system
Given the following post, respond as if you're commenting on Reddit:
human
Why don't the fusion chain reactions powering the sun get out of control, causing it to supernova?

Why do stars "burn steadily" over billions of years without the chain reactions exponentially increasing in magnitude causing a supernova? I understand fusion is a very different process to fission, but nuclear reactors require control rods to control the reaction. Also if the reaction rate doesn't increase, how come the reactions don't decrease to the point stars just burn out?
gpt
Reactions - chemical or nuclear, fission or fusion - produce heat, and heat produces pressure. Increasing the pressure causes the gas to expand. If you've doing this in a reactor, this can be a bad thing: if things get too hot, you have too much pressure, and the whole thing can burst. So we have all sorts of measures to stop this from happening.

But in space, there are no walls to hold the pressure in. So the gas is free to expand. As it expands, the gas gets cooler and less dense, and this naturally reduces the reaction rate, which reduces the amount of heat you're producing.

The star's gravity stops it from continuing to expand forever. You end up with an *equilibrium* where gravity balances out the pressure produced by the nuclear reactions. If the reactions are a little too fast, the star expands, which reduces the reaction rate. If the reactions are too slow, the star shrinks, which increases the reaction rate. So there's a "feedback" loop that keeps the star nicely settled at a roughly constant size for most of its life.

>Also if the reaction rate doesn't increase, how come the reactions don't decrease to the point stars just burn out?

Some stars basically do something like that. Stellar evolution is really very complicated, and what happens to a star depends strongly on how massive it is. But essentially, a supernova is when a star runs out of fuel and collapses dramatically, because even shrinking and increasing its pressure and density doesn't help you speed up the reaction if there's nothing left to burn. It shrinks until things are dense and hot enough to start new types of reaction, which produce a huge amount of energy in a short amount of time. If you add little bits of energy at a time then the star can expand and reach a nice equilibrium. But if you add a huge amount at once, you just blow the whole thing apart.