Sigma Sagittarii

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Sigma Sagittarii
Diagram showing star positions and boundaries of the Sagittarius constellation and its surroundings
Cercle rouge 100%.svg
Location of σ Sagittarii (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension  18h 55m 15.92650s[1][2]
Declination –26° 17′ 48.2068″[1][2]
Apparent magnitude (V) 2.05[3]
Spectral type B2.5 V[4]
U−B color index –0.761[5]
B−V color index –0.204[5]
Radial velocity (Rv)–11.2[3] km/s
Proper motion (μ) RA: +15.14[1][2] mas/yr
Dec.: –53.43[1][2] mas/yr
Parallax (π)14.32 ± 0.29[1][2] mas
Distance228 ± 5 ly
(70 ± 1 pc)
Absolute magnitude (MV)−2.17[6]
Mass7.8 ± 0.2[7] M
Radius4.5[8] R
Luminosity3,300[9] L
Temperature18,890[9] K
Rotational velocity (v sin i)165[10] km/s
Age31.4 ± 0.4[7] Myr
Other designations
Nunki, Sadira, σ Sagittarii, σ Sgr, Sigma Sgr, 34 Sagittarii, CCDM J18552-2618A, CPD-27  5241, FK5 706, GC 25941, HD 175191, HIP 92855, HR 7121, IDS 18491-2625 A, PPM 269078, SAO 187448, WDS J18553-2618Aa,Ab.
Database references

Sigma Sagittarii (σ Sagittarii, abbreviated Sigma Sgr, σ Sgr), formally named Nunki /ˈnʌŋki/,[11][12] is the second-brightest star in the constellation of Sagittarius. It has an apparent magnitude of +2.05,[3] making it readily visible to the naked eye. The distance to this star, determined using parallax measurements from the Hipparcos astrometry satellite,[13] yields an estimated value of 228 light-years (70 parsecs) from the Sun.[1][2]


Sigma Sagittarii has a spectrum matching a stellar classification of B2.5 V,[4] which indicates this is a B-type main-sequence star. Its total luminosity is 3300[14] times that of the Sun while it has a surface temperature of 18,890 K.[9] X-ray emission has been detected from this star, which has an estimated X-ray luminosity of 1.2 × 1028 erg s−1.[15]

It has a 10th magnitude optical companion located 5.2 arcminutes away.[16]

It is 3.45 degrees south of the ecliptic, so it can be occulted by the Moon and rarely by planets. The last occultation by a planet took place on November 17, 1981, when it was occulted by Venus.[17] This is the brightest star that can be principally occulted by an exterior planet between 5000 BC and 5000 AD. However, only Mars can do this, and only rarely; the last time was on September 3, 423.[citation needed]


σ Sagittarii (Latinised to Sigma Sagittarii) is the star's Bayer designation.

It bore the traditional name of Nunki, which was an Assyrian or Babylonian name recovered by archaeologists and made public by R. H. Allen.[18] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[19] to catalogue and standardize proper names for stars. The WGSN approved the name Nunki for this star on 21 August 2016 and it is now so included in the List of IAU-approved Star Names.[12]

This star, together with :

Zeta Sagittarii and Pi Sagittarii may have been the Akkadian Gu-shi-rab‑ba, the Yoke of the Sea.[21]

In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Thanih al Sadirah, which was translated into Latin as Secunda τού al Sadirah, meaning second returning ostrich.[22]

In Chinese, (Dǒu), meaning Dipper, refers to an asterism consisting of Sigma Sagittarii, Phi Sagittarii, Lambda Sagittarii, Mu Sagittarii, Tau Sagittarii and Zeta Sagittarii. Consequently,the Chinese name for Sigma Sagittarii itself is 斗宿四 (Dǒu Sù sì, English: the Fourth Star of Dipper.)[23]


