cite book
 | first=Richard Hinckley |last=Allen
 | title=Star-names and their meanings |date=1936 |pages=314–315 }}</ref>

<ref name=Knobel>{{cite journal
 | title=The name of epsilon Orionis|last=Knobel |first=E. B.
 | journal=The Observatory |volume=32 |page=357|date=September 1909
 | bibcode=1909Obs....32..357K}}</ref>

<ref name="puebla">{{cite journal
 | author=Puebla, R.E. |author2=Hillier, D.J. |author3=Zsargó, J.
 | author4=Cohen, D.H. |author5=Leutenegger, M.A.|date=2015
 | title=X-ray, UV and optical analysis of supergiants: ϵ Ori
 | journal= Monthly Notices of the Royal Astronomical Society|volume=456|issue=3|pages=2907–2936
 | doi=10.1093/mnras/stv2783 |doi-access=free |bibcode=2016MNRAS.456.2907P
 | arxiv = 1511.09365 |s2cid=7887625 }}</ref>

<ref name=perryman>{{cite journal
 | bibcode=1997A&A...323L..49P
 | title=The HIPPARCOS Catalogue|journal=Astronomy and Astrophysics |volume=323
 | pages=L49|last1=Perryman|first1=M. A. C.|last2=Lindegren|first2=L.|last3=Kovalevsky|first3=J.
 | last4=Hoeg|first4=E.|last5=Bastian|first5=U.|last6=Bernacca|first6=P. L.
 | last7=Crézé|first7=M.|last8=Donati|first8=F.|last9=Grenon|first9=M.
 | last10=Grewing|first10=M.|last11=Van Leeuwen|first11=F.|last12=Van Der Marel|first12=H.
 | last13=Mignard|first13=F.|last14=Murray|first14=C. A.|last15=Le Poole
 | first15=R. S.|last16=Schrijver|first16=H.|last17=Turon|first17=C.|last18=Arenou|first18=F.
 | last19=Froeschlé|first19=M.|last20=Petersen|first20=C. S.|date=1997}}</ref>

<ref name="Voss 2010">{{cite journal
 | author= Voss, R.|author2= Diehl, R.|author3= Vink, J. S.|author4= Hartmann, D. H.
 | date=2010
 | title=Probing the evolving massive star population in Orion with kinematic and radioactive tracers
 | journal= Astronomy and Astrophysics|volume=520|pages=10|id=A51 | doi=10.1051/0004-6361/201014408
 | bibcode=2010A&A...520A..51V|arxiv = 1005.3827 |s2cid= 38599952}}</ref>

<ref name="Searle 2008">{{cite journal
 | author=Searle, S. C.|author2=Prinja, R. K.|author3=Massa, D.
 | author4=Ryans, R.|date=2008
 | title=Quantitative studies of the optical and UV spectra of Galactic early B supergiants. I. Fundamental parameters
 | journal=Astronomy and Astrophysics|volume=481|issue=3|pages=777–97|doi=10.1051/0004-6361:20077125
 | bibcode=2008A&A...481..777S |arxiv = 0801.4289 |s2cid=1552752}}</ref>

<ref name="crowther">{{cite journal
 | last1=Crowther|first1=P. A.|last2=Lennon|first2=D. J.|last3=Walborn|first3=N. R.
 | date=January 2006
 | title=Physical parameters and wind properties of galactic early B supergiants
 | journal=Astronomy & Astrophysics|volume=446|issue=1|pages=279–293|arxiv=astro-ph/0509436
 | bibcode=2006A&A...446..279C|doi=10.1051/0004-6361:20053685|s2cid=18815761}}</ref>

<ref name=mkk>{{cite journal
 | title=An Atlas of Stellar Spectra |doi=10.1038/152147a0
 | url=https://www.ucl.ac.uk/ucl-observatory/sites/ucl-observatory/files/mkkbook.pdf
 | journal=Astrophysical Monographs |volume=152 |issue=3849 |pages=147 |date=1943
 | last1=Morgan |first1=W. W. |last2=Keenan |first2=Philip C. |last3=Kellman |first3=Edith
 | bibcode=1943Natur.152..147. |s2cid=4109100 |doi-access=free }}</ref>

<ref name=GCVSQF>{{Cite web
 | url=http://www.sai.msu.su/gcvs/cgi-bin/search.cgi?search=eps+Ori
 | title=GCVS Query forms|website=www.sai.msu.su|access-date=2019-01-12}}</ref>

