In “Sun Promises Five Billion More Years, Apologises for Summer”, the Sun explained its working conditions. Its core, it said, is at about fifteen million degrees, where protons slam into each other, and although almost none have enough energy to overcome their mutual repulsion, every now and then one tunnels through anyway. “Physicists call this quantum tunnelling. We call it Tuesday.” Four hydrogen nuclei go in, one helium nucleus comes out, and about seven tenths of a per cent of the mass becomes sunlight. That’s the summary. The details are stranger, and they explain why the Sun is not a bomb.
The Sun’s main route from hydrogen to helium is called the proton-proton chain, and it has three steps. The first is the hard one. Two protons have to get close enough for the strong force to grab them. Tunnelling makes that possible, rarely. But even when two protons do touch, they can’t simply stick together, because a nucleus made of two protons and nothing else is unstable and falls apart instantly. To make something lasting, one of the two protons has to turn into a neutron during the brief moment they’re together. That change is governed by the weak nuclear force, which lives up to its name: it happens so rarely that almost every encounter fails, and the protons just fly apart again.
Occasionally, though, it works. One proton becomes a neutron, and the pair becomes deuterium, a heavy form of hydrogen. The change throws out two particles: a positron, the antimatter twin of an electron, which almost immediately meets an electron and turns into energy, and a neutrino, a tiny, nearly massless particle we’ll come back to.
After that, things move fast. Within about a second, the deuterium meets another proton and they fuse into helium-3, a light form of helium, releasing a burst of gamma rays. Then, typically after hundreds of thousands of years, two helium-3 nuclei meet, fuse into the ordinary helium-4 nucleus, and hand back two protons. Count it all up and four protons have become one helium nucleus, two neutrinos have escaped, and the leftover mass has become energy.
The bottleneck is that very first step. The odds of a given proton making deuterium are so tiny that an average proton in the Sun’s core waits around ten billion years before it happens. That sounds absurd, but there are so many protons in the core, something like ten to the power of fifty-six of them, that even with those odds, an enormous number succeed every second, enough to turn about six hundred million tonnes of hydrogen into helium.
And here’s the surprising consequence. Because each proton is so reluctant, the Sun’s core is not producing energy very intensely at all. Divide the Sun’s total power by the volume of its core and you get only a few tens of watts per cubic metre on average. Even at the very centre, where fusion is fastest, it’s only a few hundred, similar to the heat coming from a well-rotted compost heap, and much less than your own body produces for its size. The Sun is dazzling not because each bit of its core is ferocious but because the core is a ball with a radius of about a hundred and forty thousand kilometres. A modest effect, multiplied by an unimaginable volume.
That slowness is exactly what we should be grateful for. If the first step were easy, the Sun would burn through its fuel in a flash, or blow itself apart. Instead, it’s held in a self-correcting balance. If the core gets slightly too hot, fusion speeds up, the core expands, cools, and fusion slows again. If it cools too much, it contracts, heats up and speeds up. The weak force’s reluctance and this balance together give the Sun a steady life of about ten billion years, which is long enough for planets to form, life to evolve, and someone to complain about summer.
Now the neutrinos. They interact with matter so weakly that almost all of them pass straight out of the Sun, through space and through the Earth as if it weren’t there. Every second, about sixty-five billion solar neutrinos pass through each square centimetre of your body, by day, and also at night, coming up through the planet beneath you. They’re the only direct messengers from the Sun’s core, arriving eight minutes after they’re made, whereas the light from the core takes tens of thousands of years to escape.
In the 1960s, scientists built a huge tank of cleaning fluid in a gold mine in the United States to catch a few of them. They found only about a third of the expected number. For decades, this “solar neutrino problem” suggested something might be wrong with our understanding of the Sun. In the end, the Sun was right. Neutrinos come in three types and, it turned out, switch between them on the way to Earth, and the early detectors could only see one type. That discovery earned two Nobel Prizes, and confirmed, particle by particle, exactly how the Sun shines.