Every second, the Sun converts about 600 million tonnes of hydrogen into helium through nuclear fusion, releasing energy equivalent to billions of nuclear bombs. Deep in its core, where temperatures reach 15 million degrees Celsius and pressures are 250 billion times Earth's atmosphere, hydrogen nuclei are forced so close together that they overcome their natural electrostatic repulsion and fuse. The resulting helium nucleus has slightly less mass than the original hydrogen nuclei combined, and that tiny missing mass (about 0.7%) is converted directly into energy following Einstein's famous E = mc². That equation explains why a little mass yields a staggering amount of energy — the "c" is the speed of light, an enormous number, squared.
Fusion is not just the Sun's trick; it is the engine of the entire visible universe. Stars fuse hydrogen into helium, then helium into carbon and oxygen, then onward through silicon all the way to iron. Elements heavier than iron require even more extreme events — supernova explosions and neutron star collisions — to form. Every atom of carbon in your DNA, every atom of calcium in your bones, every atom of iron in your blood was forged by fusion inside a star that exploded long before our Solar System existed. As Carl Sagan put it, we are literally made of star stuff.
Here on Earth, scientists have been trying to harness fusion for clean energy since the 1950s. The appeal is irresistible: fusion fuel (hydrogen isotopes deuterium and tritium) is virtually limitless, the process produces no greenhouse gases, and the waste is far less problematic than nuclear fission. The challenge is recreating stellar conditions on Earth. The ITER project in southern France, a collaboration of 35 nations, is building the world's largest tokamak — a donut-shaped magnetic containment vessel designed to heat plasma to 150 million degrees and sustain fusion reactions. If successful, fusion could provide humanity with an essentially inexhaustible source of clean energy.