The final stage occur when a massive star begin producing iron . Since iron nuclei are more tightly bound than any heavier nuclei , any fusion beyond iron do not produce a net release of energy . To a very limited degree such a process proceed , but it consume energy . Likewise , since they are more tightly bound than all lighter nuclei , such energy cannot be released by fission . In relatively old , very massive stars , a large core of inert iron will accumulate in the center of the star . The heavier elements in these stars can work their way to the surface , forming evolved objects known as Wolf @-@ Rayet stars that have a dense stellar wind which shed the outer atmosphere .