Jump to content

Sigma Sagittarii

From Wikipedia, the free encyclopedia
Sigma Sagittarii
Location of σ Sagittarii (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 18h 55m 15.92650s[1]
Declination –26° 17′ 48.2068″[1]
Apparent magnitude (V) +2.05[2]
Characteristics
Spectral type B2.5 V[3]
U−B color index −0.761[4]
B−V color index −0.204[4]
Astrometry
Radial velocity (Rv)−11.2[2] km/s
Proper motion (μ) RA: +15.14 mas/yr[1]
Dec.: −53.43 mas/yr[1]
Parallax (π)14.32±0.29 mas[1]
Distance228 ± 5 ly
(70 ± 1 pc)
Absolute magnitude (MV)−2.17[5]
Details[6]
Aa
Mass6.5±0.2 M☉
Radius4.1±0.5 R☉
Temperature18,890[7] K
Rotational velocity (v sin i)165[8] km/s
Age31.4±0.4[9] Myr
Ab
Mass6.3±0.2 M☉
Radius3.9±0.5 R☉
Temperature18,600 K
Other designations
Nunki, Sadira[10][11], 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/,[12][13] 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 parallax measurements from the Hipparcos astrometry satellite, is 228 light-years (70 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.[14]

Properties

[edit]

Sigma Sagittarii has a spectrum matching a stellar classification of B2.5 V,[3] 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.[15]

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 temperatures are 18,900 K and 18,600 K respectively, over three times the Sun's effective temperature of 5,772 K. They are separated by about 0.6 astronomical units and take 50 days to complete an orbit around each other.[6] Estimates suggest an age of about 30 million years.[9][6]

Sigma Sagittarii is the nearest star expected to explode in a core-collapse supernova. 20 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 about 12 solar masses, a mass sufficient to explode as a supernova.[6]

Sigma Sagittarii has a 10th magnitude optical companion located 5.2 arcminutes away,[16] but this is an unrelated background star.[17] Spectroscopic observations in 2007-2008 hinted at the presence of a yet unseen stellar companion, estimated to have a mass of 1.2±0.2 M☉ and a temperature of 6,100 K,[18] but follow-up observations did not confirm this. Instead, it was found in 2025 that Nunki is made up of two nearly identical stars. The separation of the components is too small to resolve them with a telescope, requiring an interferometer to be used.[6]

Nomenclature

[edit]

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.[19]

It bore the traditional name of Nunki, which was an Assyrian or Babylonian name recovered by archaeologists and made public by R. H. Allen.[20] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[21] 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.[13]

This star, together with :

Zeta Sagittarii and Pi Sagittarii may have been the Akkadian Gu-shi-rab‑ba, the Yoke of the Sea.[23]

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.[24]

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.)[25]

See also

[edit]
  • Spica – Second-closest star that will explode in a supernova.

References

[edit]
  1. ^ a b c d 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. ^ a b 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. ^ 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, 3, Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan, Bibcode:1982mcts.book.....H
  4. ^ 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
  5. ^ 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.
  6. ^ a b c d e 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.
  7. ^ 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, S2CID 14969137
  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. ^ 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, 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. ^ a b "Naming Stars". IAU.org. Retrieved 16 December 2017.
  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. ^ 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. ^ "CD-26 13591". SIMBAD. Centre de données astronomiques de Strasbourg.
  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. ^ Bayer, Johann (1603). Uranometria omnium asterismorum continens schemata, nova methodo delineata aereis laminis expressa. Bibcode:1603uoac.book.....B.
  20. ^ Allen, Richard Hinckley, Star Names, their lore and meaning, p. 359
  21. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  22. ^ "Teapot". constellation-guide.com. Retrieved 2017-05-13.
  23. ^ 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.
  24. ^ 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.
  25. ^ (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
Cite error: A list-defined reference named "kaler" is not used in the content (see the help page).
Retrieved from "https://en.wikipedia.org/w/index.php?title=Sigma_Sagittarii&oldid=1284314992"