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]
  | |
| 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 mas[1] |
| Distance | 146 ± 2 ly (44.7 ± 0.5 pc) |
| Absolute magnitude (MV) | −4.90[7] |
| Details[8] | |
| η Sgr A | |
| Mass | 2.1±0.7 M☉ |
| Radius | 66±12 R☉ |
| Luminosity | 525±6 L☉ |
| Surface gravity (log g) | 1.13±0.05 cgs |
| Temperature | 3638±125 K |
| Other designations | |
| 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.[13]
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.[14] 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°.[15] 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.[16] 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°.[14]
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.[17] 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[18]
- 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.[19]
- 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.)[20]