A celestial body. Our Sun is an example of a star. Defined as a large massive ball of mostly hot Hydrogen and Helium gas that radiates energy derived from thermonuclear reactions in the interior.

A star is so massive that its core is extremely dense and hot. At the high stellar core temperatures, Atom move so fast that they sometimes stick to other atoms when they collide with them, forming more massive atoms and releasing a great amount of energy. This process is known as nuclear fusion.