What is the source of elements in the universe that are more massive than helium but less massive than iron? What is the source of elements in the universe that are more massive than iron? Solution The source of elements in the universe that are more massive than helium but less massive than iron is \' Fusion process going on inside a massive star\'. But all of the post-iron elements in the universe are formed in \' supernova explosions\' themselves. So much energy is released during a supernova explosion that the freed energy and copious free neutrons streaming from the collapsing core drive massive fusion reactions, long past the formation of iron. .