How the Sun shines

The Sun is not on fire. A fire needs air, and there is none in space. A Sun made of burning coal would have gone out long ago.

The Sun shines because of what happens in its centre. There the gas is squeezed so hard, and is so hot, that tiny particles of hydrogen are forced together to make helium. Each time that happens, a little of their weight disappears and turns into energy.

A little goes a long way. Every second the Sun gets about four million tonnes lighter, and all of that becomes light and heat.

It has been doing this for 4.6 billion years. It is so big that it has hardly noticed.

An old puzzle. In the 1800s nobody knew what kept the Sun hot. The best idea was that it was slowly shrinking, and that the squeeze warmed it. That would keep it going for a few tens of millions of years. Geologists and biologists, though, were finding that the Earth was far older than that. Something was missing.

The answer came in the 1920s and 1930s. Arthur Eddington pointed out that four hydrogen atoms weigh slightly more than one helium atom, and that if the Sun could turn one into the other, the missing weight would appear as energy. Hans Bethe and others then worked out how it could happen.

The furnace. In the Sun’s core the temperature is nearly 16 million degrees and the gas is 150 times denser than water. Hydrogen nuclei, which are single protons, fly about at great speed. They all carry the same electric charge, so they push each other away, and even at this heat they should hardly ever touch. They manage it only because, on the very small scale, particles do not have sharp edges, and now and then two get close enough to merge. For any one proton that takes billions of years. The Sun has so many that it happens constantly.

Step by step, four protons become one helium nucleus. The helium has 0.7 per cent less mass than the protons that went in. That difference is the Sun’s entire output.

Each second, the Sun
Fuses	About 600 million tonnes of hydrogen
Turns into energy	About 4 million tonnes of it
Shines with	380 million million million million watts

The slow way out. The energy starts as gamma rays, and it does not get far. The inside of the Sun is so crowded that a ray of light travels a tiny distance before it is absorbed and sent off again in a new direction. It staggers outwards like someone lost in a crowd. Estimates of how long the journey takes run from about ten thousand years to more than a hundred thousand.

For the last third of the way the gas itself carries the heat, rising and sinking like water in a pan. At the visible surface the light finally gets clear, and reaches the Earth 8 minutes and 19 seconds later. The sunlight on your hand today is energy that was made before there were cities.

Proof. How can anyone know what goes on in the middle of the Sun? Fusion also makes neutrinos, ghostly particles that pass through almost anything. They leave the core and cross the whole Sun in about two seconds. Tens of billions of them go through every square centimetre of you each second, day and night, since the Earth does not stop them either.

In the 1960s Raymond Davis set out to catch a few, with a tank of cleaning fluid deep in a gold mine. He found them, which showed that fusion really is happening. But there were only about a third as many as predicted. For thirty years nobody knew whether the theory of the Sun was wrong or the theory of neutrinos. In 2001 an experiment in Canada settled it: the Sun was right. Neutrinos come in three types and change from one to another on the way here, and Davis’s tank could see only one type.