Massive balls of plasma that are the gravitational cores of every star system.
Stars are gigantic spheres of plasma, which serve as the gravitational cores of a star system. All celestial objects, planets, asteroids, comets ultimately orbit the central star(s)
At the end of a star's live the go through a phase where they become red giants, swelling up and turning from orange/yellow, to red. After this one of two things could happen, the explode or go Nova, or an even bigger Super Nova and become a white dwarf, a miniature white star which is formed from the old star's remains or they collapse into a singularity if they are especially large.