The core of the star is around the mass of the Sun but is the size of the Earth and composed of iron. The core is surrounded by the shell sources ignited after each phase of core burning ran to completion. Because iron is the last element which can be profitably used as fuel for the fusion process, something catastrophic is poised to happen.
Instability Look at the iron core.
It is made out of hot iron nuclei and degenerate electrons. It is supported by degenerate electron pressure and the hot iron nuclei. When its temperature reaches around 5 billion Kelvins, the core becomes so hot that the radiation in the core starts to break apart the iron nuclei in a process known as photo-disintegration . It takes a lot of energy to break apart an iron nucleus and this process steals a energy from the core. At around the same time another interesting thing happens.