Stars that formed in nebulae that had 2 previous cycles of star formation and super novae. This means that the star’s solar system at that point has sufficient

Stars that formed in nebulae that had 2 previous cycles of star formation and super novae. This means that the star’s solar system at that point has sufficient quantities of heavy elements which cannot be produced by stellar fusion, as these elements are produced by super novae and from solar radiation. Basically all the neutrons that are produced when a star goes boom create the elements that are above iron in the periodic table.