What is nuclear fusion?

Nuclear fusion supplies the stars with their energy, allowing them to generate light.

Stars, like our sun, are powered by nuclear fusion. (Image credit: DrPixel/Getty Images)

Nuclear fusion is the process of forcing together two light atomic nuclei and creating a heavier one, in the process taking a tiny amount of matter and turning it into massive amounts of energy.


It is nuclear fusion that supplies the stars  —  including the sun — with their energy, allowing them to generate light. The vast majority of energy that Earth receives comes from the sun, and without it, life itself on our planet would be impossible.


This energy is directed at our planet from what can loosely be described as our star's surface, the photosphere. This layer of the ball of superheated plasma we call the sun is heated by the star's core, where the majority of nuclear fusion takes place. This source of energy is so ubiquitous and so vital here on Earth, that it's little wonder that physicists are desperate to emulate it in reactors on our planet. A future powered by fusion could mean humanity's growing power needs are met by clean and highly efficient  fusion energy.



The key to understanding how fusion generates energy is Albert Einstein's infamous equation explaining how energy equals mass times the speed of light squared (E=mc²). This tells us that matter and energy are interchangeable, while the term c² tells us that a little mass creates a lot of energy.


When matter particles fuse, the particles going into the process have slightly more mass than the daughter particles that are created, with the difference in mass 'liberated' as energy.


Even with the considerable mass-to-energy yield of fusion, each incidence of fusion releases only a tiny amount of energy. Fortunately, stars offset that by having a lot of raw material to power fusion, and these processes run at incredible rates.


The main fusion process that provides the vast majority of the sun's energy is the proton-proton I (PPI) chain. There are two other branches of the PP chain (II and III) but these only account for around 15 percent of the thermonuclear fusion in the sun.


The PPI chain process involves four hydrogen atoms smashing together and creating a helium atom, two electrons, two neutrinos, and two highly energetic gamma-ray photons.


An illustration of  the process of nuclear fusion, specifically the creation of helium from hydrogen. Four protons (hydrogen nuclei) are combining on the left, releasing in the process two protons and two neutrons (a helium nucleus).  (Image credit: Mark Garlick/Getty Images)

While some of the energy is carried away as the kinetic energy of the daughter particle, the majority is carried by the two gamma-ray photons. These photons will struggle to escape the star's dense interior, however  —  taking over 30,000 years to move from the core to the surface. During this time the photons are undergoing a series of collisions, absorptions, and re-emissions, which 'downgrade' their energy to photons of visible light eventually radiated out by the photosphere.


Each occurrence of the PPI radiates about 0.0000000000044 Joules, which means  — ignoring the other fusion process going on in the sun  —  our star has to complete this process about 9x10³⁷ (9 followed by 37 zeroes) times every second to maintain its luminosity!


If four grams of hydrogen were converted to helium through this process, only 0.0028 grams would escape as energy. That equates to about 260 billion Joules, enough energy to power a 60-watt light bulb for about 100 years.


Because of its tremendous hydrogen content, the sun has maintained this fusion rate for around four and a half billion years and will continue to do so for a further four and a half billion years until the hydrogen in its center is exhausted.


This hydrogen-burning helium forging phase is what astrophysicists call the main sequence lifetime of a star. But, helium isn't the only chemical element being forged in the sun.



Astronomers describe stars as containing hydrogen, helium and everything else (with elements heavier than helium described as 'metals' by astronomers) and these other elements also play a role in fusion.


The PPI isn't the main fusion reaction in more massive stars than the sun, however. Instead, most of these stars' energy comes from the carbon-nitrogen-oxygen (CNO) cycle which requires the higher temperatures of more massive stars to get started.


The CN cycle begins with the nucleus of a carbon-12 atom using it as a catalyst  —  an element that speeds up a reaction but is unchanged at the end of it  —  for fusion. Carbon-12 through proton capture goes through various stages until a helium atom is emitted and carbon-12 is recovered. The NO cycle is similar but uses nitrogen-14 as a catalyst.


The energy generated by fusion serves a vital purpose within stars, providing the outward pressure that balances the ball of plasma against the inward force of gravity. That means that when fusion ceases, so goes the outward pressure; this results in the collapse of the star and the swelling and loss of its outer layers.


For stars more massive than the sun  —  which will end its life as a smoldering white dwarf  —  this gravitational collapse creates enough pressure to trigger the nuclear fusion of helium created by the main sequence lifetime in its core, fusing it to create carbon, neon and oxygen.


When helium is exhausted, collapse occurs again triggering the fusion of even heavier elements. As this continues, the star develops an onion-like structure with lighter elements fusing in its outer layers and subsequently heavier elements being created towards the core.


A close-up of the sun depicting solar surface activity and the corona. (Image credit: DrPixel/Getty Images)

This progression of nuclear fusions ends even for the most massive stars when iron dominates the stellar core. This is because iron is an extremely stable element and stars aren't massive enough to trigger its fusion.


When all nuclear fusion ceases, the star undergoes a final and catastrophic gravitational collapse. This triggers a supernova that flings the elements the star has forged during its lifetime out into the universe.


This material from these dead stars becomes the building blocks of the next generation of stars, the planets, and everything around us, including our own human bodies.


Additionally, shockwaves from the compressing iron core  —  which will eventually birth a neutron star or even a black hole  —  hit gas shed by the supernova triggering further nuclear fusion creating elements heavier than iron and radioactive materials as well as blasting out x-rays and gamma-rays.

Reference: "DOEExplains...Nuclear Fusion Reactions." U.S. Department of Energy Office of Science (2022).

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