<ref name=PCRV>{{cite journal
 | title=Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system
 | first=G. A. |last=Gontcharov|journal=Astronomy Letters|date=November 2006 |volume=32
 | issue=11 |pages=759–771|bibcode=2006AstL...32..759G|doi = 10.1134/S1063773706110065
 | arxiv = 1606.08053 |s2cid=119231169 }}</ref>

<ref name=ruban>{{cite journal
 | title=Spectrophotometric observations of variable stars
 | first1=E. V. |last1=Ruban |first2=G. A. |last2=Alekseeva
 | first3=A. A. |last3=Arkharov |first4=E. I. |last4=Hagen-Thorn
 | first5=V. D. |last5=Galkin |first6=I. N. |last6=Nikanorova
 | first7=V. V. |last7=Novikov |first8=V. P. |last8=Pakhomov
 | first9=T. Yu. |last9=Puzakova|s2cid=121747360
 | journal=Astronomy Letters|date=September 2006 |volume=32 |issue=9
 | pages=604–607|doi=10.1134/S1063773706090052
 | bibcode = 2006AstL...32..604R}}</ref>

<ref name=ducati>{{cite journal
 | title=VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system
 | journal=CDS/ADC Collection of Electronic Catalogues |volume=2237 |last=Ducati |first=J. R.
 | date=2002|bibcode=2002yCat.2237....0D}}</ref>

<ref name="zsargo2020">{{Citation |last1=Zsargó |first1=J. |last2=Fierro-Santillán |first2=C. R. |last3=Klapp |first3=J. |last4=Arrieta |first4=A. |last5=Arias |first5=L. |last6=Valencia |first6=J. M. |last7=Sigalotti |first7=L. Di G. |last8=Hareter |first8=M. |last9=Puebla |first9=R. E. |date=2020-11-01 |title=Creating and using large grids of precalculated model atmospheres for a rapid analysis of stellar spectra |url=https://ui.adsabs.harvard.edu/abs/2020A&A...643A..88Z |journal=Astronomy and Astrophysics |volume=643 |pages=A88 |doi=10.1051/0004-6361/202038066 |issn=0004-6361|arxiv=2009.10879 |bibcode=2020A&A...643A..88Z }}</ref>

<ref name=Kunitzsch>{{cite book
  | last1=Kunitzsch |first1=Paul
  | last2=Smart |first2=Tim
  | date = 2006 |edition = 2nd rev.
  | title = A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations
  | publisher = Sky Pub |location = Cambridge, Massachusetts
  | isbn = 978-1-931559-44-7
}}</ref>

<ref name="Krtička">{{cite journal
 | last1=Krtička |first1=J. |last2=Feldmeier |first2=A. |bibcode=2018A&A...617A.121K
 | title=Light variations due to the line-driven wind instability and wind blanketing in O stars
 | journal=Astronomy & Astrophysics |date=September 2018 |volume=617 |page=A121
 | doi=10.1051/0004-6361/201731614 |arxiv=1807.09407 |s2cid=119388848 |url=https://www.aanda.org/articles/aa/pdf/2018/09/aa31614-17.pdf
 | access-date=11 August 2022}}</ref>

<ref name=hipparcos>{{cite journal
 | title=Validation of the new Hipparcos reduction
 | first=F.| last=van Leeuwen | journal=Astronomy & Astrophysics|volume=474 |issue=2
 | date=November 2007|pages=653–664|bibcode=2007A&A...474..653V|arxiv = 0708.1752
 | doi = 10.1051/0004-6361:20078357 |s2cid=18759600}}</ref>

}}

==External links==
* {{APOD |date=29 September 2009 |title=Image of Alnilam}}

{{Stars of Orion}}
{{Portal bar|Astronomy|Stars|Outer space}}
{{Sky|05|36|12.8|-|01|12|06.9|800}}

{{DEFAULTSORT:Alnilam}}
<!-- Properties -->
[[Category:B-type supergiants]]
[[Category:Alpha Cygni variables]]
<!-- Position/Catalogues -->
[[Category:Orion (constellation)]]
[[Category:Bayer objects|Orionis, Epsilon]]
[[Category:Bright Star Catalogue objects|1903]]
[[Category:Durchmusterung objects|BD-01 0969]]
[[Category:Flamsteed objects|Orionis, 46]]
[[Category:Henry Draper Catalogue objects|037128]]
[[Category:Hipparcos objects|026311]]
[[Category:Stars with proper names|Alnilam]]