Elements with more protons in their nuclei require higher temperatures: for instance, fusing carbon requires a temperature of about one billion degrees Most of the heavy elements, from oxygen up through iron, are thought to be produced in stars ten times larger than our Sun.

After hydrogen in the star's core is exhausted, the star "burns" helium to form progressively heavier elements, carbon and oxygen and so on, until iron and nickel form. The star is layered ("onion") with the hottest "burning" deepest. Up to this point the process releases energy so star keeps "burning".