The Sun's Power Trick: How nuclear fusion makes energy
CHEMISTRY · 4 MIN
How does the Sun burn for billions of years without any fire?
There's no oxygen up there and nothing to set alight. Instead, tiny specks crash and stick together. No science needed: we'll start from what everything is made of and build up from there.
8 steps · 9 quick challenges
Step 1 of 8 · The journey in
Our sky engine
Feel the warmth of sunlight on your face? It comes from a reaction we're still trying to copy. Let's follow it from the sky down to the tiniest particles.
Every sunbeam on your face started deep inside the Sun. But the Sun isn't burning like a campfire. It's doing something far more powerful.
- Huge ball. The Sun is a giant glowing ball of super-hot gas. There's no solid ground anywhere. You could fit about a million Earths inside it. If the Sun were a front door, Earth would be a coin lying next to it.
It's about 109 times wider than Earth, and it holds more than 99% of all the mass in our solar system.
It's hot gas all the way down, getting hotter and more tightly packed toward the middle.
- Always on. It has been shining for about 4.6 billion years, and it has enough fuel left for billions more. No fire could ever last that long. If the Sun were a giant pile of coal on fire, it would have burnt out before the pyramids were built.
A fire is a chemical reaction: fuel joins with oxygen and gives off heat. That's far too weak to power a star for billions of years. Something much stronger must be going on.
- Big idea. That something is nuclear fusion. Everything is made of tiny bits called atoms. Fusion smashes the centres of atoms together so hard that they join, releasing loads of energy. Fire is like swapping LEGO bricks between models. Fusion is like melting two bricks into one brand-new brick.
Fusion means joining. It's the opposite of the fission used in today's nuclear power stations, which splits big, heavy atoms apart.
Scroll down and we'll dive into the Sun to watch fusion happen, one tiny step at a time.
- Age
- 4.6 billion yrs
- Size
- ≈1M Earths
Did you know? The sunlight you see is already about 8 minutes old. That's how long light takes to cross the 150 million km between the Sun and Earth.
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Everything around you, from your phone to the air, is made of atoms: tiny building blocks far too small to see. Each atom has a tiny centre, called the nucleus, with even tinier electrons buzzing around it.
A campfire is chemistry: atoms swap partners (wood joins with oxygen), but each atom stays the same kind of atom. Fusion is different. It changes the nuclei themselves, joining small ones into a bigger one.
When small nuclei join, a tiny bit of their mass turns into a huge amount of energy. That energy slowly works its way out of the Sun and leaves as sunlight. Some of it reaches your face about eight minutes later.
The Sun turns about 4 million tonnes of its mass into energy every second.
Step 2 of 8 · The journey in
Into the core
So the Sun runs on fusion. But fusion only happens in one place: right at the very centre. Let's dive in.
Dive down into the very middle of the Sun. It's the hottest, most crushed place in the solar system, and the only place where the magic happens.
- Wild heat. Heat is just how fast tiny particles are moving: hotter means faster. The core is about 15 million °C, so its particles are racing at hundreds of kilometres every second. A hot oven is 200 °C. The core is like 75,000 ovens' worth of heat stacked together.
At these temperatures, atoms can't hold on to their electrons, so everything turns into plasma: a soup of bare nuclei and loose electrons. More on that soon.
- Huge squeeze. Gravity, the same pull that keeps your feet on the ground, pulls all of the Sun's gas toward its middle. The core has the whole Sun's weight pressing down on it from every side. Like being at the bottom of a giant pile of mattresses, with more piled on from every direction.
The deeper you go, the more gas is pressing down from above, so the core feels the biggest squeeze of all.
That squeeze makes the core more than 10 times denser than lead, even though it isn't solid.
- Perfect mix. Heat makes particles move fast, and squeezing packs them close together. Fast plus crowded means lots and lots of crashes. Like a packed bumper-car arena where every car is flat out: crashes happen constantly.
Only about the inner quarter of the Sun's width is hot and squeezed enough for fusion. Outside the core, the energy is just carried outward.
- Core heat
- 15M °C
- Core size
- ¼ Sun
Did you know? A sugar cube's worth of the Sun's core would weigh about 150 grams, as much as a smartphone, even though it isn't solid at all.
