Star is a massive, luminous ball of gas or plasma, held together by its own gravity, that replenishes the heat it loses to space by means of nuclear energy generated in its core. Almost all of the elements heavier than hydrogen and helium were created by the nuclear fusion processes in stars. There are many different types of stars including binary stars, proto-stars, dwarf stars (like our nearest star which we call the Sun), supergiants, supernovas, neutron stars, pulsars, quasars, etc. There are a roughly estimated 10,000 billion billion stars (1022) in the observable universe.