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Alnilam

Coordinates:Sky map05h36m12.8s,−01° 12′ 06.9″
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Alnilam
Location of Alnilam (circled)
Observation data
EpochJ2000EquinoxJ2000
Constellation Orion
Pronunciation /ælˈnaɪlæm/[1]
Right ascension 05h36m12.8s[2]
Declination −01° 12′ 06.9″[2]
Apparent magnitude(V) 1.69[3](1.64 – 1.74[4])
Characteristics
Spectral type B0 Ia[5]
U−Bcolor index −1.03[3]
B−Vcolor index −0.18[3]
Variable type α Cygni[4]
Astrometry
Radial velocity(Rv)25.9[6]km/s
Proper motion(μ)RA:1.49[2]mas/yr
Dec.:−1.06[2]mas/yr
Parallax(π)1.65 ± 0.45mas[2]
Distanceapprox. 2,000ly
(approx. 600pc)
Absolute magnitude(MV)−6.89[7]
Details[8]
Mass40M☉
Radius30.62R☉
Luminosity419,600L☉
Surface gravity(logg)3.02cgs
Temperature26,540K
Rotational velocity(vsini)80 km/s
Age4.47Myr
Other designations
Alnilam,ε Ori,46 Orionis,Orionis,BD−01°969,FK5210,HD37128,HIP26311,HR1903,SAO132346, TD1 4963,Tham túc nhị
Database references
SIMBADdata

Alnilamis the centralstarofOrion's Beltin theequatorialconstellationofOrion.It has theBayer designationε Orionis,which isLatinisedtoEpsilon Orionisand abbreviatedEpsilon Oriorε Ori.This is a massive,blue supergiantstar some 2,000light-yearsdistant. It is estimated to be 832,000 times asluminous as the Sun,and 64.5 timesas massive.

Observation[edit]

Alnilam is the middle and brightest of the three stars of Orion's Belt.

It is the29th-brightest starin the sky (the fourth brightest in Orion) and is ablue supergiant.Together withMintakaandAlnitak,the three stars make upOrion's Belt,known by many names across many ancient cultures. Alnilam is the middle star.

Since 1943, thespectrumof this star has served as one of the stable anchor points by which other stars are classified, for the spectral class B0Ia.[5]Although the spectrum shows variations, particular in theH-alphaabsorption lines,this is considered typical for this type of luminous hot supergiant.[9]It is also one of the58 starsused incelestial 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 anα Cygni variable.[10]Its spectrum also varies, possibly due to unpredictable changes in mass loss from the surface.[9]

Physical characteristics[edit]

Ablue bandlight curvefor Epsilon Orionis, adapted from Krtička and Feldmeier (2018)[11]

Estimates of Alnilam's properties vary. Searle and colleagues, using CMFGEN code to analyse the spectrum in 2008, calculated a luminosity of 537,000L☉,an effective temperature of 27,500 ± 100Kand a radius of 32.4 ± 0.75R☉.[7]Analysis of the spectra and age of the members of the Orion OB1 association yields a mass 34.6 times that of the Sun (40.8M☉on themain sequence) and an age of 5.7 million years.[12]A more recent detailed analysis of Alnilam across multiple wavelength bands produced very high luminosity, radius, and mass estimates, assuming the distance of 606parsecssuggested by theHipparcosnew reduction.[2]Adopting the larger parallax from the originalHipparcosreduction gives a distance of 412 parsecs[13]and physical parameters more consistent with earlier publications. The luminosity of 832,000L☉and the mass of 64.5M☉at 606 parsecs is the highest ever derived for this star.[9]A newer study from 2020 found smaller values for luminosity (420,000L☉), radius (30.61R☉), and mass (40M☉).[8]

Alnilam's relatively simple spectrum has made it useful for studying theinterstellar medium.Within the next million years, this star may turn into aWolf-Rayet starand explode as asupernova.Alnilam's high mass means that due to high mass loss, it will not become ared supergiantstar.[14]It is surrounded by amolecular cloud,NGC 1990,which it illuminates to make areflection nebula.Itsstellar windsmay reach up to 2,000km/s,causing it to losemassabout 20 million times more rapidly than the Sun.[15]

Nomenclature and history[edit]

ε Orionisis the star'sBayer designationand46 OrionisitsFlamsteed designation.

