Deep in its core the star is fighting a futile battle against its own gravity. As it desperately tries to stop itself collapsing under its own weight new elements are made in a sequence of separate stages.

Stage-1 , is while there is still a supply of Hydrogen to burn. Whilst the stars burning Hydrogen to Helium in the core, vast amounts of energy are released and that energy escapes literally creating an outward pressure which balances the force of gravity and it holds the star up and keeps it Stable. But eventually the Hydrogen in the core will run out and at that point the fusion reactions will stop. No more energy will be released and that Outward pressure will disappear. Now at that point the core will start to collapse very rapidly leaving a shell of Hydrogen and Helium behind. Beneath this shell as the core collapses the temperature rises again.

Stage-2 , at a hundred million degrees Helium nuclii inter-fuse together. Helium fusion does two things. Firstly more energy is released and so the collapse is halted. But secondly two more elements are produced in that process, Carbon Oxygen, two elements vital for life. So this is where all the carbon in the universe comes from. Every atom of Carbon in my hand, every atom of Carbon in every living thing on the planet was produced in the heart of a dying star. Compared to the lifetime of the star the creation process of carbon and oxygen is over in a blink of an eye. Because in only about a million years the supply of Helium in the core is used up and the stars as massive as the Sun, that's where fusion stops. Because there isn't enough gravitational energy to compress the core any further and restart fusion. But for massive stars like Betelgeuse the fusion process can continue. When the Helium runs out gravity takes over again and the collapse continues. The temperature rises once more launching next stage.

Stage-3, in which Carbon fuses into Magnesium Neon Sodium and Aluminium. And so it goes on core-collapse. Followed by the next stage of fusion to create more elements. Each stage hotter and shorter than the last. And eventually in a final stage that lasts only a couple of days the heart of the star is transformed Into almost pure Iron. Whose chemical symbol is Fe. This is where the fusion process stops. In its millions of years of life the star has made all the common elements. The stuff that makes up 99% of the Earth. The core is now a solid ball of those elements stacked on top of each other in layers. On the outside there's a shell of Hydrogen, beneath it a layer of Helium, then Carbon and Oxygen and all the other elements all the way down to the very heart of the star. And once that has fused into solid iron, the star has only seconds left to live. When a star runs out of fuel then it can, no longer release energy through fusion reactions and then there's only one thing that can happen, EXPLOSION!

Fin