How Does the Sun Shine? The Story of Nuclear Fusion in Its Core

The Sun that rises every morning is the energy source for almost all life on Earth. Photosynthesis in plants, the water cycle that feeds the rain, the winds and even the energy stored in fossil fuels are all, ultimately, forms of sunlight. So how does this giant light source work — and how has it kept shining for billions of years?

How big is the Sun?

The Sun is an ordinary but enormous star:

The Sun is mostly hydrogen (about 73%) and helium (about 25%). These gases are so hot that their atoms have lost their electrons, forming an electrically charged state called plasma.

The layers of the Sun

Inside, the Sun is layered like an onion, and each layer has a different job:

• Core: the centre, at about 15 million °C, where all the energy is produced.
• Radiative zone: energy travels outwards as particles of light — photons. But the material is so dense that each photon is constantly absorbed and re-emitted in random directions, so crossing this zone can take tens of thousands of years.
• Convective zone: here energy is carried by vast currents of hot gas that rise and sink like water boiling in a pan.
• Photosphere (surface): the layer at about 5,500 °C from which the light we see is emitted.
• Corona: the Sun’s outer atmosphere, visible as a white halo during a total solar eclipse. Surprisingly, it is 1–3 million °C — far hotter than the surface.

The Sun’s engine: nuclear fusion

In the Sun’s core, temperature and pressure are so extreme that the nuclei of hydrogen atoms can collide and fuse. This is called nuclear fusion.

Normally, atomic nuclei repel each other because they are positively charged. To fuse them, that repulsion must be overcome. The core’s 15-million-degree heat gives particles speeds of hundreds of kilometres a second, while the immense pressure from the Sun’s own weight squeezes them together. Even so, fusion is not easy: without a quantum effect called “tunnelling”, even the Sun’s heat would not be enough.

The process takes several steps, but in summary:

• Four hydrogen nuclei (protons) combine to form one helium nucleus.
• The resulting helium nucleus is about 0.7% lighter than the four protons that made it.
• That lost mass becomes energy according to Einstein’s famous E = mc². Because c — the speed of light — is such a huge number, even a tiny mass means an enormous amount of energy.

Every second the Sun turns about 600 million tonnes of hydrogen into helium, and about 4 million tonnes of mass are converted entirely into energy. That sounds a lot, but the Sun is so vast that it can keep going at this rate for billions of years.

Why doesn’t the Sun explode or collapse?

The Sun is in a constant balance:

• the gravity of its enormous mass tries to squeeze it inwards;
• the heat and pressure from the energy produced in the core push outwards.

The two forces balance each other. If fusion speeds up slightly, the Sun expands a little, its core cools and fusion slows. If fusion slows, the Sun contracts slightly, its core heats up and fusion speeds up again. This self-regulating system lets the Sun shine steadily for billions of years.

Light’s long journey

Once sunlight leaves the surface, it reaches Earth in about 8 minutes and 20 seconds. So when you look at the Sun, you see it as it was 8 minutes ago.

But that light’s real journey began much earlier. Energy produced in the core wanders through the radiative zone as photons are absorbed and re-emitted again and again, as if lost in a maze. Scientists estimate it takes tens of thousands, perhaps hundreds of thousands, of years to reach the surface. The energy in the sunlight warming your face today may have been produced in the Sun’s core before humans painted the first cave walls.

The Sun’s future

The Sun is about 4.6 billion years old and has used about half the hydrogen in its core. Scientists predict its future like this:

1. Over the next billion years: the Sun will slowly brighten, which in the distant future could boil away Earth’s oceans.
2. In about 5 billion years: as hydrogen in the core runs low, the Sun will swell into a red giant, swallowing Mercury and Venus and perhaps reaching Earth’s orbit.
3. The final stage: it will shed its outer layers into space as a planetary nebula, leaving behind an Earth-sized, extremely dense and slowly cooling white dwarf.

The Sun is not heavy enough to explode as a supernova or form a black hole — it would need to be at least several times more massive. We explain how black holes form in What is a black hole?.

Is fusion energy possible on Earth?

For decades scientists have tried to recreate the Sun’s working principle on Earth. Fusion energy promises a power source with no carbon emissions, no long-lived radioactive waste and fuel that can be extracted from seawater.

But Earth lacks the Sun’s immense pressure, so fusion reactors must reach temperatures above 100 million °C — hotter even than the Sun’s core. The international ITER project under construction in France and experimental reactors in several countries are working towards that goal. In 2022 a US laboratory achieved, for the first time, more energy from a fusion experiment than the laser energy delivered to its fuel. Commercial fusion power stations, though, are still some way off.

In short

The Sun is a giant nuclear fusion reactor that fuses hydrogen nuclei into helium in its core. The small amount of mass lost becomes enormous energy according to E = mc². That energy takes tens of thousands of years to travel from the core to the surface, then just 8 minutes to reach Earth. Thanks to the balance between gravity and internal pressure, the Sun will keep shining steadily for another 5 billion years or so.