Epsilon Ophiuchi or ε Ophiuchi, formally named Yed Posterior (), is a red giant star in the constellation of

Ophiuchus Ophiuchus () is a large constellation straddling the celestial equator. Its name comes from the Ancient Greek (), meaning "serpent-bearer", and it is commonly represented as a man grasping a snake. The serpent is represented by the constella ...

. Located less than five degrees south of the

celestial equator The celestial equator is the great circle of the imaginary celestial sphere on the same plane as the equator of Earth. This plane of reference bases the equatorial coordinate system. In other words, the celestial equator is an abstract proj ...

in the eastern part of the constellation, it forms a

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying, light-collecting optical instrument, such as a telescope or microscope, or eye protection. Vision corrected to normal ...

optical double In observational astronomy, a double star or visual double is a pair of stars that appear close to each other as viewed from Earth, especially with the aid of optical telescopes. This occurs because the pair either forms a binary star (i.e. a bi ...

with

Delta Ophiuchi Delta Ophiuchi (δ Ophiuchi, abbreviated Delta Oph, δ Oph), formally named Yed Prior , is a star in the constellation of Ophiuchus. It forms a naked-eye optical double with Epsilon Ophiuchi (named ''Yed Posterior''). The apparent visual ...

(named ''Yed Prior''). With an

apparent visual magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's lig ...

of 3.220, the star can be seen with the naked eye from most of the

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

under suitably dark skies. Parallax measurements yield an estimated distance of from the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

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 Arabic (, ' ; , ' or ) is a Semitic language spoken primarily across the Arab world.Semitic languages: an international handbook / edited by Stefan Weninger; in collaboration with Geoffrey Khan, Michael P. Streck, Janet C. E.Watson; Walter ...

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 The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreac ...

organized a

Working Group on Star Names The International Astronomical Union (IAU) established a Working Group on Star Names (WGSN) in May 2016 to catalog and standardize proper names for stars for the international astronomical community. It operates under Division C – Education ...

(WGSN) 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. Epsilon Ophiuchi was a member of the indigenous Arabic asterism ''al-Nasaq al-Yamānī'', the "Southern Line" of ''al-Nasaqān'' the "Two Lines", along with

Alpha Serpentis Alpha Serpentis or α Serpentis, formally named Unukalhai (), is a double star in the head (Serpens Caput) of the equatorial constellation of Serpens. With an apparent visual magnitude of 2.6, this star is the brightest in the constell ...

Delta Serpentis Delta Serpentis, Latinized from δ Serpentis, is a binary star system in the constellation Serpens, in its head (Serpens Caput). The light from the two stars in the system give a combined apparent magnitude of +3.80, which is bright enou ...

Epsilon Serpentis Epsilon Serpentis, Latinized from ε Serpentis, is a single, white-hued star in the constellation Serpens, in its head (Serpens Caput). It is visible to the naked eye with an apparent visual magnitude of +3.69. Based upon an annual parall ...

, Delta Ophiuchi,

Zeta Ophiuchi Zeta Ophiuchi (ζ Oph, ζ Ophiuchi) is a single star located in the constellation of Ophiuchus. It has an apparent visual magnitude of 2.6, making it the third-brightest star in the constellation. Parallax measurements give an estimate ...

and

Gamma Ophiuchi Gamma Ophiuchi, Latinized from γ Ophiuchi, is a fourth-magnitude star in the constellation Ophiuchus. Together with Beta Ophiuchi, it forms the serpent-holder's right shoulder. The staris visible to the naked eye with an apparent visual ...

. In Chinese, (), 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 Gamma Herculis, Latinized from γ Herculis, is a magnitude 3.74 binary star system in the northern constellation of Hercules. Properties This is known to be a spectroscopic binary system, although there is no information about the seco ...

Kappa Herculis Kappa Herculis (κ Herculis, abbreviated Kappa Her, κ Her) is an optical double star in the constellation of Hercules. The two components, Kappa Herculis A (Marsic , the traditional name of the system) and B, were 27.3 arc seconds apa ...

, Gamma Serpentis,

Beta Serpentis Beta Serpentis, Latinized from β Serpentis, is a binary star system in the constellation Serpens, in its head (Serpens Caput). It is visible to the naked eye with a combined apparent visual magnitude of +3.65. Based upon an annual par ...

, Alpha Serpentis, Delta Serpentis, Epsilon Serpentis, Epsilon Ophiuchi and Zeta Ophiuchi. Consequently, the

Chinese name Chinese names or Chinese personal names are names used by individuals from Greater China and other parts of the Chinese-speaking world throughout East and Southeast Asia (ESEA). In addition, many names used in Japan, Korea and Vietnam are ofte ...

for Epsilon Ophiuchi itself is (, en, the Tenth Star of Right Wall of Heavenly Market Enclosure), representing the state Chu (楚) (or Tsoo), together with

Phi Capricorni Phi Capricorni (φ Cap, φ Capricorni) is a solitary star in the southern constellation of Capricornus. It is visible to the naked eye with an apparent visual magnitude of +5.16. Based upon an annual parallax shift of 5.07 mas as se ...

(or 24 Capricorni in R.H.Allen's versionStar Names - R.H.Allen p.142
/ref>) in the '' Twelve States'' (asterism).

Properties

Epsilon Ophiuchi has a stellar classification of G9.5 IIIb, with the

luminosity class In astronomy, stellar classification is the classification of stars based on their spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a prism or diffraction grating into a spectrum exhibiting the ...

of III indicating that this is a

giant star A giant star is a star with substantially larger radius and luminosity than a main-sequence (or ''dwarf'') star of the same surface temperature.Giant star, entry in ''Astronomy Encyclopedia'', ed. Patrick Moore, New York: Oxford University Press ...

that has exhausted the hydrogen and

evolved Evolution is change in the heritable characteristics of biological populations over successive generations. These characteristics are the expressions of genes, which are passed on from parent to offspring during reproduction. Variation ...

away from the main sequence. This red giant has nearly double the

Sun's mass The solar mass () is a standard unit of mass in astronomy, equal to approximately . It is often used to indicate the masses of other stars, as well as stellar clusters, nebulae, galaxies and black holes. It is approximately equal to the mass of ...

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

. 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 Thermonuclear fusion is the process of atomic nuclei combining or “fusing” using high temperatures to drive them close enough together for this to become possible. There are two forms of thermonuclear fusion: ''uncontrolled'', in which the re ...

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. The

projected rotational velocity Stellar rotation is the angular motion of a star about its axis. The rate of rotation can be measured from the spectrum of the star, or by timing the movements of active features on the surface. The rotation of a star produces an equatorial bulg ...

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°.

References

External links

* {{DEFAULTSORT:Epsilon Ophiuchi G-type giants Double stars Ophiuchus (constellation) Ophiuchi, Epsilon Durchmusterung objects Ophiuchi, 02 146791 079882 6075 Yed Posterior