What Happens Inside the Sun

If the Sun were a bonfire, it would have run out long ago: chemical burning releases far too little energy per gram - fuel the size of the Sun would burn out in thousands of years. But the Sun is about 4.6 billion years old, and it has traveled about half its path.

Something else is at work. The weight of the outer layers compresses the center until matter there is about 150 times denser than water, and the temperature is about 15 million degrees. In several steps, hydrogen nuclei combine into a helium nucleus, which is slightly lighter than the sum of the originals. The difference becomes light: about four million tons per second stop being matter.

It happens slowly. Protons have the same charge and repel each other, and even 15 million degrees isn't enough to make them collide directly: fusion happens through quantum tunneling, meaning by rare chance. On average, an individual proton waits billions of years for its reaction. That is why the Sun doesn't flare up all at once, but shines steadily.

A cubic meter of the solar core releases about 276 watts on average - roughly the same order of magnitude as a compost heap. The Sun is dazzling not because of the power at one point, but because of its size: there are so many such cubic meters inside that together they make a star.

Next, the energy has to get out. The core takes up about a quarter of the radius, above it is the radiative zone - reaching 70% of the radius - and the outer third is convective. Along the way, the temperature drops by almost a factor of three thousand.

The layers are arranged this way for a reason. As long as matter is transparent, radiation carries the heat. Closer to the surface, the gas cools and becomes opaque, so radiation cannot get through - then the gas itself moves: hot gas rises, cooled gas sinks. From outside, you can see this as a grainy pattern: each “granule” is about a thousand kilometers across and lasts about ten minutes.

A gamma ray from the core is almost immediately absorbed and emitted again - billions of times, each time in a random direction. Estimates of how long it takes energy to reach the surface vary: from thousands to hundreds of thousands of years. The remaining 150 million kilometers to Earth, light travels in 8 minutes 20 seconds.

No one has looked inside, but there are two workarounds. Neutrinos are born in the same reactions and interact with almost nothing: they leave the core in a couple of seconds and reach detectors on Earth - a direct report from the center. And the Sun also makes sounds: its surface oscillates, and the pattern of frequencies reveals density and the speed of sound at depth.