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.
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