The Big Bang produced hydrogen, helium and traces of lithium, while all heavier elements are synthesised in stars, supernovae, and collisions between neutron stars (thus being indirectly due to supernovae). Supernovae tend to enrich the surrounding interstellar medium with elements other than hydrogen and helium, which usually astronomers refer to as "metals". These ejected elements ultimately enrich the molecular clouds that are the sites of star formation. Thus, each stellar generation has a slightly different composition, going from an almost pure mixture of hydrogen and helium to a more metal-rich composition. Supernovae are the dominant mechanism for distributing these heavier elements, which are formed in a star during its period of nuclear fusion. The different abundances of elements in the material that forms a star have important influences on the star's life, and may influence the possibility of having planets orbiting it: more giant planets form around stars of higher metallicity.