The traditional nameAlnilamderives from theArabicالنظامal-niẓām'arrangement/string (of pearls)'. Related spellings areAlnihanandAlnitam:[16]all three variants are evidently mistakes in transliteration or copy errors, the first perhaps due to confusion with النيلمal-nilam'sapphire'.[17]In 2016, theInternational Astronomical Unionorganized aWorking Group on Star Names(WGSN)[18]to catalog and standardize proper names for stars. The WGSN's first bulletin of July 2016[19]included a table of the first two batches of names approved by the WGSN; which includedAlnilamfor this star. It is now so entered in the IAU Catalog of Star Names.[20]

Orion's Belt[edit]

Main article:Orion's Belt

The three belt stars were collectively known by many names in many cultures. Arabic terms includeAl 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'). InChinese mythology,they were also known as the Weighing Beam.[16]

InChinese,Tham túc(Shēn Sù), meaningThree Stars(asterism),refers to anasterismconsisting of Alnilam,AlnitakandMintaka(Orion's Belt), withBetelgeuse,Bellatrix,SaiphandRigellater added.[21]Consequently, theChinese namefor Alnilam isTham túc nhị(Shēn Sù èr,English:the Second Star of Three Stars).[22]It is one of the western mansions of theWhite Tiger.

See also[edit]

References[edit]

