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
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.
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