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
Distance 228 ± 5 ly
(70 ± 1 pc)
Mass 7.8 ± 0.2[6] M
Radius 4.5[7] R
Luminosity 3,300[8] L
Temperature 18,890[8] K
Rotational velocity (v sin i) 165[9] km/s
Age 31.4 ± 0.4[6] 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.

Sigma Sagittarii (Sigma Sgr, σ Sagittarii, σ Sgr) is the second brightest star in the constellation Sagittarius. Its modern name Nunki is an Assyrian or Babylonian name recovered by archaeologists and made public by R. H. Allen.[10] Nunki 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,[11] yields an estimated value of 228 light-years (70 parsecs) from Earth.[1][2]

Nunki has a spectrum matching a stellar classification of B2.5 V,[4] which indicates this is a B-type main sequence star. The total luminosity of σ Sgr is 3300[12] times that of the Sun while it has a surface temperature of 18,890 K.[8] X-ray emission has been detected from this star, which has an estimated X-ray luminosity of 1.2 × 1028 erg s−1.[13] Sigma Sgr has a magnitude +9.5 optical companion located 5.2 arcminutes away.[citation needed]

Because it is close to the ecliptic, Nunki can be occulted by the Moon and very rarely by planets. The last occultation of Nunki by a planet took place on November 17, 1981, when it was occulted by Venus. Furthermore, Nunki 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]

Name and etymology[edit]


  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, F. (November 2007). "Validation of the new Hipparcos reduction". Astronomy and 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, Bibcode:1982MSS...C03....0H 
  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. ^ 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 
  7. ^ 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: 601–605, Bibcode:1979MNRAS.189..601U 
  8. ^ 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, Bibcode:2009A&A...501..297Z, doi:10.1051/0004-6361/200811147 
  9. ^ 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 
  10. ^ Allen, Richard Hinckley, Star Names, their lore and meaning, p. 359 
  11. ^ Perryman, Michael (2010), 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 
  12. ^ James B. Kaler, "NUNKI (Sigma Sagatarii)", Stars (University of Illinois), retrieved 2012-01-31 
  13. ^ 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 
  14. ^ "Sagittarius". Retrieved 30 August 2008. 
  15. ^ skywatchers[dead link]
  16. ^ 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. 
  17. ^ 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: 430. Bibcode:1895MNRAS..55..429K. 
  18. ^ (Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日