A star is a massive, luminous ball of plasma held together by gravity. Stars form from clouds of gas and dust, and their size and characteristics are determined by their mass. They produce energy through the process of nuclear fusion, which converts lighter elements into heavier ones and releases a tremendous amount of energy in the process. The different stages of a star’s life are determined by the balance between gravity and the energy produced by fusion reactions. Eventually, stars run out of fuel and either collapse into a very dense object called a white dwarf or explode in a catastrophic event known as a supernova.