What is the sun?
Imagine … before the solar system … a vast cold cloud of gas and dust, spreading out over billions of km in all directions. On our scale of 150million to 1, the cloud covers the whole of Europe and much, much further. But is a little thicker over London. The cloud has mass, so the denser cloud over London develops its own gravity, pulling gas and dust in like a whirlpool centred on the London Eye. Everything caught in the spiral falls towards the centre, like water spiralling down a plug hole, into a dense ball in the middle.
So far, this could be the story of Jupiter, but when it reaches a certain size, about 70 Jupiter masses, the tremendous pressure at the centre causes something that sets its fate on a dramatically different course to the planets. The ‘nuclei’ of atoms (their centres) crush together so hard that occasionally the tiniest nuclei (hydrogen) can squash together to make one slightly larger (helium) nucleus. It's a single nuclear reaction, releasing a tiny burst of heat.
But there is nowhere for the heat to escape, so the Sun begins to superheat from inside. The temperature soars, the core of liquid metal hydrogen vaporises into plasma, which expands, overcoming incredible pressure of weight of the sun all around. It has ignited to become a star.
Nuclear fusion
The nuclear heat changes everything. Instead of Jupiter’s swirling turbulent oceans of dense liquid metal hydrogen cooked up to 25,000K, the Sun’s plasma core is 600 times more savage at 15 million K.
But this nuclear energy is trapped by a half-million km thick blanket called the 'radiation zone'. This is where heat begins its slow journey to escape the sun, that will take an incredible 171,000 years, bumping and pushing its way through densely packed hydrogen atoms. Finally it reaches the more fluid 'convection zone' where it meets hydrogen at 1/10th of the temperature, just 1.5 million K, and much lower pressure. Vast bubbles of ultra-hot hydrogen break off, like icebergs from a glacier, free to float upward in ferocious convection currents that rise the last 300,000km all the way to the sun's surface, to break like bubbles in a pan of boiling water. The surface of this cauldron of boiling hydrogen is what we see from earth. Here the energy is freed at last, as heat and light shooting out into space, enabling the sun to cool down.
But the sun’s ferocity doesn’t end there. The heat is enough to rip atoms apart into electrically charged ions. The star’s stormy turbulence generates massive unstable magnetic fields that can fling a billion tonnes of these ions out into space at up to 2000 km per second. Travelling at that speed, the deadly charged particles could reach earth in just two hours.
Nuclear reactions are the most potent energy source known. The nuclear-powered sun is so hot that it heats the entire solar system. But the Sun’s core has a big surprise. The nuclear heat from a cubic meter of the Sun’s core would barely heat your living room. Weight for weight, the power it kicks out is a puny 270W (per cubic meter), less than a typical compost heap. This isn’t nuclear power as we know it! What happened to a single nuclear bomb with more explosive power than all the bombs of World War II put together, or a nuclear reactor using just 5m3 of uranium to power a city with a billion watts?! How could this be true, and if it is, how can the sun be so hot?
The answer is that the core’s size is huge – billions upon billions of cubic km – and that the heat is trapped, so the centre of the Sun just gets hotter and hotter until the heat can escape.