  1. ^Kunitzsch, Paul; Smart, Tim (2006).A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations(2nd rev. ed.). Cambridge, Massachusetts: Sky Pub.ISBN978-1-931559-44-7.
  2. ^abcdefvan Leeuwen, F. (November 2007). "Validation of the new Hipparcos reduction".Astronomy & Astrophysics.474(2): 653–664.arXiv:0708.1752.Bibcode:2007A&A...474..653V.doi:10.1051/0004-6361:20078357.S2CID18759600.
  3. ^abcDucati, J. R. (2002). "VizieR Online Data Catalog: Catalogue of Stellar Photometry in Johnson's 11-color system".CDS/ADC Collection of Electronic Catalogues.2237.Bibcode:2002yCat.2237....0D.
  4. ^abRuban, E. V.; Alekseeva, G. A.; Arkharov, A. A.; Hagen-Thorn, E. I.; Galkin, V. D.; Nikanorova, I. N.; Novikov, V. V.; Pakhomov, V. P.; Puzakova, T. Yu. (September 2006). "Spectrophotometric observations of variable stars".Astronomy Letters.32(9): 604–607.Bibcode:2006AstL...32..604R.doi:10.1134/S1063773706090052.S2CID121747360.
  5. ^abMorgan, W. W.; Keenan, Philip C.; Kellman, Edith (1943)."An Atlas of Stellar Spectra"(PDF).Astrophysical Monographs.152(3849): 147.Bibcode:1943Natur.152..147..doi:10.1038/152147a0.S2CID4109100.
  6. ^Gontcharov, G. A. (November 2006). "Pulkovo Compilation of Radial Velocities for 35 495 Hipparcos stars in a common system".Astronomy Letters.32(11): 759–771.arXiv:1606.08053.Bibcode:2006AstL...32..759G.doi:10.1134/S1063773706110065.S2CID119231169.
  7. ^abSearle, S. C.; Prinja, R. K.; Massa, D.; Ryans, R. (2008). "Quantitative studies of the optical and UV spectra of Galactic early B supergiants. I. Fundamental parameters".Astronomy and Astrophysics.481(3): 777–97.arXiv:0801.4289.Bibcode:2008A&A...481..777S.doi:10.1051/0004-6361:20077125.S2CID1552752.
  8. ^abZsargó, J.; Fierro-Santillán, C. R.; Klapp, J.; Arrieta, A.; Arias, L.; Valencia, J. M.; Sigalotti, L. Di G.; Hareter, M.; Puebla, R. E. (2020-11-01),"Creating and using large grids of precalculated model atmospheres for a rapid analysis of stellar spectra",Astronomy and Astrophysics,643:A88,arXiv:2009.10879,doi:10.1051/0004-6361/202038066,ISSN0004-6361
  9. ^abcPuebla, R.E.; Hillier, D.J.; Zsargó, J.; Cohen, D.H.; Leutenegger, M.A. (2015). "X-ray, UV and optical analysis of supergiants: ϵ Ori".Monthly Notices of the Royal Astronomical Society.456(3): 2907–2936.arXiv:1511.09365.Bibcode:2016MNRAS.456.2907P.doi:10.1093/mnras/stv2783.S2CID7887625.
  10. ^"GCVS Query forms".www.sai.msu.su.Retrieved2019-01-12.
  11. ^Krtička, J.; Feldmeier, A. (September 2018)."Light variations due to the line-driven wind instability and wind blanketing in O stars"(PDF).Astronomy & Astrophysics.617:A121.arXiv:1807.09407.Bibcode:2018A&A...617A.121K.doi:10.1051/0004-6361/201731614.S2CID119388848.Retrieved11 August2022.
  12. ^Voss, R.; Diehl, R.; Vink, J. S.; Hartmann, D. H. (2010). "Probing the evolving massive star population in Orion with kinematic and radioactive tracers".Astronomy and Astrophysics.520:10.arXiv:1005.3827.Bibcode:2010A&A...520A..51V.doi:10.1051/0004-6361/201014408.S2CID38599952.A51.
  13. ^Perryman, M. A. C.; Lindegren, L.; Kovalevsky, J.; Hoeg, E.; Bastian, U.; Bernacca, P. L.; Crézé, M.; Donati, F.; Grenon, M.; Grewing, M.; Van Leeuwen, F.; Van Der Marel, H.; Mignard, F.; Murray, C. A.; Le Poole, R. S.; Schrijver, H.; Turon, C.; Arenou, F.; Froeschlé, M.; Petersen, C. S. (1997). "The HIPPARCOS Catalogue".Astronomy and Astrophysics.323:L49.Bibcode:1997A&A...323L..49P.
  14. ^Humphreys, Roberta M.; Helmel, Greta; Jones, Terry J.; Gordon, Michael S. (2020-09-01)."Exploring the Mass-loss Histories of the Red Supergiants*".The Astronomical Journal.160(3): 145.arXiv:2008.01108.Bibcode:2020AJ....160..145H.doi:10.3847/1538-3881/abab15.ISSN0004-6256.
  15. ^Crowther, P. A.; Lennon, D. J.; Walborn, N. R. (January 2006). "Physical parameters and wind properties of galactic early B supergiants".Astronomy & Astrophysics.446(1): 279–293.arXiv:astro-ph/0509436.Bibcode:2006A&A...446..279C.doi:10.1051/0004-6361:20053685.S2CID18815761.
  16. ^abAllen, Richard Hinckley (1936).Star-names and their meanings.pp. 314–315.
  17. ^Knobel, E. B. (September 1909). "The name of epsilon Orionis".The Observatory.32:357.Bibcode:1909Obs....32..357K.
  18. ^"IAU Working Group on Star Names (WGSN)".Retrieved22 May2016.
  19. ^"Bulletin of the IAU Working Group on Star Names, No. 1"(PDF).Retrieved28 July2016.
  20. ^"IAU Catalog of Star Names".Retrieved28 July2016.
  21. ^(in Chinese)Trung quốc tinh tọa thần thoại,written by trần cửu kim. Published by đài loan thư phòng xuất bản hữu hạn công tư, 2005,ISBN978-986-7332-25-7.
  22. ^(in Chinese)AEEA (Activities of Exhibition and Education in Astronomy) thiên văn giáo dục tư tấn võng 2006 niên 5 nguyệt 25 nhật

External links[edit]

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