Living things are star stuff. Other than hydrogen, which comes from the Big Bang, which marked the birth of the universe, the familiar elements of which flesh is composed—carbon, oxygen, nitrogen and so on—were created by the energy-releasing process of nuclear fusion that powers stars. But fusion has its limits. The balance of forces inside an atomic nucleus means that creating an element heavier than iron (number 26 on the periodic table) consumes energy, rather than releasing it. Further up the table, beyond lead (number 82), nuclei tend to fall apart spontaneously. In other words, they become radioactive.