The early Universe consisted of only the light elements formed during Big Bang nucleosynthesis. Stellar structure and the Hertzsprung–Russell diagram indicate that the length of the lifetime of a star depends greatly on its initial mass, with the most massive stars being very short-lived, and less massive stars are longer-lived. The B FH paper argued that when a star dies, it will enrich the interstellar medium with 'heavy elements' (in this case all elements heavier than lithium), from which newer stars are formed.