Question

Why does fusion eventually end in a star?

Answer 1

Fusion ends in stars when they run out of fusible material.

Explanation:

The fusion process ends differently in small and large stars. All main sequence stars fuse Hydrogen into Helium. It is when the Hydrogen supply in the core runs out that fusion begins to come to an end.

If the star is less than about 8 solar masses, once the Hydrogen in the core is exhausted the core starts to collapse under gravity. As it collapses the temperatures and pressures rise to the point where Helium fusion can take place. Helium fusion produces Carbon and Oxygen. Once the supply of Helium in the core is exhausted the core isn't massive enough to start Carbon fusion. At this point fusion stops and the star becomes a white Dwarf.

In larger stars, as the supply of Hydrogen in the core runs out, the star starts Helium fusion. As the Helium runs out it starts Carbon fusion. The process continues until the core is mainly Iron. Fusion reactions involving Iron don't release energy, they require extra energy. Once the core is Iron fusion reactions stop. The iron core will then collapse under gravity into a neutron star or a black hole.

So, all stars fusion reactions come to an end when the run out of fusible material in their core.