Sol ((ソル, soru?)) is the name given to the mother star in Ar Ciel's solar system. Its name comes from an Hymmnos word that means shining/radiant. It doesn't seem to be particularly different in any way from the Sun existent in the real universe.
It was personified in the ancient times as Nayudara/Juklizda, but during the First Era, it was discovered that Juklizda was just one of the Wills of Ar Ciel, to be more specific, the one in charge of the light and radiation that reaches the planet, and the control of the stratosphere. In the other hand, Sol does have its own Will, but no one currently knows how it's composed, or what is its name.
Sol, like any of the other celestial bodies, has a core within itself, similar to the Heart of the Land that the planets possess. However, this core is a level higher than the Heart of the Land, and receives the name of Autostabilizing Penteractal Nuclear Loop.