Sigma Sagittarii

Sigma Sagittarii
Location of σ Sagittarii (circled)
Observation data
Epoch J2000      Equinox ICRS
Constellation Sagittarius
Right ascension 18h 55m 15.92650s
Declination −26° 17′ 48.2068″
Apparent magnitude (V) +2.05
Characteristics
Spectral type B2.5 V
U−B color index −0.761
B−V color index −0.204
Astrometry
Radial velocity (Rv)−11.2 km/s
Proper motion (μ) RA: +15.14 mas/yr
Dec.: −53.43 mas/yr
Parallax (π)14.32±0.29 mas
Distance224.7 ± 9.8 ly
(68.9±3.0 pc)
Absolute magnitude (MV)−2.17
Orbit
Period (P)134.779±0.025 days
Semi-major axis (a)(18.0±0.2)×10−3"
(1.26±0.05 au)
Eccentricity (e)0.492±0.003
Inclination (i)19.7±1.9°
Longitude of the node (Ω)45.9±8.0°
Periastron epoch (T)48,588.1±2.2 MJD
Argument of periastron (ω)
(secondary)		76.4±7.1°
Details
Aa
Mass6.5±0.2 M☉
Radius4.1±0.5 R☉
Surface gravity (log g)4.0 cgs
Temperature18,500 K
Rotational velocity (v sin i)165 km/s
Age31.4±0.4 Myr
Ab
Mass6.3±0.2 M☉
Radius3.9±0.5 R☉
Surface gravity (log g)4.0 cgs
Temperature18,500 K
Rotational velocity (v sin i)160 km/s
Age30 Myr
Other designations
Nunki, Sadira, σ Sgr, Sigma Sgr, 34 Sagittarii, CD−26 13595, CPD−27 5241, FK5 706, GC 25941, HD 175191, HIP 92855, HR 7121, SAO 187448, PPM 269078, CCDM J18552-2618A, WDS J18553-2618Aa,Ab
Database references
SIMBADdata

Sigma Sagittarii, Latinized from σ Sagittarii; formally named Nunki /ˈnʌŋki/, is the second-brightest star in the constellation of Sagittarius. It is a binary star system, viewed as a single star of combined apparent magnitude +2.05, about the same brightness as Saiph in Orion. The distance to this system, determined using a dynamical parallax measurement, is 225 light-years (68.9 parsecs).

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.

Properties

Sigma Sagittarii has a spectrum matching a stellar classification of B2.5 V, which indicates its components are B-type main-sequence stars. X-ray emission has been detected from this star, which has an estimated X-ray luminosity of 1.2 × 1028 erg s−1.

The two component stars have masses of 6.5 and 6.3 solar masses (M☉) and radii of 4.1 and 3.9 solar radii (R☉), respectively. The effective temperature of both components is about 18,500 K, over three times the Sun's effective temperature of 5,772 K. They take 134.779 days to complete an orbit, following a moderately eccentric path with e = 0.492 and an orbital semi-major axis of 1.26 astronomical units (au), implying a periastron of 0.64 au. There is strong evidence that the orbit, which is inclined at 20° relative to Earth, is misaligned with the stellar spin axes. The age of the system is estimated at about 30 million years.

Nunki is the nearest star expected to explode in a core-collapse supernova. 25 million years in the future, the primary star will evolve to a red giant, fill its roche lobe, and start to transfer mass to the secondary star. This will result in the system merging into a single star of over 10 solar masses, a mass sufficient to explode as a supernova.

The binary nature of Sigma Sagittarii was uncovered in 1974 with interferometric observations, which found the components to have a magnitude difference of 0.46. Interferometric data published in 1994 partially resolved the system, finding a separation of 11.5 milliarcseconds under the assumption of equally bright components. Spectroscopic observations in 2007-2008 hinted at the presence of a yet unseen stellar companion with a possible orbital period of 6.8 days, estimated to have a mass of 1.2±0.2 M☉ and a temperature of 6,100 K, but follow-up observations in 2016 by the same authors did not confirm this. Interferometric observations published in 2025 again found Nunki to be a close binary of similar components, which were detected at an angular separation of 8.644 milliarcseconds. There is a 10th magnitude optical companion located 5.2 arcminutes away, but this is an unrelated background star.

Nomenclature

σ Sagittarii (Latinised to Sigma Sagittarii) is the star's Bayer designation. In his Uranometria star atlas, Johann Bayer placed this star in the fourth magnitude class, although it is a second-magnitude star by modern measurements.

It bore the traditional name of Nunki, which was an Assyrian or Babylonian name recovered by archaeologists and popularized by R. H. Allen, though it is now thought that this name originally referred to an asterism in the area of Vela. The name Nunki is of Sumerian origin and, in the Sumerian written form NUNKI (‘'Divine Place of the Earth’'), is the name of the ancient holy city Eridu, seat of the great underground freshwater ocean Apzu. In Sumerian, the star Canopus was called MUL.NUNKI, which translates to “Star of the City of Eridu” and can be linked to this star becoming visible after the city’s founding due to the precession of the Earth’s axis.

