Beta Sagittarii

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β¹ Sagittarii
Diagram showing star positions and boundaries of the Sagittarius constellation and its surroundings
Cercle rouge 100%.svg

Location of Beta Sagittarii (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Sagittarius
Right ascension 19h 22m 38.3s
Declination −44° 27' 32"
Apparent magnitude (V) +3.96
Distance 378 ± 42 ly
(116 ± 13 pc)
Spectral type B9V
Other designations
Arkab Prior, CCDM J19226-4428A, CPD-40  8955, FK5 1502, GC 26703, HD 181454, HIP 95241, HR 7337, PPM 325041 , SAO 229646, WDS J19226-4428A
β² Sagittarii
Observation data
Epoch       Equinox
Constellation Sagittarius
Right ascension 19h 23m 13.2s
Declination -44° 47' 59"
Apparent magnitude (V) +4.27
Distance 139 ± 5 ly
(43 ± 2 pc)
Spectral type F2III
Other designations
Arkab Posterior, CPD-45  9701, GC 26718, HD 181623, HIP 95294, HR 7343, PPM 325051, SAO 229654,

Beta Sagittarii (β Sagittarii, abbreviated Beta Sgr, β Sgr) is the common designation shared by two star systems in the constellation of Sagittarius, themselves designated β¹ Sagittarii (itself a binary star) and β² Sagittarii. β¹ and β² Sagittarii are approximately 378 and 139 light years, respectively, from Earth. The two systems are separated by 0.36° in the sky.

β¹ Sagittarii's two components are designated β¹ Sagittarii A, also named Arkab Prior,[1] and β¹ Sagittarii B (sometimes designated Arkab Prior A and B). β² Sagittarii is named Arkab Posterior.[1] Beta Sagittarii is also referred to by the traditional name Arkab.


β Sagittarii (Latinised to Beta Sagittariii) is the system's Bayer designation; β¹ and β² Sagittarii, those of its two constituents. The designations of β¹'s components - β¹ Sagittarii A and B - derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[2]

The system's traditional name Arkab derives from the Arabic عرقوب carqūb meaning Achilles Tendon.[3] The two constituents bore the traditional names Arkab Prior and Arkab Posterior since β¹ leads β² (or β² follows β¹) across the sky.[citation needed] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[4] to catalogue and standardize proper names for stars. The WGSN states that in the case of multiple stars the name should be understood to be attributed to the brightest component by visual brightness.[5] The WGSN approved the names Arkab Prior and Arkab Posterior for β¹ Sagittarii A and β² Sagittarii on 5 October 2016 and they are now so entered in the IAU Catalog of Star Names.[1]

β¹ and β² Sagittarii, together with Alpha Sagittarii, were Al Ṣuradain (السردين), the two Surad, "desert birds".[6]

In Chinese, 天淵 (Tiān Yuān), meaning Celestial Spring, refers to an asterism consisting of β¹ Sagittarii, β² Sagittarii, and Alpha Sagittarii, Consequently, β¹ and β² Sagittarii themselves are known as 天淵一 (Tiān Yuān yī, English: the First Star of Celestial Spring.) and 天淵二 (Tiān Yuān èr, English: the Second Star of Celestial Spring.)[7]


USS Arkab (AK-130) was a United States Navy Crater class cargo ship named after the system.


β¹ Sagittarii A is a spectral type B9 main sequence dwarf which has an apparent magnitude of +3.96. β¹ Sagittarii B is a type A3 dwarf with an apparent magnitude of +7.4. The two stars are separated in the sky by 28 arcseconds, which puts them at least 3300 astronomical units apart.

β² Sagittarii is a spectral type F2 giant which has an apparent magnitude of +4.27.


  1. ^ a b c "IAU Catalog of Star Names". Retrieved 13 October 2016. 
  2. ^ Hessman, F. V.; Dhillon, V. S.; Winget, D. E.; Schreiber, M. R.; Horne, K.; Marsh, T. R.; Guenther, E.; Schwope, A.; Heber, U. (2010). "On the naming convention used for multiple star systems and extrasolar planets". arXiv:1012.0707Freely accessible [astro-ph.SR]. 
  3. ^ - تعريف و معنى عرقوب في معجم المعاني الجامع - معجم عربي عربي تعريف و معنى عرقوب في معجم المعاني الجامع - معجم عربي عربي
  4. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016. 
  5. ^ "Bulletin of the IAU Working Group on Star Names, No. 2" (PDF). Retrieved 12 October 2016. 
  6. ^ Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 357. ISBN 0-486-21079-0. Retrieved 4 September 2012. 
  7. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 7 月 2 日