Epoch J2000 Equinox J2000
|Right ascension||16h 18m 19.28974s|
|Declination||–04° 41′ 33.0345″|
|Apparent magnitude (V)||3.220|
|Spectral type||G9.5 IIIb|
|U−B color index||+0.762|
|B−V color index||+0.972|
|Radial velocity (Rv)||–10.3 km/s|
|Proper motion (μ)||RA: +83.40 mas/yr
Dec.: +40.58 mas/yr
|Parallax (π)||30.64 ± 0.20 mas|
|Distance||106.4 ± 0.7 ly
(32.6 ± 0.2 pc)
|Absolute magnitude (MV)||+0.55|
|Mass||1.85 ± 0.05 M☉|
|Radius||10.39 ± 0.07 R☉|
|Surface gravity (log g)||2.59 ± 0.08 cgs|
|Temperature||4,918 ± 28 K|
|Metallicity [Fe/H]||–0.13 ± 0.06 dex|
|Rotational velocity (v sin i)||5.7 km/s|
Epsilon Ophiuchi (ε Oph, ε Ophiuchi) is a red giant star in the constellation Ophiuchus. It has the traditional name Yed Posterior. The star Delta Ophiuchi, with which it forms a naked eye optical double, is named Yed Prior. The name Yed comes from Arabic for "the hand".
Epsilon Ophiuchi is located less than five degrees south of the celestial equator in the eastern part of the constellation. With an apparent visual magnitude of 3.220, this allows the star to be seen with the naked eye from most of the Earth under suitably dark skies. Parallax measurements yield an estimated distance of 106.4 light-years (32.6 parsecs). It has a stellar classification of G9.5 IIIb, with the luminosity class of III indicating that this is a giant star that has exhausted the hydrogen and evolved away from the main sequence. This red giant has nearly double the Sun's mass and has expanded to an estimated radius of over ten times the radius of the Sun, giving it a luminosity of about 54 times the Sun. It is about a billion years old.
Unusually for a class G giant, it is cyanogen-deficient and carbon-deficient. The outer envelope of this star displays solar-type oscillations with a period of 0.19 days, allowing the methods of asteroseismology to be applied. However, the models for this star have not been able to distinguish whether this star is generating energy by the thermonuclear fusion of hydrogen along a shell, or the fusion of helium at its core. Either mode produces a good fit to the star's physical properties. The projected rotational velocity of the star is 5.7 km s−1, and the inclination of the rotation axis to the line of sight from the Earth lies in the range of 41–73°.
The traditional name Yed is derived from Arabic meaning "the hand", and the two stars are the left hand of Ophiuchus (the Serpent Bearer) that holds the head of the serpent Serpens Caput. It was a member of indigenous Arabic asterism al-Nasaq al-Yamānī, "the Southern Line" of al-Nasaqān "the Two Lines", along with α Ser (Unukalhai), δ Ser (Qin, Tsin), ε Ser (Ba, Pa), δ Oph (Yed Prior), ζ Oph (Han) and γ Oph (Tsung Ching).
According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, al-Nasaq al-Yamānī or Nasak Yamani were the title for two stars :δ Ser as Nasak Yamani I and ε Ser as Nasak Yamani II (exclude this star, α Ser, δ Oph, ζ Oph and γ Oph).
In Chinese, 天市右垣 (Tiān Shì Yòu Yuán), meaning Right Wall of Heavenly Market Enclosure, refers to an asterism which is represent eleven old states in China which is marking the right borderline of the enclosure, consisting of ε Ophiuchi, β Herculis, γ Herculis, κ Herculis, γ Serpentis, β Serpentis, α Serpentis, δ Serpentis, ε Serpentis, δ Ophiuchi and ζ Ophiuchi. Consequently, ε Ophiuchi itself is known as 天市右垣十 (Tiān Shì Yòu Yuán shí, English: the Tenth Star of Right Wall of Heavenly Market Enclosure), represent the state Chu (楚) (or Tsoo), together with φ Capricorni (or 24 Capricorni in R.H.Allen's version) in Twelve States (asterism).
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