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Temperature measures how fast particles are moving. Touch a hot mug and its fast-moving particles bash into the particles of your hand. That's what heat feels like.
The Sun's core is hot because gravity squeezes it hard. Squeezing a gas heats it up, the same way a bike pump gets warm when you pump hard.
Even in the core, fusion isn't easy. Most particles miss or bounce away. The Sun shines because there are so unbelievably many chances every second.
Energy made in the core can zigzag for thousands of years before it escapes the Sun.
Step 3 of 8 · Meet the fuel
Tiny fuel bits
The core is hot and crushed. But what exactly is being crushed down there? Let's zoom in on the fuel.
What fuel could keep a star going for billions of years? It's the simplest, lightest atom there is: hydrogen.
- Tiny centre. Every atom has a tiny centre called the nucleus, with a cloud of electrons whizzing around it. Fusion happens in the nucleus, not in the cloud. If an atom were a football stadium, its nucleus would be a pea on the centre spot.
Yet almost all of the atom's weight is in that pea. The rest of the atom is mostly empty space with a few very light electrons zipping about.
- One proton. Particles can carry an electric charge: positive (+) or negative (−). A normal hydrogen nucleus is just one proton, a tiny particle with a + charge. Nothing could be simpler. Hydrogen is the single LEGO brick of the universe: every bigger atom is built from more bricks.
Inside the Sun, the heat has stripped the electrons away, so what's zooming around is mostly bare protons.
- Heavy types. Some hydrogen nuclei also carry neutrons: particles just like protons but with no charge. They add weight, not charge. These heavier kinds are called deuterium and tritium. Same person, different backpack: still hydrogen, just carrying extra weight.
Deuterium has one proton and one neutron. Tritium has one proton and two neutrons.
Fusion machines on Earth usually burn a mix of deuterium and tritium, because they fuse much more easily than plain protons.
- Sun
- ≈73% H
- Proton
- + charge
Did you know? Most of the hydrogen in the water you drink was made just after the Big Bang, so those atoms are about 13.8 billion years old.
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Atoms are built from three kinds of particle. Protons (+ charge) and neutrons (no charge) huddle together in the nucleus. Electrons (− charge) whizz around outside.
The number of protons decides what kind of atom it is. One proton: hydrogen. Two protons: helium. Six: carbon. So joining nuclei together literally makes a new kind of atom.
The Sun mainly uses a slow chain of steps called the proton-proton chain. Step by step, hydrogen nuclei end up as helium.
About three quarters of the Sun's mass is hydrogen.
Step 4 of 8 · Meet the fuel
They push away
So the fuel is protons, and fusion means joining them together. Sounds easy. There's just one big problem.
Here's the problem: protons really, really don't want to touch. So how does the Sun get them to join?
- Same repels. Electric charges follow one simple rule: opposites attract, matching charges push apart. Every proton is +, so any two protons shove each other away. Just like pushing the matching ends of two magnets together: the closer you get, the harder they fight.
- Hidden wall. The closer two protons get, the harder they push apart. It acts like an invisible wall that almost never lets them meet. Like rolling a ball up a steep hill: unless it's going really fast, it rolls back down.
Scientists call this the Coulomb barrier, after the electric force between charges. Halve the distance between two protons and the push gets four times stronger.
- Need speed. To get past the wall, protons must slam together incredibly fast. Even then, they need a strange shortcut from the quantum world, the rulebook for the tiniest things. Like a ghost sometimes slipping through a wall instead of climbing over it.
That shortcut is called quantum tunnelling. Tiny particles act a bit like fuzzy waves, so sometimes one just appears on the other side of the wall without climbing over it.
Without tunnelling, even the Sun's core wouldn't be hot enough for fusion to happen.
- Rule
- + pushes +
- Need
- speed
Did you know? Fusion is so hard to start that a single proton in the Sun's core may wait billions of years before it finally fuses. Luckily, there are an unimaginable number of protons.
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Electric charge has one rule: opposites attract, but matching charges repel. You've felt it if a balloon rubbed on your hair has made your hair stand up. Protons are all positive, so they push each other away.
