The Sun is a star, and as all other stars, the Sun is a light object in the universe. Being a light object means that the Sun produces light, unlike the planets and the moons that are absorbing and reflecting the light. The reason why the Sun is light and it shines is because of the constant burning of gasses, which constantly creates reactions that produce light, and with the light being able to spread out in all directions it gives the Sun a shining appearance.