A star is a massive, luminous ball of gas that is held together by its own gravity. The nearest star to Earth is the Sun. A star begins as a collapsing cloud of material composed primarily of hydrogen, along with helium and trace amounts of heavier elements. Once the stellar core is sufficiently dense, some of the hydrogen is steadily converted into helium through the process of nuclear fusion. For most of its life, a star shines due to thermonuclear fusion in its core releasing energy that traverses the star's interior and then radiates into space. Almost all elements heavier than hydrogen and helium were created by fusion processes in stars.