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Epsilon Ophiuchi
	Location of ε Ophiuchi (circled)
Observation data; Epoch J2000 Equinox J2000
Constellation	Ophiuchus
Right ascension	16h 18m 19.28974s
Declination	–04° 41′ 33.0345″
Apparent magnitude (V)	3.220
Characteristics
Spectral type	G9.5 IIIb
U−B color index	+0.762
B−V color index	+0.972
Astrometry
Radial velocity (Rv)	–10.3 km/s
Proper motion (μ)	RA: +83.40 mas/yr ; Dec.: +40.58 mas/yr
Parallax (π)	30.5136 ± 0.1878 mas
Distance	106.9 ± 0.7 ly ; (32.8 ± 0.2 pc)
Absolute magnitude (MV)	+0.55
Details
Mass	1.85±0.05 M☉
Radius	10.39±0.07 R☉
Luminosity	54 L☉
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
Age	0.95–1.01 Gyr
Other designations
	Yed Posterior, ε Oph, Epsilon Oph, 2 Ophiuchi, BD–04 4086, FK5 605, HD 146791, HIP 79882, HR 6075, SAO 141086
Database references
SIMBAD	data

Epsilon Ophiuchi or ε Ophiuchi, formally named Yed Posterior (/ˌjɛd pɒˈstɪəriər/),^[12]^[13] is a red giant^[7] star in the constellation of Ophiuchus. Located less than five degrees south of the celestial equator in the eastern part of the constellation,^[1] it forms a naked eye optical double with Delta Ophiuchi (named Yed Prior^[13]). With an apparent visual magnitude of 3.220,^[2] the star can 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) from the Sun.

Nomenclature

ε Ophiuchi (Latinised to Epsilon Ophiuchi) is the star's Bayer designation.

It bore the traditional name Yed Posterior. Yed derives from the Arabic يد yad meaning "hand". Epsilon and Delta Ophiuchi comprise the left hand of Ophiuchus (the Serpent Bearer) that holds the head of the serpent (Serpens Caput). Epsilon is Yed Posterior as it follows Delta across the sky. In 2016, the International Astronomical Union organized a Working Group on Star Names (WGSN)^[14] to catalogue and standardize proper names for stars. The WGSN approved the name Yed Posterior for this star on 5 October 2016 and it is now so included in the List of IAU-approved Star Names.^[13]

Epsilon Ophiuchi was a member of the indigenous Arabic asterism al-Nasaq al-Yamānī, the "Southern Line" of al-Nasaqān the "Two Lines",^[15] along with Alpha Serpentis, Delta Serpentis, Epsilon Serpentis, Delta Ophiuchi, Zeta Ophiuchi and Gamma Ophiuchi.^[16]

In Chinese, 天市右垣 (Tiān Shì Yòu Yuán), meaning Right Wall of Heavenly Market Enclosure, refers to an asterism which represents eleven ancient states in China and which mark the right borderline of the enclosure, consisting of Delta Ophiuchi, Beta Herculis, Gamma Herculis, Kappa Herculis, Gamma Serpentis, Beta Serpentis, Alpha Serpentis, Delta Serpentis, Epsilon Serpentis, Epsilon Ophiuchi and Zeta Ophiuchi.^[17] Consequently, the Chinese name for Epsilon Ophiuchi itself is 天市右垣十 (Tiān Shì Yòu Yuán shí, English: the Tenth Star of Right Wall of Heavenly Market Enclosure), representing the state Chu (楚) (or Tsoo),^[18]^[19]^[20] together with Phi Capricorni (or 24 Capricorni in R.H.Allen's version^[21]) in the Twelve States (asterism).

Properties

Epsilon Ophiuchi 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,^[10] giving it a luminosity of about 54 times the Sun.^[8] It is about a billion years old.^[10]

Unusually for a class G giant, it is cyanogen-deficient and carbon-deficient.^[22] 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.^[7] 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 model produces a good fit to the star's physical properties.^[7] 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°.^[9]

References

^ ^a ^b ^c ^d ^e 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.
^ ^a ^b ^c ^d Jennens, P. A.; Helfer, H. L. (September 1975), "A new photometric metal abundance and luminosity calibration for field G and K giants", Monthly Notices of the Royal Astronomical Society, 172 (3): 667–679, Bibcode:1975MNRAS.172..667J, doi:10.1093/mnras/172.3.667
^ ^a ^b ^c ^d Wu, Yue; et al. (January 2011), "Coudé-feed stellar spectral library - atmospheric parameters", Astronomy and Astrophysics, 525: A71, arXiv:1009.1491, Bibcode:2011A&A...525A..71W, doi:10.1051/0004-6361/201015014, S2CID 53480665
^ Wilson, Ralph Elmer (1953), "General Catalogue of Stellar Radial Velocities", Washington, Washington: Carnegie Institution of Washington, Bibcode:1953GCRV..C......0W
^ Brown, A. G. A.; et al. (Gaia collaboration) (2021). "Gaia Early Data Release 3: Summary of the contents and survey properties". Astronomy & Astrophysics. 649: A1. arXiv:2012.01533. Bibcode:2021A&A...649A...1G. doi:10.1051/0004-6361/202039657. S2CID 227254300. (Erratum: doi:10.1051/0004-6361/202039657e). Gaia EDR3 record for this source at VizieR.
^ Takeda, Yoichi; Sato, Bun'ei; Murata, Daisuke (August 2008), "Stellar parameters and elemental abundances of late-G giants", Publications of the Astronomical Society of Japan, 60 (4): 781–802, arXiv:0805.2434, Bibcode:2008PASJ...60..781T, doi:10.1093/pasj/60.4.781, S2CID 16258166
^ ^a ^b ^c ^d ^e Mazumdar, A.; et al. (August 2009), "Asteroseismology and interferometry of the red giant star ɛ Ophiuchi", Astronomy and Astrophysics, 503 (2): 521–531, arXiv:0906.3386, Bibcode:2009A&A...503..521M, doi:10.1051/0004-6361/200912351, S2CID 15699426
^ ^a ^b Massarotti, Alessandro; et al. (January 2008), "Rotational and Radial Velocities for a Sample of 761 HIPPARCOS Giants and the Role of Binarity", The Astronomical Journal, 135 (1): 209–231, Bibcode:2008AJ....135..209M, doi:10.1088/0004-6256/135/1/209, S2CID 121883397
^ ^a ^b Hekker, S.; Aerts, C. (June 2010), "Line-profile variations of stochastically excited oscillations in four evolved stars", Astronomy and Astrophysics, 515: A43, arXiv:1002.2212, Bibcode:2010A&A...515A..43H, doi:10.1051/0004-6361/200912777, S2CID 30911563
^ ^a ^b ^c Bi, Shao-Lan; et al. (December 2010), "Asteroseismic study of the red giant in Ophiuchi", Research in Astronomy and Astrophysics, 10 (12): 1265–1274, Bibcode:2010RAA....10.1265B, doi:10.1088/1674-4527/10/12/007, S2CID 250665105
^ "eps Oph -- Star in double system", SIMBAD, Centre de Données astronomiques de Strasbourg, retrieved 2012-01-08
^ Kunitzsch, Paul; Smart, Tim (2006). A Dictionary of Modern star Names: A Short Guide to 254 Star Names and Their Derivations (2nd rev. ed.). Cambridge, Massachusetts: Sky Pub. ISBN 978-1-931559-44-7.
^ ^a ^b ^c "Naming Stars". IAU.org. Retrieved 16 December 2017.
^ IAU Working Group on Star Names (WGSN), International Astronomical Union, retrieved 22 May 2016.
^ Kunitzsch, P.; Smart, T. (2006), A Dictionary of Modern Star names: A Short Guide to 254 Star names and Their Derivations (Second Revised ed.), Cambridge, MA: Sky Publishing, p. 31, ISBN 1-931559-44-9
^ Allen, R. H. (1963), Star Names: Their Lore and Meaning (Reprint ed.), New York, NY: Dover Publications Inc, p. 243, ISBN 0-486-21079-0, retrieved 2010-12-12
^ (in Chinese) 中國星座神話, written by 陳久金. Published by 台灣書房出版有限公司, 2005, ISBN 978-986-7332-25-7.
^ Star Names - R.H.Allen p.302
^ (in Chinese) 香港太空館 - 研究資源 - 亮星中英對照表 Archived August 19, 2010, at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
^ (in Chinese) English-Chinese Glossary of Chinese Star Regions, Asterisms and Star Name Archived August 10, 2010, at the Wayback Machine, Hong Kong Space Museum. Accessed on line November 23, 2010.
^ Star Names - R.H.Allen p.142
^ Luck, R. Earle (1991). "Chemical abundances for cyanogen-weak giants". Astrophysical Journal Supplement Series. 75: 579. Bibcode:1991ApJS...75..579L. doi:10.1086/191542.