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).
Because fusing hydrogen is really, really hard. To use your wood fire analogy, you’re trying to maintain a fire in a pile of soaking wet wood, only the wetness never dries out.
If you want to get a bit more technical about it, the normal result of fusing hydrogen is an atom containing two protons and no neutrons, which is an isotope of helium called Helium-2. Helium-2 is catastrophically unstable, with a half-life of less than a billionth of a second, so it almost immediately decays back into the two hydrogens it came from, taking back the energy generated from the fusion as it does so. The only way it can survive long enough to be useful is if one of the protons spontaneously undergoes beta particle emission and turns into a neutron, producing an atom of deuterium, which doesn’t happen very often. It’s only because the Sun is so absolutely massive and contains so *many* fusing hydrogen atoms that the process produces as much energy as it does.
Fusion requires some pretty specific conditions to get started, and if you don’t maintain the insanely high pressures and temperatures required then the fusion will stop.
This is different from say burning a stick where the nearby fire provides enough heat to start up more little fires around it where all you have to do is start it and it will continue until it runs out of fuel.
Stars begin their life as a big cloud of gas that is slowly compressing into a big ball. The outer layers of the ball push inwards on the core and compress it causes high pressure and temperature in the center. When it gets high enough the hydrogen in the core will begin to fuse and make the core even hotter, but this hot core passes some heat off to the surrounding layers which pushes them outwards and keeps the pressure in those layers lower.
If too much fusion starts happening in the core then the outer layers get pushed away which drops the pressure, then fusion slows letting the pressure and temperature come down so the outer layers move back in and fusion rates ramp up again.
For most stars the compression occurs at a slow rate so when fusion starts up it finds a happy medium quickly and remains as a star. Towards the end of a star’s life though the fusion rate in the core can drop off suddenly causing the outer layers to pick up a good bit of speed on their way in so that when fusion does start back up their inertia is still carrying them inward quickly and fusion rates skyrocket and blow the star to bits in a supernova
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