nuclear fusion

Nuclear fusion is the joining of two light nuclei into a single heavier one, releasing an enormous amount of energy. Everyday image: like two small drops of water merging into a bigger drop, two light nuclei such as hydrogen can merge into a heavier one like helium, and in doing so they shed energy. The question it answers: what makes the Sun and the stars shine, and could we harness that same power on Earth?

Precisely, when very light nuclei fuse, the product is more tightly bound (it climbs toward the iron peak of the binding-energy curve), so a little mass is lost and released as energy through E = m c^2. But there is a fierce obstacle: both nuclei are positively charged and repel each other strongly, so they must collide at tremendous speed to touch. That requires extreme temperatures, millions of degrees, so that the nuclei move fast enough to overcome the repulsion; this is why fusion is a process of hot, dense plasma. In the Sun, hydrogen nuclei fuse in steps into helium, powered by the crushing heat and pressure of its core.

Why it matters: fusion is the power source of every star and the origin of most light elements in the universe; it releases even more energy per unit mass than fission and produces far less long-lived radioactive waste. Honest caveat: fusion is extraordinarily hard to achieve and control on Earth, because containing a plasma at tens of millions of degrees for long enough to release net energy is a formidable engineering challenge, and despite decades of effort, a commercially viable fusion power plant does not yet exist.