But nuclei hold a second, secret force: the strong force. It's far stronger than the electric push, but it only reaches about the width of a proton. If two protons get that close, the strong force grabs them and holds on.
So fusion is a race: get close enough for the strong force to win before the electric push throws you back. The Sun also gets help from quantum tunnelling, where a particle sometimes sneaks through a wall it couldn't climb.
Without quantum tunnelling, the Sun would struggle to shine at all.
Step 5 of 8 · The squeeze
Hot means zoomy
Protons push each other away, so they need to crash in fast. And in the core, speed comes from heat.
At millions of degrees, ordinary gas turns into something else entirely: plasma, the fourth state of matter.
- Plasma soup. Heat ice and you get water; heat water and you get steam. Heat steam to thousands of degrees and electrons get knocked off their atoms. That glowing soup of charged bits is plasma. Like a dance floor so wild that every couple gets split up and everyone dances alone.
Solid, liquid, gas, then plasma: each one is what you get by adding more heat. Because plasma is made of charged bits, it can be steered with magnets, which matters later in our story.
- Popcorn move. Heat is really just particles moving. Hotter means faster, so in the core, protons zip around at hundreds of kilometres per second. Like popcorn in a pan: the hotter the pan, the higher and faster the kernels fly.
- Many crashes. The faster and more crowded the particles are, the more often two nuclei get close enough for a chance to fuse. Like a lottery: each crash is one ticket, and the core buys trillions of tickets every second.
- State
- plasma
- Core speed
- hundreds km/s
Did you know? More than 99% of all the ordinary matter you can see in the universe is plasma, mostly inside stars.
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Plasma is called the fourth state of matter, after solid, liquid and gas. It's what you get when a gas becomes so hot that electrons break loose from their atoms.
Because plasma is full of charged particles, magnets can push and guide it. That's very useful for fusion machines on Earth.
In the Sun, heat means nonstop motion. Nuclei zip around, collide, bounce away, and very occasionally get close enough to fuse.
Lightning, neon signs and the Sun are all made of plasma.
Step 6 of 8 · The squeeze
Stick, then shine
Fast, crowded protons crash again and again. And every so often, one crash gets close enough…
Finally, two nuclei get close enough to stick. After a few more steps, four protons have turned into one helium nucleus, and energy bursts out.
- Close enough. Up really close, a much stronger force takes over: the strong force. It grabs the nuclei and glues them together. Like Velcro: it does nothing from across the room, but press the two sides together and they lock tight.
The strong force is the strongest force in nature, but it only reaches about the width of a proton. That's why the nuclei have to get so incredibly close.
- Step chain. In the Sun, it happens in steps. Two protons join (one turns into a neutron) to make deuterium. Add a proton: helium-3. Two helium-3s join: helium-4. Like a recipe: you can't bake the cake in one go, you mix, then rise, then bake.
This is the proton-proton chain. Along the way, some protons change into neutrons, giving off ghostly little particles called neutrinos.
Overall: four hydrogen nuclei in, one helium nucleus out, plus energy.
- Energy out. Each step gives off energy as light and fast-moving particles. That energy heats the Sun and eventually escapes as sunlight. Like two magnets snapping together with a loud click: joining releases energy.
- In
- hydrogen
- Out
- helium + energy
Did you know? Helium was discovered in sunlight in 1868, before anyone found it on Earth. Its name comes from Helios, the Greek word for Sun.
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The Sun's main recipe isn't one simple crash. It's a chain: protons join, one changes into a neutron, and later helium forms.
That chain is called the proton-proton chain. Four hydrogen nuclei eventually make one helium nucleus, plus energy and tiny particles.
The strong force is the glue. It only works over a tiny distance, so nuclei must get almost touching-close, which is why the Sun needs all that heat and squeeze.
Trillions of neutrinos from the Sun pass through your body every second, almost all without touching anything.
Step 7 of 8 · The payoff
Missing mass
Four protons went in, one helium came out, and energy burst out. But where did that energy actually come from?
Weigh everything before and after fusing, and something is missing. That missing bit is where all the energy comes from.
