The universe's three lightest elements — hydrogen, helium and lithium — were created in the earliest moments of the cosmos, just after the Big Bang, and can also be formed from nuclear fusion. Most of the quantities of elements heavier than lithium, up to iron on the periodic table, were forged billions of years later, in the cores of stars. But how elements heavier than iron, such as gold and uranium, were created has long been uncertain.
A supernova explosion expels into space not only the elements formed inside the star through fusion, but elements forged in the supernova blast wave itself. In the explosion, nuclei are bombarded with neutrons, until elements all the way up to uranium are formed within seconds. All these heavier elements are then spread throughout the galaxy by the immense force of the supernova. Supernovas are responsible for changing the composition of gas from which each generation of stars form. Without supernova explosions, there would be no heavy elements in the interstellar gas. In particular, there would be no silicon to form rocky planets, no oxygen to form water, none of the elements we depend on here on Earth. It is the stuff of which we are made.
