Eli5. Our sun will expand to an orange giant in the future. Why is that?

Question

I have read that it has something to do with the depletion of fuel. Seems like the star would collapse rather than shrink.

In: 105

Answer 1

The star converts helium to heavier atoms to generate energy. When helium runs out it fuses the heavier elements generating more mass and expansion. Eventually the mas cannot be sustained by the reactions and it explodes and collapses on itself.