Epoch J2000 Equinox J2000
|Right ascension||17h 10m 22.68689s|
|Declination||–15° 43′ 29.6639″|
|Apparent magnitude (V)||2.43 (3.05/3.27)|
|Spectral type||A1 V + A3 V|
|U−B color index||0.09|
|B−V color index||0.05|
|Radial velocity (Rv)||–0.9 km/s|
|Proper motion (μ)||RA: +40.13 mas/yr
Dec.: +99.17 mas/yr
|Parallax (π)||36.91 ± 0.80 mas|
|Distance||88 ± 2 ly
(27.1 ± 0.6 pc)
|Period (P)||87.58 ± 1.00 yr|
|Semi-major axis (a)||1.396 ± 0.010"|
|Eccentricity (e)||0.950 ± 0.020|
|Inclination (i)||95.2 ± 2.0°|
|Longitude of the node (Ω)||38.9 ± 1.0°|
|Periastron epoch (T)||2024.68 ± 0.50|
|Argument of periastron (ω)
|274.8 ± 2.0°|
|η Oph B|
|Rotational velocity (v sin i)||23 km/s|
η Oph is actually a binary star system that is difficult to resolve in amateur telescopes but whose true nature has been determined through use of more advanced techniques. The "primary" star (whose observational data make up the table in this article), which could nominally be referred to as η Oph A, is actually only slightly larger and warmer than its companion, η Oph B. Individually each star is a fairly unremarkable A class main sequence star, but as a binary pair they are unusual. Each star orbits around a common center in a close and highly elliptical orbit, making planetary formation impossible in this system and some stellar data imprecise.
In Chinese, this star is considered part of 天市左垣 (Tiān Shì Zuǒ Yuán), meaning Left Wall of Heavenly Market Enclosure, which refers to an asterism (pattern of stars) representing eleven old states in China that mark the left borderline of the enclosure, consisting of η Ophiuchi, δ Herculis, λ Herculis, μ Herculis, ο Herculis, 112 Herculis, ζ Aquilae, θ1 Serpentis, η Serpentis, ν Ophiuchi and ξ Serpentis. Consequently, η Ophiuchi itself is known as 天市左垣十一 (Tiān Shì Zuǒ Yuán shíyī, English: the Eleventh Star of Left Wall of Heavenly Market Enclosure), representing the state Song(宋).
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