I’ve always wondered this, if fusion creates energy, why doesn’t the whole star just fuse everything together as quickly as possible? My only logical guess is maybe there’s pockets that can’t reach each other to be able to fuse, but if the fusion creates energy, surely it would be extremely turbulent and mixing things up even more, allowing more fusion to happen, creating more energy to make more fusion happen, etc. Or a better comparison to how my brain thinks it should work, if you add more wood to a fire, it gets hotter and burns faster. The more wood you add, the hotter it gets, the more burning that happens. Why aren’t stars like this?
In: Physics
Fusion in stars is mainly driven by gravity pulling inwards, rather than the temperature of the stuff it’s fusing.
When stuff like hydrogen fuses, the explosion pushes outwards, fighting gravity, and slowing down the fusion. The star gets bigger, but colder.
This leads to a few counter-intuitive things.
Like lighter stars lasting longer than heavier ones, while often being larger.
And when it’s done fusing the stuff that gives out a lot of energy, when it fuses the stuff that doesn’t give off much energy. The energy doesn’t fight gravity well, so it fuses ridiculously fast (like a star larger than the sun might fuse ALL of it’s silicon in literally 5 days), causing the star to balloon up before it all comes crashing down in a [massive explosion.](https://en.wikipedia.org/wiki/Type_II_supernova).
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