  1. ^ a b c d e f 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. (July 1997), "The Hipparcos Catalogue", Astronomy and Astrophysics, 323: L49–L52, Bibcode:1997A&A...323L..49P
  2. ^ a b c d e f Van Leeuwen, Floor (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.
  3. ^ a b c Wielen, R.; et al. (1999), "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions", Veröff. Astron. Rechen-Inst. Heidelb, Astronomisches Rechen-Institut Heidelberg, 35 (35): 1, Bibcode:1999VeARI..35....1W
  4. ^ a b Houk, Nancy (1979), "Michigan catalogue of two-dimensional spectral types for the HD stars", Michigan Catalogue of Two-dimensional Spectral Types for the HD Stars. Volume_3. Declinations -40_ƒ0 to -26_ƒ0, Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan, 3,
  5. ^ a b Gutierrez-Moreno, Adelina; Moreno, Hugo (June 1968), "A photometric investigation of the Scorpio-Centaurus association", Astrophysical Journal Supplement, 15: 459, Bibcode:1968ApJS...15..459G, doi:10.1086/190168
  6. ^ Anderson, E.; Francis, Ch. (2012), "XHIP: An extended hipparcos compilation", Astronomy Letters, 38 (5): 331, arXiv:1108.4971, Bibcode:2012AstL...38..331A, doi:10.1134/S1063773712050015.
  7. ^ a b Tetzlaff, N.; Neuhäuser, R.; Hohle, M. M. (January 2011), "A catalogue of young runaway Hipparcos stars within 3 kpc from the Sun", Monthly Notices of the Royal Astronomical Society, 410 (1): 190–200, arXiv:1007.4883, Bibcode:2011MNRAS.410..190T, doi:10.1111/j.1365-2966.2010.17434.x
  8. ^ Underhill, A. B.; et al. (November 1979), "Effective temperatures, angular diameters, distances and linear radii for 160 O and B stars", Monthly Notices of the Royal Astronomical Society, 189 (3): 601–605, Bibcode:1979MNRAS.189..601U, doi:10.1093/mnras/189.3.601
  9. ^ a b c Zorec, J.; et al. (July 2009), "Fundamental parameters of B supergiants from the BCD system. I. Calibration of the (λ_1, D) parameters into Teff", Astronomy and Astrophysics, 501 (1): 297–320, arXiv:0903.5134, Bibcode:2009A&A...501..297Z, doi:10.1051/0004-6361/200811147
  10. ^ Abt, Helmut A.; Levato, Hugo; Grosso, Monica (July 2002), "Rotational Velocities of B Stars", The Astrophysical Journal, 573 (1): 359–365, Bibcode:2002ApJ...573..359A, doi:10.1086/340590
  11. ^ Davis, George A. (1944). "The pronunciations, derivations, and meanings of a selected list of star names". Popular Astronomy. 52: 8–30.
  12. ^ a b "Naming Stars". Retrieved 16 December 2017.
  13. ^ Perryman, Michael (2010), "The Making of History's Greatest Star Map", The Making of History's Greatest Star Map, Astronomers’ Universe, Heidelberg: Springer-Verlag,, doi:10.1007/978-3-642-11602-5, ISBN 978-3-642-11601-8
  14. ^ James B. Kaler, "NUNKI (Sigma Sagatarii)", Stars, University of Illinois, retrieved 2012-01-31
  15. ^ Cassinelli, J. P.; et al. (February 1994), "X-ray emission from near-main-sequence B stars", Astrophysical Journal, Part 1, 421 (2): 705–717, Bibcode:1994ApJ...421..705C, doi:10.1086/173683
  16. ^ Mason, Brian D.; Wycoff, Gary L.; Hartkopf, William I.; Douglass, Geoffrey G.; Worley, Charles E. (2001). "The 2001 US Naval Observatory Double Star CD-ROM. I. The Washington Double Star Catalog". The Astronomical Journal. 122 (6): 3466. Bibcode:2001AJ....122.3466M. doi:10.1086/323920.
  17. ^ Können, G. P.; Van Maanen, J. (April 1981). "Planetary occultations of bright stars". Journal of the British Astronomical Association. 91: 148–157. Bibcode:1981JBAA...91..148K.
  18. ^ Allen, Richard Hinckley, Star Names, their lore and meaning, p. 359
  19. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  20. ^ "Teapot". Retrieved 2017-05-13.
  21. ^ a b Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint ed.). New York: Dover Publications Inc. p. 355. ISBN 0-486-21079-0. Retrieved 2012-09-04.
  22. ^ Knobel, E. B. (June 1895). "Al Achsasi Al Mouakket, on a catalogue of stars in the Calendarium of Mohammad Al Achsasi Al Mouakket". Monthly Notices of the Royal Astronomical Society. 55 (8): 430. Bibcode:1895MNRAS..55..429K. doi:10.1093/mnras/55.8.429.
  23. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日