thermonuclear fusion

Stars shine for billions of years, far longer than any ordinary fire or chemical reaction could last. Their power source is not burning in the everyday sense but something deeper: the joining of atomic nuclei. Thermonuclear fusion is the process of squeezing light atomic nuclei together so hard that they merge into a heavier nucleus, releasing a burst of energy each time. It is the furnace at the heart of every star, including the Sun.

Nuclei are all positively charged, and like charges repel fiercely, so they normally refuse to touch. Only at the staggering temperatures of a stellar core — about 15 million kelvin in the Sun — do nuclei slam together hard enough (with a quantum assist) to fuse. When four hydrogen nuclei are ultimately turned into one helium nucleus, the helium weighs very slightly less than the four hydrogen did, and that missing mass is converted into energy according to Einstein's E = mc^2. The fraction lost is tiny — about 0.7 percent — but with c (the speed of light) squared as the multiplier, even a whisker of mass yields enormous energy.

Fusion is what makes stars stable, long-lived, and the factories of the chemical elements. The energy it releases provides the pressure that holds a star up against gravity, and it does so steadily for a very long time precisely because the reactions are so slow and hard to start. It is also, quite literally, where most atoms in your body came from — the carbon, oxygen, and nitrogen forged in earlier generations of stars. A common confusion: fusion (joining light nuclei) releases energy for elements up to iron, whereas fission (splitting heavy nuclei) is the different reaction used in today's power plants.