Nuclear Fusion
Nuclear fusion is the process in which light atomic nuclei, such as hydrogen, combine into heavier nuclei and release enormous energy, powering stars.
Definition
Nuclear fusion is when tiny atomic nuclei squeeze together to form a bigger nucleus and release a huge amount of energy, which is how the Sun and all stars make their light and heat. The energy comes from the fact that the product nucleus has slightly less mass than the starting nuclei; in the Sun's core four hydrogen nuclei fuse into one helium nucleus, and this requires temperatures of millions of degrees so the nuclei move fast enough to overcome their electrical repulsion. Fusion releases energy whenever the binding energy per nucleon of the product exceeds that of the reactants, which holds for nuclei lighter than iron-56. Sun-like stars use the proton-proton chain, more massive stars use the CNO cycle, and core fusion produces every element from helium up to iron, with heavier elements forged in supernovae and neutron star mergers.
Example
Deep inside the Sun, hydrogen is squeezed and heated until it becomes helium, and the released energy is what warms your face on a sunny day. When four hydrogen nuclei become one helium nucleus, about $0.7\%$ of their mass turns into energy, and the Sun converts about $600$ million tonnes of hydrogen into helium every second. The net reaction $4\,{}^1\text{H} \rightarrow {}^4\text{He} + 2e^+ + 2\nu_e$ releases about $26.7\ \text{MeV}$ from a mass deficit of about $0.029\ \text{u}$, converted by $E = mc^2$.
Key Insight
Fusion needs incredible heat and pressure, which is why it happens in a star's center and not on Earth's surface, and why fusion power plants are still under development. Fusion is the opposite of fission: fission splits heavy atoms like uranium, fusion joins light atoms, and both release energy because the products are more tightly bound. The Sun's core is not even hot enough for classical physics to allow fusion; quantum tunneling lets protons pass through the Coulomb barrier they could not climb over, and even then a given proton waits billions of years on average before fusing.