Epoch J2000 Equinox J2000
|Right ascension||16h 14m 20.73853s|
|Declination||–03° 41′ 39.5612″|
|Apparent magnitude (V)||2.75|
|Spectral type||M0.5 III|
|U−B color index||+1.96|
|B−V color index||+1.59|
|Radial velocity (Rv)||–19.9 km/s|
|Proper motion (μ)||RA: –47.54 mas/yr
Dec.: –142.73 mas/yr
|Parallax (π)||19.06 ± 0.16 mas|
|Distance||171 ± 1 ly
(52.5 ± 0.4 pc)
|Absolute magnitude (MV)||–0.90|
|Surface gravity (log g)||1.4 cgs|
|Metallicity [Fe/H]||0.32 dex|
|Rotational velocity (v sin i)||7.0 km/s|
Delta Ophiuchi (δ Oph, δ Ophiuchi) is a star in the constellation Ophiuchus. It has the traditional name Yed Prior. The star Epsilon Ophiuchi, with which it forms a naked eye optical double, is Yed Posterior. The apparent visual magnitude is 2.75, making this a third-magnitude star and the fourth brightest in the constellation. Parallax measurements from the Hipparcos spacecraft yield a distance estimate of around 171 light-years (52 parsecs) from Earth.
Delta Ophiuchi is 170 light years from Earth, while Epsilon is 108; hence they are a mere optical double. This star has a stellar classification of M0.5 III, making this a red giant star that has undergone expansion of its outer envelope after exhausting the supply of hydrogen at its core. The measured angular diameter of this star, after correction for limb darkening, is 10.47 ± 0.12 mas. At the estimated distance of Delta Ophiuchi, this yields a physical size of about 59 times the radius of the Sun. In spite of its enlarged size, this star has only 1.5 times the mass of the Sun and hence a much lower density. The effective temperature of the outer atmosphere of Delta Ophiuchi is a relatively cool 3,679 K, which is what gives it the orange-red hue of an M-type star.
Yed Prior is listed as a suspected variable star that may change by 0.03 in visual magnitude. It has a low projected rotational velocity of 7.0 km s−1, which gives a minimum value for the azimuthal velocity along the star's equator. The abundance of elements other than hydrogen and helium, what astronomers term the star's metallicity, is more than double the abundance in the Sun's photosphere.
The star has a high optical linear polarisation that increases from red to blue wavelengths and displays some variability, this has been ascribed to either an asymmetric distribution of dust grains in an envelope expelled from it, or the presence of photometric hot spots.
It was a member of the indigenous Arabic asterism al-Nasaq al-Yamānī, the "Southern Line" of al-Nasaqān the "Two Lines", along with α Ser (Unukalhai), δ Ser (Qin, Tsin), ε Ser (Ba, Pa), ε Oph (Yed Posterior), ζ Oph (Han) and γ Oph (Tsung Ching).
According to the catalogue of stars in the Technical Memorandum 33-507 - A Reduced Star Catalog Containing 537 Named Stars, al-Nasaq al-Yamānī or Nasak Yamani were the titles for two stars :δ Ser as Nasak Yamani I and ε Ser as Nasak Yamani II (exclude this star, α Ser, ε Oph, ζ Oph and γ Oph[clarification needed] ).
In Chinese, 天市右垣 (Tiān Shì Yòu Yuán), meaning Right Wall of Heavenly Market Enclosure, refers to an asterism which is represent eleven old states in China which is marking the right borderline of the enclosure, consisting of δ Ophiuchi, β Herculis, γ Herculis, κ Herculis, γ Serpentis, β Serpentis, α Serpentis, δ Serpentis, ε Serpentis, ε Ophiuchi and ζ Ophiuchi. Consequently, δ Ophiuchi itself is known as 天市右垣九 (Tiān Shì Yòu Yuán jiǔ, English: the Ninth Star of Right Wall of Heavenly Market Enclosure), representing the state of Liang (梁) (or Leang).
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