Zeta Sagittarii

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ζ Sagittarii
Sagittarius constellation map.svg
Red circle.svg
Location of ζ Sagittarii (circled)
Observation data
Epoch J2000.0      Equinox J2000.0
Constellation Sagittarius
Right ascension  19h 02m 36.73024s[1]
Declination –29° 52′ 48.2279″[1]
Apparent magnitude (V) +2.59[2] (3.27/3.48)[3]
Spectral type A2.5 Va[4]
U−B color index +0.05[2]
B−V color index +0.08[2]
Radial velocity (Rv)+22[5] km/s
Proper motion (μ) RA: +10.79[1] mas/yr
Dec.: +21.11[1] mas/yr
Parallax (π)36.98 ± 0.87[1] mas
Distance88 ± 2 ly
(27.0 ± 0.6 pc)
Absolute magnitude (MV)1.11/1.32[3]
Period (P)21.00 ± 0.01 years
Semi-major axis (a)0.489 ± 0.001″
Eccentricity (e)0.211 ± 0.001
Inclination (i)111.1 ± 0.1°
Longitude of the node (Ω)74.0 ± 0.1°
Periastron epoch (T)2005.99 ± 0.03
Argument of periastron (ω)
7.2 ± 0.6°
Mass5.26 ± 0.37[3] M
Surface gravity (log g)3.90[6] cgs
Temperature8,799[6] K
Rotational velocity (v sin i)77[7] km/s
Age500 - 710[3] Myr
Other designations
Ascella, ζ Sagittarii, ζ Sgr, Zeta Sgr, 38 Sagittarii, CCDM J19026-2953AB, CPD-30  5798, GC 26161, HD 176687, HIP 93506, HR 7194, IDS 18562-3001 AB, PPM 269230, SAO 187600, WDS J19026-2953AB
Database references

Zeta Sagittarii (ζ Sagittarii, abbreviated Zeta Sgr, ζ Sgr) is a triple star system and the third-brightest star in the constellation of Sagittarius. Based upon parallax measurements, it is about 88 ly (27 pc) from the Sun.[1]

The three components are designated Zeta Sagittarii A (officially named Ascella /əˈsɛlə/, the traditional name for the entire system)[8][9] and B, themselves forming a binary pair, and a smaller companion star, C.


ζ Sagittarii (Latinised to Zeta Sagittarii) is the system's Bayer designation. The designations of the two components as ζ Sagittarii A, B and C derive from the convention used by the Washington Multiplicity Catalog (WMC) for multiple star systems, and adopted by the International Astronomical Union (IAU).[10]

It bore the traditional name Ascella, from a Late Latin word meaning armpit. In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Thalath al Sadirah, which was translated into Latin as Tertia τού al Sadirah, meaning third returning ostrich.[11] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[12] to catalogue and standardize proper names for stars. The WGSN decided to attribute proper names to individual stars rather than entire multiple systems.[13] It approved the name Ascella for the component Zeta Sagittarii A on 12 September 2016 and it is now so included in the List of IAU-approved Star Names.[9]

This star, together with Gamma Sagittarii, Delta Sagittarii, Epsilon Sagittarii, Lambda Sagittarii, Sigma Sagittarii, Tau Sagittarii and Phi Sagittarii comprise the Teapot asterism.[14]

In Chinese, (Dǒu), meaning Dipper, refers to an asterism consisting of Zeta Sagittarii, Phi Sagittarii, Lambda Sagittarii, Mu Sagittarii, Sigma Sagittarii and Tau Sagittarii. Consequently,the Chinese name for Zeta Sagittarii itself is 斗宿一 (Dǒu Sù yī, English: the First Star of Dipper.)[15]


Ascella was a United States Navy Crater class cargo ship named after the star.


Zeta Sagittarii has a combined apparent visual magnitude of +2.59.[2] It is moving away from the Solar System with a radial velocity of 22 km s−1,[5] and some 1.0–1.4 million years ago, came within 7.5 ± 1.8 ly (2.30 ± 0.55 pc) of the Sun.[16]

The two components Zeta Sagittarii A and B orbit each other over a period of 21 years at an eccentricity of 0.211. The combined mass of the binary pair is 5.26 ± 0.37 times the mass of the Sun[3] and their blended stellar classification is A2.5 Va. A is a spectral class A2 giant with an apparent magnitude of +3.27, and B is an A4 subgiant with apparent magnitude of +3.48. The pair have a mean separation of 13.4 AU.[17]

The binary pair have a faint, 10th magnitude companion, C, separated from them by a distance of 75 arcseconds.


  1. ^ 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
  2. ^ a b c d Johnson, H. L.; et al. (1966). "UBVRIJKL photometry of the bright stars". Communications of the Lunar and Planetary Laboratory. 4 (99). Bibcode:1966CoLPL...4...99J.
  3. ^ a b c d e f De Rosa, Robert J.; et al. (2011), "The VAST Survey -- II. Orbital motion monitoring of A-type star multiples", Monthly Notices of the Royal Astronomical Society, 422: 2765–2785, arXiv:1112.3666, Bibcode:2012MNRAS.422.2765D, doi:10.1111/j.1365-2966.2011.20397.x
  4. ^ "* zet Sgr". SIMBAD. Centre de données astronomiques de Strasbourg. Retrieved 2012-02-18.
  5. ^ a b Wilson, R. E. (1953). General Catalogue of Stellar Radial Velocities. Carnegie Institute of Washington D.C. Bibcode:1953GCRV..C......0W.
  6. ^ a b Gray, R. O.; et al. (October 2003), "Contributions to the Nearby Stars (NStars) Project: Spectroscopy of Stars Earlier than M0 within 40 Parsecs: The Northern Sample. I.", The Astronomical Journal, 126 (4): 2048–2059, arXiv:astro-ph/0308182, Bibcode:2003AJ....126.2048G, doi:10.1086/378365
  7. ^ Royer, F.; Zorec, J.; Gómez, A. E. (February 2007), "Rotational velocities of A-type stars. III. Velocity distributions", Astronomy and Astrophysics, 463 (2): 671–682, arXiv:astro-ph/0610785, Bibcode:2007A&A...463..671R, doi:10.1051/0004-6361:20065224
  8. ^ Davis, George A. (1944). "The pronunciations, derivations, and meanings of a selected list of star names". Popular Astronomy. 52: 8–30.
  9. ^ a b "Naming Stars". IAU.org. Retrieved 16 December 2017.
  10. ^ 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:1012.0707 [astro-ph.SR].
  11. ^ 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. doi:10.1093/mnras/55.8.429.
  12. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
  13. ^ "WG Triennial Report (2015-2018) - Star Names" (PDF). p. 5. Retrieved 2018-07-14.
  14. ^ "Teapot". constellation-guide.com. Retrieved 2017-05-13.
  15. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日
  16. ^ Dybczyński, P. A. (April 2006), "Simulating observable comets. III. Real stellar perturbers of the Oort cloud and their output", Astronomy and Astrophysics, 449 (3): 1233–1242, Bibcode:2006A&A...449.1233D, doi:10.1051/0004-6361:20054284
  17. ^ Kaler, James B., "ASCELLA (Zeta Sagittarii)", Stars, University of Illinois, retrieved 2012-02-18