Zeta Sagittarii

From Wikipedia, the free encyclopedia
Jump to: navigation, search
Zeta Sagittarii
Diagram showing star positions and boundaries of the Sagittarius constellation and its surroundings
Cercle rouge 100%.svg

Location of ζ Sagittarii (circled) near the center
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
Distance 88 ± 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°
Mass 5.26 ± 0.37[3] M
Surface gravity (log g) 3.90[6] cgs
Temperature 8,799[6] K
Rotational velocity (v sin i) 77[7] km/s
Age 500 - 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), also named Ascella,[8] is a binary star 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]


ζ Sagittarii (Latinised to Zeta Sagittarii) is the system's Bayer designation.

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.[9] In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)[10] to catalogue and standardize proper names for stars. The WGSN approved the name Ascella for this star on 12 September 2016 and it is now so entered in the IAU Catalog of Star Names.[8]

This star, together with Gamma Sagittarii, Delta Sagittarii, Epsilon Sagittarii, Lambda Sagittarii, Sigma Sagittarii, Tau Sagittarii and Phi Sagittarii comprise the Teapot asterism.[11]

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, Zeta Sagittarii itself is known as 斗宿一 (Dǒu Sù yī, English: the First Star of Dipper.)[12]


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


Zeta Sagittarii has an 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.[13]

The two components orbit each other over a period of 21 years at an eccentricity of 0.211. The combined mass of the system is 5.26 ± 0.37 times the mass of the Sun[3] and their blended stellar classification is A2.5 Va.

The system consists of a spectral class A2 giant with an apparent magnitude of +3.27, and an A4 subgiant with apparent magnitude of +3.48. The pair have a mean separation of 13.4 astronomical units (AU).[14] The primary has a faint, 10th magnitude companion, separated from it 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:0708.1752Freely accessible, 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:1112.3666Freely accessible, Bibcode:2012MNRAS.422.2765D, doi:10.1111/j.1365-2966.2011.20397.x 
  4. ^ "CCDM J19026-2953AB -- Double or multiple star", SIMBAD Astronomical Object Database, 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/0308182Freely accessible, 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/0610785Freely accessible, Bibcode:2007A&A...463..671R, doi:10.1051/0004-6361:20065224 
  8. ^ a b "IAU Catalog of Star Names". Retrieved 28 July 2016. 
  9. ^ 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. 
  10. ^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016. 
  11. ^ "Teapot". constellation-guide.com. Retrieved 2017-05-13. 
  12. ^ (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日
  13. ^ 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 
  14. ^ Kaler, James B., "ASCELLA (Zeta Sagittarii)", Stars, University of Illinois, retrieved 2012-02-18