- Weigh before. Mass is how much 'stuff' something is made of. Four protons have a little more mass than the one helium nucleus they make. About 0.7% vanishes. Like building a LEGO model, weighing it, and finding it's lighter than the pile of bricks you started with.
- Tiny loss. The missing mass isn't gone. It has been turned into energy: light and heat. That's what makes the Sun shine. Like swapping a coin for a huge stack of notes: the stuff changes form, it doesn't disappear.
Mass and energy are two forms of the same thing. Normally mass stays mass, but in nuclear reactions a little of it gets turned into energy.
- Big rule. Einstein's famous E = mc² gives the exchange rate. E is energy, m is mass, and c is the speed of light: huge. So a tiny bit of mass makes a LOT of energy. Like an exchange rate where one penny buys you a mansion.
c is about 300,000 km per second, and in the formula it's squared, so it's multiplied by itself. That giant number is why nuclear energy is so powerful compared with burning things.
- Mass lost
- ≈0.7%
- Rule
- E=mc²
Did you know? If you could turn just 1 gram of mass (about a paperclip) completely into energy, it could power around 2,500 homes for a whole year.
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When four hydrogen nuclei end up as one helium nucleus, the helium is a little lighter than the starting pieces. The particles hold together so tightly inside helium that some of their mass is set free.
The missing mass turns into energy. Einstein's equation, E = mc², tells us why even a tiny bit of mass makes a huge amount of energy: you multiply it by the speed of light, twice.
Across the whole Sun, this adds up fast. About 4 million tonnes of mass become energy every second, and the Sun is so big it can keep that up for billions of years.
One gram of fusion fuel can release energy similar to burning about eight tonnes of oil.
Step 8 of 8 · On Earth
Star in a donut
That energy leaves the core, escapes as light and reaches your face 8 minutes later. Could we build the same thing here?
Can we build a mini Sun on Earth? Scientists are trying, using magnetic cages to hold plasma hotter than the Sun's core.
- Magnet bottle. We can't squeeze plasma with gravity like the Sun does, so we make it even hotter instead. Nothing solid can touch it, so magnets hold it in mid-air. Like holding a scoop of ice cream on a hot day without touching it, using invisible hands.
Earth machines need around 100 to 150 million °C, about 10 times hotter than the Sun's core, to make up for not having the Sun's huge squeeze.
Remember: plasma is made of charged bits, and magnets can push charged bits around. So magnetic fields can steer the plasma and keep it away from the walls.
- Donut machine. A tokamak is a donut-shaped machine. Giant magnets wrap around it and keep the plasma racing round and round in a ring. Like a racetrack with no exits: the plasma laps forever without ever hitting a wall.
The word tokamak comes from Russian, where the design was invented in the 1950s. The donut shape means the plasma has no ends to leak out of.
- Hard goal. Huge projects like ITER, being built in France by over 30 countries, are testing whether fusion can become a clean power source for everyone. Like learning to keep a campfire going in a storm: lighting it is easy, keeping it alive is the hard part.
Fusion makes no carbon dioxide, and its fuel is plentiful. The hard part is getting more energy out than you put in, steadily, for a long time.
In 2022, the US National Ignition Facility got more energy out of a fusion fuel pellet than its lasers put in. That was a big first, but power plants are still years away.
- Plasma
- 100–150M °C
- Big test
- ITER
Did you know? One fusion fuel, deuterium, is found in ordinary seawater: about 1 in every 6,400 hydrogen atoms in the ocean is the heavy kind.
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The Sun has two helpers we don't: enormous gravity to squeeze its fuel, and billions of years to wait. On Earth we have to cheat, so we use fuel that fuses more easily (deuterium and tritium) and heat it far hotter.
A tokamak uses strong magnetic fields to keep the plasma in a donut-shaped path. The plasma can reach 100 to 150 million °C, yet the walls just a few metres away stay safe.
Fusion research is real but hard. ITER is a giant international tokamak, while NIF in the U.S. reached fusion ignition in 2022 using lasers.
NIF reported fusion ignition in December 2022: the fusion fuel gave out more energy than the lasers delivered to it.