Eta Sagittarii

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Eta Sagittarii
Diagram showing star positions and boundaries of the Sagittarius constellation and its surroundings
Cercle rouge 100%.svg

Location of η Sagittarii (circled)
Observation data
Epoch J2000      Equinox J2000
Constellation Sagittarius
Right ascension 18h 17m 37.63505s[1]
Declination −36° 45′ 42.0667″[1]
Apparent magnitude (V) +3.11[2]/+7.8
Spectral type M2 III[3] + F7 V[4]
U−B color index +1.71[2]
B−V color index +1.56[2]
Variable type Irregular[5]
Radial velocity (Rv) +0.5[6] km/s
Proper motion (μ) RA: −129.56[1] mas/yr
Dec.: −166.26[1] mas/yr
Parallax (π) 22.35 ± 0.24[1] mas
Distance 146 ± 2 ly
(44.7 ± 0.5 pc)
Absolute magnitude (MV) −4.90[5]
η Sgr A
Radius 57[7] R
Other designations
η Sagittarii, η Sgr, Eta Sgr, CCDM J18176-3646AB, CPD-36 8128, FK5 683, GC 24944, HD 167618, HIP 89642, HR 6832, IDS 18109-3647 AB, PPM 297508, SAO 209957, WDS J18176-3646AB.[8]

Eta Sagittarii (Eta Sgr, η Sagittarii, η Sgr) is a binary star system in the southern zodiac constellation of Sagittarius. Based upon parallax measurements, it is located at a distance of 146 light-years (45 parsecs) from Earth.[1] It was formerly known as "Beta Telescopii" (β Tel). In India, where part of the constellation of Sagittarius represents an Elephant, this star forms the creature's tail.[9]

The primary component, η Sagittarii A, is a red giant star with a stellar classification of M2 III.[3] It is an evolved star that is currently at a stage called the asymptotic giant branch, having exhausted both the hydrogen and the helium at its core.[10] This star is classified as an oxygen-rich irregular variable,[5] as it undergoes small magnitude fluctuations between +3.08 and 3.12.[4] The measured angular diameter of this star is 11.9 ± 2.1 mas.[11] At the estimated distance of Eta Draconis,[1] this yields a physical size of about 57 times the radius of the Sun.[7]

The companion, η Sagittarii B, was first noted by American astronomer S. W. Burnham in 1879. The two stars share a common proper motion and hence are probably gravitationally bound to each other.[12] The secondary is likely an F-type main sequence star with an apparent magnitude of +7.77. It located at an angular separation of 3.6 arcseconds from the primary, along a position angle of 108°.[13] This star is at a projected distance of 165 Astronomical Units from the red giant primary and the pair take a minimum of 1,270 years to complete an orbit.[4]

Within the context of the Milky Way galaxy, this system is a member of the faint old disk group.[5] Because of proper motion, this star will move into constellation Corona Australis around 6300 CE.[14] Eta Sagittarii has two optical companions that are not physically associated with the system. The first is a 10th magnitude star at an angular separation of 93 arcseconds with a position angle of 303°. There is a fainter, 13th magnitude star at an angular separation of 33 arcseconds along a position angle of 276°.[12]

Name and etymology[edit]

  • This star, together with γ Sgr, δ Sgr and ε Sgr were Al Naʽām al Wārid (النعم الوارد), the Going Ostriches.[15] According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, Al Naʽām al Wārid or Namalwarid was the title for this star[16]
  • In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Rabah al Waridah, which was translated into Latin as Quarta τού al Warida, meaning fourth going ostrich.[17]
  • In Chinese, (), meaning Winnowing Basket, refers to an asterism consisting of η Sagittarii, γ Sagittarii, δ Sagittarii and ε Sagittarii. Consequently, η Sagittarii itself is known as 箕宿四 (Jī Sù sì, English: the Fourth Star of Winnowing Basket.)[18]


  1. ^ a b c d e f g 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 Nicolet, B. (1978), "Photoelectric photometric Catalogue of homogeneous measurements in the UBV System", Astronomy and Astrophysics Supplement Series 34: 1–49, Bibcode:1978A&AS...34....1N. 
  3. ^ a b Houk, Nancy (1979), Michigan catalogue of two-dimensional spectral types for the HD stars 3, Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan, 
  4. ^ a b c Kaler, James B., "Eta Sagittarii", Stars (University of Illinois), retrieved 2012-02-10. 
  5. ^ a b c d Mennessier, M. O. et al. (August 2001), "Long period variable stars: galactic populations and infrared luminosity calibrations", Astronomy and Astrophysics 374: 968–979, arXiv:astro-ph/0105552, Bibcode:2001A&A...374..968M, doi:10.1051/0004-6361:20010764. 
  6. ^ Wilson, R. E. (1953), General Catalogue of Stellar Radial Velocities, Carnegie Institute of Washington D.C., Bibcode:1953GCRV..C......0W. 
  7. ^ a b Lang, Kenneth R. (2006), Astrophysical formulae, Astronomy and astrophysics library 1 (3 ed.), Birkhäuser, ISBN 3-540-29692-1.  The radius (R*) is given by:
    \begin{align} 2\cdot R_*
 & = \frac{(44.7\cdot 11.9\cdot 10^{-3})\ \text{AU}}{0.0046491\ \text{AU}/R_{\bigodot}} \\
 & \approx 114\cdot R_{\bigodot}
  8. ^ "HD 167618 -- Variable Star", SIMBAD Astronomical Database, retrieved 2007-03-29. 
  9. ^ O'Meara, Stephen James (2011), Deep-Sky Companions: The Secret Deep, Cambridge University Press, p. 341, ISBN 0-521-19876-3. 
  10. ^ Eggen, Olin J. (July 1992), "Asymptotic giant branch stars near the sun", Astronomical Journal 104 (1): 275–313, Bibcode:1992AJ....104..275E, doi:10.1086/116239. 
  11. ^ Richichi, A.; Percheron, I. (May 2005), "First results from the ESO VLTI calibrators program", Astronomy and Astrophysics 434 (3): 1201–1209, arXiv:astro-ph/0501532, Bibcode:2005A&A...434.1201R, doi:10.1051/0004-6361:20042257 
  12. ^ a b Burnham, Robert (1978), Burnham's Celestial Handbook: an observer's guide to the universe beyond the solar system, Dover books explaining science 3 (2nd ed.), Courier Dover Publications, ISBN 0-486-23673-0. 
  13. ^ Jasinta, D. M. D.; Soegiartini, E. (October 1994), "Photographic observations of visual double stars", Astronomy and Astrophysics Supplement 107: 235–241, Bibcode:1994A&AS..107..235. 
  14. ^ Moore, Patrick; Rees, Robin (2011), Patrick Moore's Data Book of Astronomy, Cambridge: Cambridge University Press, p. 296. 
  15. ^ 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. 
  16. ^ Jack W. Rhoads - Technical Memorandum 33-507-A Reduced Star Catalog Containing 537 Named Stars, Jet Propulsion Laboratory, California Institute of Technology; November 15, 1971
  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