Why does the Sun shine?
The Sun isn’t burning
The Sun looks like a gigantic fire, but it isn’t burning. If it were made of coal, it would burn out in a few thousand years. Yet it has been shining for almost five billion years.
What the Sun is made of
The Sun is mostly hydrogen, the simplest element. A hydrogen atom has a single positively charged particle at its centre, a proton. Around it is one negatively charged electron.
Gravity crushes the Sun
The Sun is three hundred thousand times heavier than the Earth. Its own gravity pulls everything towards the centre and squeezes the gas inside. Squeezed gas heats up, just as a bike pump gets warm when you pump up a tyre.
An inferno in the core
The centre of the Sun, its core, is the most squeezed and the hottest part: over 15 million degrees. The hotter something is, the faster its particles move. In the core they whizz about so fast that the electrons are torn off the protons and fly around on their own.
Protons repel each other
Like charges repel, so protons push each other away. The faster they fly at each other, the closer they get, but in the end the repulsion always wins. In the Sun’s core they are so fast that now and then they even touch.
See for yourself
Blow up two balloons, rub them on your hair and hang them side by side on threads. The balloons push each other apart. They both have the same charge, negative in this case, so they repel just like two protons.
Once in billions of years
After touching, though, they almost always fly apart again. They only join if, at that very moment, one of them shoots out a positron, a tiny particle like an electron but with a positive charge, and turns into a neutron. A proton and a neutron don’t repel each other, and unlike two protons they stick together. But this change is incredibly rare: a proton waits billions of years for it on average.
Packed tight
But there are unimaginably many protons in the core, and gravity has packed them tightly together. A litre of that stuff would weigh 150 kilos. So even this rare event happens over and over again.
Four protons make helium
The joined pair then flies around the Sun’s core until two more protons join it. One of them turns into a neutron too, and helium is born, the gas that makes fairground balloons float. This joining in the heat is called nuclear fusion. The helium then stays in the core and, for now, doesn’t fuse any further.
Helium’s turn comes later
A helium nucleus has two protons, so two helium nuclei repel each other even more strongly than two protons. They will start fusing when the Sun runs out of hydrogen in its core in about five billion years and the core heats up to 100 million degrees. Then three helium nuclei will make carbon, the element that pencil lead is made of, for example.
A bit of mass turns into light
Helium weighs a little less than the four protons it was made from. So from every kilogram of hydrogen, about seven grams turn into light and heat. That is as much energy as burning 20,000 tonnes of coal.
The Sun is losing weight
Every second, the Sun turns 600 million tonnes of hydrogen into helium this way. About 4 million tonnes of that become light. For the Sun that is just a crumb: its hydrogen will last for about another five billion years.
A long way out
Energy from the core reaches the surface slowly. Light keeps bouncing back and forth inside the dense Sun, and its journey out takes tens of thousands of years, maybe over a hundred thousand. From the surface it then reaches us in just eight minutes.
A Sun on Earth
We are trying to get nuclear fusion going on Earth too and make electricity from it. But we don’t have the Sun’s gravity, so we heat the gas to 150 million degrees, ten times hotter than the Sun’s core. Gas that hot can only be held by strong magnets in a doughnut-shaped machine called a tokamak. No wall could withstand it.
So why does the Sun shine?
The Sun isn’t burning. Gravity squeezes and heats its core, and nuclear fusion goes on inside it: hydrogen protons slowly join to make helium. A bit of mass turns into light along the way, and there is so much hydrogen that it will last for billions of years.