Eta Sagittarii

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.[10]

η Sagittarii
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
Characteristics
Spectral type M2 III[3] + F7 V[4]
U−B color index +1.71[2]
B−V color index +1.56[2]
Variable type Lb[5]
Astrometry
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
Distance146 ± 2 ly
(44.7 ± 0.5 pc)
Absolute magnitude (MV)−4.90[7]
Details
η Sgr A
Radius57[8] R☉
Other designations
Sephdar, η Sagittarii, η Sgr, CCDM J18176-3646AB, CD−36°12423, CPD-36°8128, FK5 683, GC 24944, HD 167618, HIP 89642, HR 6832, IDS 18109-3647 AB, PPM 297508, SAO 209957, WDS J18176-3646AB.[9]
Database references
SIMBADdata

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.[11] This star is classified as an oxygen-rich irregular variable,[7] 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.[12] At the estimated distance of Eta Draconis,[1] this yields a physical size of about 57 times the radius of the Sun.[8]

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.[13] 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°.[14] 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.[7] Because of proper motion, this star will move into constellation Corona Australis around 6300 CE.[15] 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°.[13]

Name and etymology
  • This star, together with γ Sgr, δ Sgr and ε Sgr were Al Naʽām al Wārid (النعم الوارد), the Going Ostriches.[16] 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[17]
  • In the catalogue of stars in the Calendarium of Al Achsasi al Mouakket, this star was designated Rabah al Waridah or Rabi al Waridah, meaning fourth of Warida.[18]
  • In Chinese, 箕 (Jī), meaning Winnowing Basket, refers to an asterism consisting of η Sagittarii, γ Sagittarii, δ Sagittarii and ε Sagittarii. Consequently, the Chinese name for η Sagittarii itself is 箕宿四 (Jī Sù sì, English: the Fourth Star of Winnowing Basket.)[19]

References
  1. 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, S2CID 18759600.
  2. 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. 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, Ann Arbor, Michigan: Dept. of Astronomy, University of Michigan, 3, Bibcode:1982mcts.book.....H.
  4. Kaler, James B., "Eta Sagittarii", Stars, University of Illinois, retrieved 2012-02-10.
  5. Samus, N. N.; Durlevich, O. V.; et al. (2009). "VizieR Online Data Catalog: General Catalogue of Variable Stars (Samus+ 2007-2013)". VizieR On-line Data Catalog: B/GCVS. Originally Published in: 2009yCat....102025S. 1. Bibcode:2009yCat....102025S.
  6. Wilson, R. E. (1953), "General Catalogue of Stellar Radial Velocities", Washington, Carnegie Institute of Washington D.C., Bibcode:1953GCRV..C......0W.
  7. Mennessier, M. O.; et al. (August 2001), "Long period variable stars: galactic populations and infrared luminosity calibrations", Astronomy and Astrophysics, 374 (3): 968–979, arXiv:astro-ph/0105552, Bibcode:2001A&A...374..968M, doi:10.1051/0004-6361:20010764, S2CID 15721872.
  8. 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:
  9. "HD 167618 -- Variable Star", SIMBAD Astronomical Database, retrieved 2007-03-29.
  10. O'Meara, Stephen James (2011), Deep-Sky Companions: The Secret Deep, Cambridge University Press, p. 341, ISBN 978-0-521-19876-9.
  11. 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.
  12. 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, S2CID 2847613
  13. 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.
  14. 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..235J.
  15. Moore, Patrick; Rees, Robin (2011), Patrick Moore's Data Book of Astronomy, Cambridge: Cambridge University Press, p. 296, ISBN 9781139495226.
  16. 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.
  17. 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
  18. 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.
  19. (in Chinese) AEEA (Activities of Exhibition and Education in Astronomy) 天文教育資訊網 2006 年 5 月 11 日
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