How the sun can burn that long?

Question

I suddenly became curious that why the sun doesn't burn out at once?

What makes the sun burn gradually?

Answer 1

Short answer: like often, it's a balance between opposite trends:

• self-gravity causing collapsing thus densification build huge pressure that permit nuclear fusion
• high-temperature and pression caused by these reactions inflate the star which decrease the amount of reaction (or might even stop them if inflating too much).

As long as everything is slow enough, it reaches a balance: the size at which collapse forces = repulsive forces. Which slows down the nuclear oven.

(NB: this balance will change when one of the nuclear fuel will get exhausted. Then the cycle again: more collapse, more density, triggers a new kind of fusion with the next atomic fuel (requiring more density), thus repulsion continue, but with balance at a shorter radius. And when there is no fusionable fuel at all, sudden collapse with no counter-force, resulting in a violent bounce at center resulting in supernovae explosion, + possibly a residual neutron star or a black hole).