A star is a massive, luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth. Other stars are visible in the night sky, when they are not outshone by the Sun. For most of its life, a star shines because thermonuclear fusion in its core releases energy that traverses the star's interior an then radiates into outer space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars.