What is nuclear fusion?

What is nuclear fusion? Nuclear fusion is the joining of two atomic nuclei to form a heavier nucleus. It is the opposite of fission, which splits a heavy nucleus apart. Fusion releases far more energy per unit of fuel than any chemical reaction, and far more than nuclear fission as well.

In the Sun's core, temperatures exceed 15 million degrees Celsius. At this extreme heat, hydrogen nuclei — which are single protons — move fast enough to overcome their mutual electrical repulsion and collide. Four hydrogen nuclei fuse together through a chain of steps to produce one helium nucleus. Here is the crucial detail: the helium nucleus weighs slightly less than the four hydrogen nuclei that formed it. That missing mass does not disappear. It converts directly into energy, according to Albert Einstein's famous equation E=mc². Because the speed of light (c) is enormous, even a tiny amount of mass produces a vast amount of energy.

The Sun converts approximately four million tonnes of matter into pure energy every single second. Despite this staggering rate, the Sun has enough hydrogen fuel to continue for another five billion years. That endurance comes from sheer scale: the Sun contains about 2 × 10³⁰ kilograms of matter.

Epivo's How the Universe Works curriculum covers nuclear fusion and stellar physics from first principles.

A star is essentially a giant balancing act. Gravity pulls billions of tonnes of gas inward, trying to crush the star to a point. Nuclear fusion pushes outward with radiation pressure, resisting that collapse. This balance is called hydrostatic equilibrium. It gives a star its stable round shape and its long, steady life.

When the balance holds, the star shines steadily. Our Sun has maintained this balance for 4.6 billion years and will continue for roughly five billion more. Smaller stars, called red dwarfs, burn their fuel so slowly that they can remain stable for trillions of years. Massive stars, by contrast, burn far hotter and exhaust their fuel in millions of years rather than billions.

What happens when fusion stops

When a Sun-like star exhausts its hydrogen, the core contracts and heats up enough to fuse helium instead. The outer layers expand enormously, turning the star into a red giant. Eventually the outer layers drift away as a glowing cloud called a planetary nebula, leaving a dense, slowly cooling core called a white dwarf.

For stars more than eight times the Sun's mass, the end is far more violent. These stars fuse progressively heavier elements — helium, carbon, oxygen, neon, silicon — each round of fusion producing a denser, hotter core. The chain ends at iron. Iron does not release energy when fused; it absorbs energy instead. The moment the core fills with iron, fusion stops providing support. Gravity wins in milliseconds. The core collapses at one-quarter the speed of light and compresses to roughly 12 kilometres across. It then rebounds in a catastrophic supernova that briefly outshines an entire galaxy.

According to OpenStax Astronomy 2e, this collapse and explosion is how the universe seeds space with heavy elements forged by nuclear fusion deep inside dying stars.

The Big Bang produced only three elements: hydrogen, helium, and traces of lithium. Every other element on the periodic table was forged later by nuclear fusion inside stars — a process called stellar nucleosynthesis.

Elements up to iron are built by fusion inside stars and scattered into space by stellar winds and supernova explosions. But some elements are too heavy to form this way. Gold, platinum, and uranium require conditions even more extreme than a supernova core. They form when two neutron stars collide — an event called a kilonova.

In August 2017, the LIGO and Virgo gravitational-wave detectors picked up a signal from two neutron stars spiralling together and merging 130 million light-years away. Telescopes worldwide then observed the kilonova that followed. As LIGO reported, the event produced quantities of gold and platinum equivalent to several times the mass of the Earth. That observation confirmed a decades-old prediction. The gold in a wedding ring was made in a neutron star collision billions of years before the Sun was born.

This is what nuclear fusion ultimately means for everyday life. The carbon in your body was fused in a red giant. The oxygen you breathe was scattered by a supernova. The iron in your blood was formed in the core of a massive star. We are, quite literally, made of stardust — the products of nuclear fusion across cosmic time.

Parents connecting science to curiosity can find support at our guide for parents.

If nuclear fusion powers the Sun so efficiently, can humanity replicate it here on Earth? That question has driven research since the 1950s. The challenge is enormous. Igniting fusion requires confining hydrogen plasma at 100 million degrees — hotter than the Sun's core — long enough for nuclei to collide and fuse.

Two main approaches are being pursued. Magnetic confinement uses powerful magnets to contain the hot plasma in a doughnut-shaped chamber called a tokamak. The international ITER project in France is a collaboration of 35 nations. It is the world's largest tokamak, designed to produce ten times more energy than it consumes. Inertial confinement fires powerful lasers at a tiny hydrogen fuel pellet from all directions. The rapid compression and heating ignites fusion for a fraction of a second.

In December 2022, the National Ignition Facility in California announced a historic milestone. Its laser experiment produced more fusion energy than the laser energy delivered to the target — called ignition. It was the first time in history that a fusion experiment generated net energy from the fuel itself.

Fusion fuel is abundant and clean. The primary fuels — deuterium and tritium — can be derived from seawater and lithium. Fusion produces no carbon dioxide and no long-lived radioactive waste. The quest for practical fusion energy has been called humanity's attempt to build a star on Earth. Continue the story with Epivo's How the Universe Works curriculum, alongside what is the solar system and what is a black hole.