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 2.75,2 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,3 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.