system
Given the following post, respond as if you're commenting on Reddit:
human
Why does stellar fusion last so long?

Even if fusion is only occuring in the core of a star, what is the hydrogen doing that is not undergoing fusion? Even the largest stars with lives of 100 million years seem long-lived. If hydrogen fused into helium in the core, what stops it from continuing to fuse into heavier elements immediately?
gpt
Stellar Evolution takes place in a really cool state of equilibrium. So, as you probably know, the fusion only happens at high temperatures. Also, the fusion creates a lot of energy, which in turn creates radiative pressure, which "pushes the star" keeping it large. These two forces keep each other balanced as such.

As pressure is increased on the core, the temperature of the core is heated, which makes fusion more likely to occur. If fusion is more likely to occur, more radiative pressure is created, thus expanding the size of the star, thus decreasing the pressure, thus slowing down fusion. If fusion is slowed down too much, there is less radiative pressure, the star shrinks a little, increasing the pressure, causing more fusion to occur, which then leads to more radiative pressure, expanding the star, cooling down the core. Stars are in a state where this is equalized. This is also why larger stars burn for a shorter period of time, their equilibrium state is one where there is, on average, higher pressure in the core, thus increasing the rate of fusion. 

Continuing, temperature is a very "macro" concept. If you only have a couple of particles, temperature doesn't really mean anything- but it is a measure of the average energy of the particles. So, the temperature of the core isn't actually hot enough for two average particles to undergo fusion. But that's just the average, there are lots of particles with less energy than average, and a lot with more energy than average. Only the particles on the high end of this spectrum have enough energy to fuse. This helps explain why it is a slow process. 

And finally, fusing lighter elements takes less energy than fusing heavier ones. Thus, while the lighter elements are fusing, the core is not hot enough to fuse the heavier elements. It is only after the lighter elements stop fusing, causing the star to contract slightly, that the core then gets hot enough for the heavier elements to fuse.