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] 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	18^h 17^m 37.63505^s^[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 mas^[1]
Distance	146 ± 2 ly (44.7 ± 0.5 pc)
Absolute magnitude (MV)	−4.90^[7]
Details
η Sgr A
Radius	57^[8] R☉
Other designations
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
SIMBAD	data

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 Sagittarii,^[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, vol. 1 (3 ed.), Birkhäuser, ISBN 3-540-29692-1. The radius (R_*) is given by:

   ${\displaystyle {\begin{aligned}2\cdot R_{*}&={\frac {(44.7\cdot 11.9\cdot 10^{-3})\ {\text{AU}}}{0.0046491\ {\text{AU}}/R_{\bigodot }}}\\&\approx 114\cdot R_{\bigodot }\end{aligned}}}$ 

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, vol. 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.
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