Many helium-4 on Earth is produced from the alpha decay of heavy elements. In the Sun and the whole universe, most helium-4 is made during the Big Bang, the event that created the universe. This "Big Bang helium-4" is known as the "primordial helium". Helium-4 is also made in nuclear fusion at the center of stars. Primordial helium is not found on Earth due to it escaping because of the extremely high temperature of Earths creation.