Coulomb barrier

Try to push the north poles of two magnets together and you feel them shove back harder and harder the closer they get. Atomic nuclei do the same thing, but for electric charge: every nucleus is positively charged, and like charges repel. To fuse, two nuclei must be brought close enough to touch, but as they approach, the repulsion rises steeply, like a hill that gets steeper and steeper. The Coulomb barrier is that hill of electrical repulsion that two nuclei must overcome before they can fuse.

The barrier is named after Coulomb's law of electric force, which says the repulsion between two charges grows as they get closer — and at nuclear distances it becomes enormous. To climb it by brute force, the nuclei need tremendous speed, which in a gas means tremendous temperature. Yet here is the catch that long puzzled physicists: even at the Sun's 15-million-kelvin core, the typical nucleus moves nowhere near fast enough to crest the barrier. By the old classical picture, the Sun simply should not be able to fuse hydrogen at all.

The resolution is that nuclei do not actually have to clear the top of the barrier — thanks to quantum tunneling they can slip through it, and the higher their charge, the taller the barrier and the rarer fusion becomes. This is why hydrogen (charge 1) fuses at relatively modest temperatures while heavier nuclei need progressively hotter cores: helium-burning needs about 100 million kelvin, carbon-burning closer to a billion. The Coulomb barrier is the gatekeeper that makes fusion slow, temperature-sensitive, and therefore the steady, long-lived power source that lets stars shine for eons.