The sunlight warming your hand right now began as a few protons fumbling together at the center of a star — and it left the core a very long time ago.

A furnace that shouldn't light

At the Sun's heart the temperature is about 15 million kelvin and the pressure is something like 250 billion times the air around you. Hydrogen there isn't a gas in any familiar sense; it's a sea of bare protons moving at hundreds of kilometres a second. Hydrogen is what the Sun mostly is — close to three-quarters of its mass — so the fuel supply is, in effect, the star itself.

You'd think that's plenty of speed to fuse them. It isn't. Two protons carry the same positive charge, and like charges repel harder the closer they get. Even at 15 million degrees, a given pair almost always slows, stops, and bounces apart well before they touch. Classically, the Sun cannot burn.

It burns anyway, because particles are also waves, and a wave can leak across a barrier it doesn't have the energy to climb. That loophole — slipping through the wall — is wildly unlikely for any single pair, but with enough protons trying enough times each second, unlikely is plenty.

Four become one

The reaction that powers the Sun is the proton–proton chain. Stripped down:

1. Two protons tunnel close. Almost always they part again — but once in a great while, one proton flips into a neutron, spitting out a positron and a barely-there neutrino, and the two stick together as deuterium. This step is so improbable that an average proton waits billions of years for its turn.
2. The deuterium quickly grabs another proton, becoming helium-3.
3. Two helium-3 nuclei meet and fuse into helium-4, kicking two protons back into the crowd.

Net result: four hydrogen nuclei become one helium nucleus, plus light. Helium, and eventually a little carbon, is about as far as our modest Sun will climb; the heavier atoms in your body were forged in bigger stars and their deaths.

The missing 0.7 percent

Here's the trick. Weigh four protons, then weigh the helium-4 nucleus they end up as. The helium is about 0.7% lighter. That sliver of mass isn't lost — it leaves as energy, the exact swap that mass and energy are the same coin describes. With a conversion rate of c² (an enormous number), a tiny mass buys a staggering amount of light. The Sun cashes in roughly four million tonnes of itself every second, and has been doing so, steadily, for 4.6 billion years.

The long climb out

A photon born in the core is a violent gamma ray, and it does not fly straight out. The interior is so dense that the photon is absorbed and re-emitted again and again, staggering outward on a random walk. Estimates for the journey to the surface run from about ten thousand to over a hundred thousand years. Along the way the energy is shared and softened, until what finally escapes — from a surface near 5,800 K — is mostly visible light. That surface temperature sets the Sun's hue, the same glowing-coal physics that lets you read a star's heat from its color.

The neutrinos take the opposite path. Indifferent to matter, they pour straight out of the core and reach Earth in about eight minutes.

The light on your skin is tens of thousands of years old. The neutrinos sleeting through it left the Sun while you read this sentence.