Epsilon Ophiuchi
Red giant star in Ophiuchus, optical double with Delta Ophiuchi.
Epsilon Ophiuchi, also designated ε Ophiuchi and formally named Yed Posterior, is a red giant star in the constellation Ophiuchus. It forms a naked-eye optical double with Delta Ophiuchi and is visible from most of Earth under suitably dark skies. Parallax measurements place it about 106.4 light-years from the Sun.
Quick Facts
- Bayer designation
- ε Ophiuchi
- Formal name
- Yed Posterior
- Apparent visual magnitude
- 3.220
- Distance from sun
- 106.4 ly
- Stellar classification
- G9.5 IIIb
- Mass
- nearly double the Sun's mass
- Radius
- over ten times the Sun's radius
- Luminosity
- about 54 times the Sun
Facts from the source article.
Nomenclature
The Bayer designation for this star is ε Ophiuchi, Latinised to Epsilon Ophiuchi. Its traditional name Yed Posterior derives from the Arabic يد (yad), meaning 'hand', as Epsilon and Delta Ophiuchi represent the left hand of Ophiuchus holding the serpent's head; Epsilon follows Delta across the sky, hence 'Posterior'. In 2016, the IAU Working Group on Star Names approved Yed Posterior as the official proper name. The star was part of the Arabic asterism al-Nasaq al-Yamānī, the 'Southern Line', along with several other stars. In Chinese astronomy, it belongs to the Right Wall of Heavenly Market Enclosure asterism, where it represents the state Chu.
Properties
Epsilon Ophiuchi has a stellar classification of G9.5 IIIb, indicating it is a giant star that has left the main sequence. It has nearly double the Sun's mass and has expanded to over ten times the Sun's radius, with a luminosity about 54 times that of the Sun and an age of roughly a billion years. Unusually for a class G giant, it is cyanogen-deficient and carbon-deficient. Its outer envelope shows solar-type oscillations with a period of 0.19 days, but models cannot determine whether it fuses hydrogen in a shell or helium at its core. The projected rotational velocity is 5.7 km s−1, with an axial inclination between 41° and 73°.
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