In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN) 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.

This star, together with:

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.

In Chinese, 斗 (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 Xiù sì, English: the Fourth Star of Dipper.)

See also

References

  1. 1 2 3 4 5 van Leeuwen, Floor (13 August 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. eISSN 1432-0746. ISSN 0004-6361.
  2. 1 2 Wielen, R.; et al. (1999). "Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions". Veröff. Astron. Rechen-Inst. Heidelb. 35 (35). Astronomisches Rechen-Institut Heidelberg: 1. Bibcode:1999VeARI..35....1W.
  3. 1 2 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. 3. Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan. Bibcode:1982mcts.book.....H.
  4. 1 2 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.
  5. 1 2 3 4 5 6 7 8 9 10 Waisberg, Idel; Katz, Boaz (2026-03-17). "Confirming Nunki as the closest core collapse progenitor candidate to the Sun". arXiv:2603.17011 [astro-ph.SR].
  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. S2CID 119257644.
  7. 1 2 3 4 5 6 7 8 9 Waisberg, Idel; Klein, Ygal; Katz, Boaz (April 2025). "Hidden Companions to Intermediate-mass Stars. XXVI. Uncovering Nunki = Sigma Sagittarii as a 6.5 M⊙ + 6.3 M⊙, 0.60 au Binary". Research Notes of the AAS. 9 (4): 71. doi:10.3847/2515-5172/adc739.
  8. ↑ 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.
  9. 1 2 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. S2CID 118629873.
  10. ↑ Paul Kunitzsch [in German] (1959). Arabische Sternnamen in Europa. Wiesbaden: Otto Harrassowitz. p. 137. Retrieved 2020-08-30.
  11. ↑ Robert Burnham, Jr. (1978). Burnham's Celestial Handbook: An Observer's Guide to the Universe Beyond the Solar System. Vol. 3. New York: Dover Publications Inc. p. 1566. ISBN 9780486318035. Retrieved 2020-08-30.
  12. ↑ Davis, George A. (1944). "The pronunciations, derivations, and meanings of a selected list of star names". Popular Astronomy. 52: 26. Bibcode:1944PA.....52....8D.
  13. 1 2 "All Skies Encyclopedia". IAU WGSN. Retrieved 9 May 2026.
  14. ↑ 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.
  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. ↑ Hanbury Brown, R.; Davis, J.; Allen, L. R. (April 1974). "The Angular Diameters of 32 Stars". Monthly Notices of the Royal Astronomical Society. 167: 121–136. Bibcode:1974MNRAS.167..121H. doi:10.1093/mnras/167.1.121. ISSN 0035-8711.
  17. ↑ Bedding, T. R.; Robertson, J. G.; Marson, R. G. (October 1994). "An optical interferometer with wavelength dispersion". Astronomy and Astrophysics. 290: 340–348. arXiv:astro-ph/9404001. Bibcode:1994A&A...290..340B. ISSN 0004-6361.
  18. ↑ Gullikson, Kevin; Dodson-Robinson, Sarah (January 2013). "Detection of Low-Mass-ratio Stellar Binary Systems". The Astronomical Journal. 145 (1): 3. arXiv:1210.6360. Bibcode:2013AJ....145....3G. doi:10.1088/0004-6256/145/1/3.
  19. ↑ Gullikson, Kevin; Kraus, Adam; Dodson-Robinson, Sarah (2016-08-01). "The Close Companion Mass-ratio Distribution of Intermediate-mass Stars". The Astronomical Journal. 152 (2): 40. arXiv:1604.06456. Bibcode:2016AJ....152...40G. doi:10.3847/0004-6256/152/2/40. ISSN 0004-6256.
  20. ↑ 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.
  21. ↑ "CD-26 13591". SIMBAD. Centre de données astronomiques de Strasbourg.
  22. ↑ Bayer, Johann (1603). Uranometria omnium asterismorum continens schemata, nova methodo delineata aereis laminis expressa. Bibcode:1603uoac.book.....B.
  23. 1 2 3 Allen, Richard Hinckley (1936) [1899]. Star Names and Their Meanings. New York: G. E. Stechert. p. 355. OCLC 9825499.
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  25. ↑ Bautsch, Markus; Pedde, Friedhelm (2023-05-01). "Canopus, the "Star of the City of Eridu"" (PDF). Dem Himmel nahe (in German). 17. Berlin: Wilhelm Foerster Observatory e.V.: 8–9. ISSN 2940-9330.
  26. ↑ "IAU Working Group on Star Names (WGSN)". International Astronomical Union. Retrieved 22 May 2016.
  27. ↑ "Teapot". constellation-guide.com. 25 August 2015. Retrieved 2017-05-13.
  28. ↑ 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.
  29. ↑ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日 Archived 2011-05-22 at the Wayback Machine