How does the Sun make its energy?
The short answer
The Sun makes energy by nuclear fusion in its core. Under crushing pressure at 15 million degrees, hydrogen nuclei are forced together and fuse into helium. The helium weighs slightly less than the hydrogen that went in, and that missing mass becomes energy. The Sun converts about 600 million tonnes of hydrogen every second.
Everything happens in the core, nowhere else
Fusion needs two conditions at once: enormous temperature and enormous pressure. Only the very centre of the Sun has both. The core is squeezed by the weight of the entire star pressing in from every direction, and the gas there is packed tighter than metal. Outside the core the conditions fall away quickly and fusion stops. So the Sun is not glowing all the way through for the same reason. It has one engine, buried at the centre, and the rest is just the long business of getting the energy out.
Protons do not want to touch each other
Hydrogen nuclei are protons, and protons carry positive charge. Like charges repel, so two protons approaching each other feel a push that grows stronger the closer they get, the way two magnets resist when you turn them the wrong way round. At ordinary temperatures they simply bounce apart. The only way past that barrier is speed, and the only way to get speed is heat. At 15 million degrees protons move fast enough that some collisions overcome the repulsion and the nuclei stick.
Four hydrogens in, one helium out, and a little missing weight
Through a short chain of steps, four hydrogen nuclei end up as one helium nucleus. Weigh what went in and weigh what came out and the helium is slightly lighter. The difference is tiny, well under one percent, but it has to go somewhere. It leaves as energy. This is what Einstein's equation describes: mass and energy are two forms of the same thing, and the exchange rate between them is enormous. A very small amount of mass becomes a very large amount of energy.
600 million tonnes a second
Every second, the Sun fuses roughly 600 million tonnes of hydrogen into helium. The energy released in that one second would run everything on Earth for something like half a million years. It is worth sitting with that for a moment, because it explains why a star can shine for billions of years without obvious change. The fuel tank is so large and the process so efficient that four and a half billion years of continuous output has barely dented it.
A tug of war that has lasted four billion years
Gravity pulls every part of the Sun inward. The energy pouring out of the core pushes outward. If gravity won, the Sun would collapse. If the outward push won, it would blow itself apart. Instead the two match almost exactly, and have done for billions of years. Astronomers call this hydrostatic equilibrium. It is not a coincidence but a self-correcting balance: squeeze the core harder and fusion speeds up, pushing back; let it expand and fusion slows, letting gravity pull in again.
Sources
Full transcript: How the Sun Makes Energy: Fusion Explained
How the Sun makes its energy! Last time, we discovered something surprising: the Sun is not on fire. It's a giant ball of glowing gas! But that leaves a mystery.
Something in there is making sunshine— enough light and heat for a whole solar system! So what's the engine? Watt wants to know, let's dive in, literally! This is Watt & Why, and today's big question: How does the Sun make its energy?
Get ready, this is the deepest dive in the universe! Down we go, through half a million kilometers of glowing gas. The deeper we sink, the hotter it gets, and the harder everything above presses down—like diving to the bottom of an impossibly deep ocean. Welcome to the core, the very middle of the Sun!
Everything here is squeezed by the weight of the whole Sun pressing in from every side. It's 15 million degrees, and the gas is packed tighter than metal! This is where the magic happens. The core is full of hydrogen, the smallest, simplest stuff in the universe.
And here's the thing about heat: hot means fast! Down here, hydrogen bits zip around at millions of kilometers per hour. Careful, Watt! But these hydrogen bits—protons—have a problem.
They're all positive, and positive pushes away positive—like two magnets turned the wrong way! Every time they get close, boing! They bounce apart. So how can they ever do anything together?
Squeeze harder, move faster, and slam! When two protons crash hard enough, they stick. That's fusion! And it doesn't stop there.
In a few quick steps, four hydrogens fuse into one brand new helium. Now for the real magic trick: put them on a scale! Four hydrogens versus one helium. The helium weighs a tiny bit less!
So where did the missing bit go? It turned into pure energy, light, and heat! A scientist named Einstein wrote the recipe: a tiny pinch of stuff makes a huge amount of energy. How huge?
Every single second, the Sun fuses about 600 million tons of hydrogen. And in that one second, it makes enough energy to power the whole Earth for half a million years! One second! With all that power, why doesn't the Sun blow itself apart?
Tug of war! Gravity pulls everything in, energy pushes everything out. They pull exactly as hard as each other— a perfect balance that keeps the Sun round, steady, and shining! Will it run out of fuel?
Don't worry, the Sun has been shining for over 4 billion years, and it still has enough hydrogen for billions more. That's longer than you can imagine. Sunshine, free every single day! So remember: deep in the core, the Sun smashes hydrogen into helium.
That's fusion! The tiny missing bit of weight becomes light and heat, and gravity and energy hold it all in perfect balance. Next time, we take the grand tour layer by layer through the Sun! See you on Watt & Why!
Transcript of the episode, lightly cleaned for readability.
Sample question
Question 1 of 10
What process produces the Sun's energy?
- Nuclear fusion, joining light atomic nuclei together correct
- Nuclear fission, splitting heavy atoms apart
- Burning hydrogen with oxygen
- Friction from the Sun's rotation
Fusion joins small nuclei into larger ones. Fission, used in power stations on Earth, does the opposite.
That is one of 10. The rest are not shown here, and the answer order changes every time you take the quiz.
Written by Watt & Why. Published 7 September 2026. Last reviewed 17 October 2026.