Epsilon Ophiuchi or ε Ophiuchi, formally named Yed Posterior (), is a red giant

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The first constellati ...

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 constellati ...

. Located less than five degrees south of the celestial equator 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 magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...

optical double with Delta Ophiuchi (named ''Yed Prior''). With an apparent visual magnitude 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 Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

under suitably dark skies.

Parallax Parallax is a displacement or difference in the apparent position of an object viewed along two different sightline, lines of sight and is measured by the angle or half-angle of inclination between those two lines. Due to perspective (graphica ...

measurements yield an estimated distance of from the Sun.

Nomenclature

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

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

. It bore the traditional name ''Yed Posterior''. ''Yed'' derives from the

Arabic Arabic (, , or , ) is a Central Semitic languages, Central Semitic language of the Afroasiatic languages, Afroasiatic language family spoken primarily in the Arab world. The International Organization for Standardization (ISO) assigns lang ...

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 Serpens () is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by the 2nd-century astronomer Ptolemy, it remains one of the IAU designated constellations, 88 modern constellations designated by the Inter ...

). ''Epsilon'' is ''Yed Posterior'' as it follows ''Delta '' across the sky. In 2016, the

International Astronomical Union The International Astronomical Union (IAU; , UAI) is an international non-governmental organization (INGO) with the objective of advancing astronomy in all aspects, including promoting astronomical research, outreach, education, and developmen ...

organized a Working Group on Star Names (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, Delta Serpentis, Epsilon Serpentis, Delta Ophiuchi, Zeta Ophiuchi and Gamma Ophiuchi. 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, Kappa Herculis, Gamma Serpentis, Beta Serpentis, Alpha Serpentis, Delta Serpentis, Epsilon Serpentis, Epsilon Ophiuchi and Zeta Ophiuchi. Consequently, the Chinese name for Epsilon Ophiuchi itself is (, ), representing the state Chu (楚) (or Tsoo), together with Phi Capricorni (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 In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction gratin ...

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 In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...

. 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 Asteroseismology is the study of oscillations in stars. Stars have many Resonance, resonant modes and frequencies, and the path of sound waves passing through a star depends on the local speed of sound, which in turn depends on local temperature a ...

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 Nuclear fusion is a reaction in which two or more atomic nuclei combine to form a larger nuclei, nuclei/neutron by-products. The difference in mass between the reactants and products is manifested as either the release or absorption of ener ...

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 bu ...

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 Ophiuchi, Epsilon Durchmusterung objects Ophiuchi, 02 146791 079882 6075 Yed Posterior