An intrinsically luminous ball of gas generating energy in its hot core through nuclear fusion processes. The minimum mass required to form a star is about one-twentieth the mass of the Sun. Below this limit, the gravitational energy released when the mass condenses is insufficient to raise the temperature to the point at which the fusion of hydrogen to form helium can begin. The most massive stars known are about 100 solar masses. Mass is the prime factor determining the temperature and luminosity the star will have during its existence as a main-sequence star, when hydrogen in the core is its nuclear fuel.
Stars are predominantly hydrogen, with helium as the other major constituent.