Nuclear fusion
Nuclear fusion is the reaction in which two light atomic nuclei combine to form a heavier nucleus. This process releases an enormous amount of energy, far greater than that produced by ordinary chemical reactions. It is the driving force behind stars, the source of the Sun’s heat, and the reaction that makes every star in the universe shine.
How does fusion work?
Atomic nuclei are positively charged and naturally repel one another. To force them to fuse, they must be brought close enough together so that the strong nuclear force—which is attractive at very short ranges— overcomes the electrical repulsion. This requires extremely high temperatures and pressures: at thethe Sun’s core, the temperature reaches approximately 15 million degrees. &At this temperature, the nuclei have enough kinetic energy to come close together despite their repulsion.
The proton-proton chain
In the Sun and stars of similar mass, the dominant reaction is the proton-proton chain. Four protons (hydrogen nuclei) fuse in several steps to form a helium-4 nucleus. The resulting nucleus is slightly less massive than the four initial protons. This mass difference is converted into energy according to Einstein’s famous equation: E = mc². The Sun thus converts approximately 600 million metric tons of hydrogen into helium every second.
Fusion and fission: two opposing reactions
Fusion should not be confused with nuclear fission, which is used in today’s nuclear power plants. Fission involves breaking down heavy nuclei (such as uranium) into lighter nuclei. Fusion brings together light nuclei. Both release energy, but fusion produces more energy per unit of mass and does not generate long-lived radioactive waste.
A concrete example: the Sun
The Sun has been burning hydrogen for about 4.6 billion years and will continue to do so for another 5 billion years beforeexhaust its reserves. At its core, the pressure is 250 billion times greater than Earth’s atmospheric pressure. These extreme conditions keep fusion active at all times.
Did you know?
Researchers have been trying to replicate nuclear fusion on Earth since the 1950s to turn it into a clean and virtually inexhaustible source of energy. The ITER experimental reactor, currently under construction in France, aims to produce more energy than it consumes— a decisive step toward commercial fusion.
Frequently Asked Questions
Why does nuclear fusion stop at iron?
Iron has the most stable nucleus of all elements. Fusing iron does not release energy—on the contrary, it consumes it. This is why stars cannot burn beyond iron and collapse when their cores are dominated by this metal.
Is nuclear fusion dangerous?
Unlike fission, fusion does not produce long-lived radioactive waste and does not pose a risk of runaway reactions. It primarily uses deuterium and tritium, isotopes of hydrogen. The only radioactive byproduct is tritium, which has a short half-life.
What is the difference between stellar fusion and a hydrogen bomb?
Both are based on nuclear fusion. In a hydrogen bomb, the reaction is explosive and instantaneous, triggered by an atomic bomb. In a star, fusion is regulated by gravity and pressure, producing stable energy over billions of years.