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.^[13]

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[3]
Characteristics
Spectral type	M2 III[4] + G8:IV:[5]
U−B color index	+1.71[2]
B−V color index	+1.56[2]
Variable type	Lb[6]
Astrometry
Radial velocity (Rv)	+0.5[7] 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[8]
Details[9]
η Sgr A
Mass	2.1±0.7 M☉
Radius	57.8[10] R☉
Luminosity	525±6 L☉
Surface gravity (log g)	1.13±0.05 cgs
Temperature	3,638±125 K
B
Temperature	5,143[11] K
Other designations
η Sagittarii, η Sgr, β Telescopii, β Tel, 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[12]
Database references
SIMBAD	data
	B

The primary component, η Sagittarii A, is a red giant star with a stellar classification of M2 III.^[4] 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.^[14] This star is classified as an oxygen-rich irregular variable,^[8] as it undergoes small magnitude fluctuations between +3.08 and 3.12.^[15] The measured angular diameter of this star is 11.9±2.1 mas.^[16] At the estimated distance of Eta Sagittarii,^[1] this yields a physical size of about 57 times the radius of the Sun.^[17]

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.^[18] 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°.^[19] 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.^[15]

Within the context of the Milky Way galaxy, this system is a member of the faint old disk group.^[8] Because of proper motion, this star will move into constellation Corona Australis around 6300 CE.^[20] 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°.^[18]

Name and etymology

• This star, together with γ Sgr, δ Sgr and ε Sgr were Al Naʽām al Wārid (النعم الوارد), the Going Ostriches.^[21] 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^[22]
• 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.^[23]
• 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.)^[24]