Physics and Reality

How we know the Sun is still burning

Sunlight reaches us in about eight minutes, and it is natural to think that this means we are seeing what is happening in the Sun almost immediately.

But that is not the whole story.

The sunlight leaving the surface today is not a direct messenger from the nuclear reactions taking place in the core today.

The Sun shines because deep in its interior hydrogen nuclei are being fused into helium.

This is what powers the star. In the process, energy is released and begins its long journey outward.

However, the inside of the Sun is not transparent in the way empty space is. The light produced in the deep interior does not simply fly out to the surface. It is absorbed, re-emitted, scattered, and redirected again and again.

So although light crosses the empty space from the Sun to the Earth in only eight minutes, the energy carried outward from the core takes hundreds of thousands of years to reach the surface.

The light we see now does not tell us, in any immediate sense, what the core is doing now.

And yet we do know that the Sun is still burning.

We know it because the same fusion reactions that power the Sun also produce neutrinos.

Neutrinos are extremely light particles that interact only very weakly with matter. Unlike photons, they do not get trapped inside the solar interior. Once produced in the core, they pass almost straight through the surrounding layers of the Sun and continue into space.

This is what makes them so difficult to detect, but also what makes them so valuable.

A neutrino arriving from the Sun today is a direct messenger from fusion in the solar core today, not from some much earlier stage of the Sun’s life. It reaches us in essentially the same few minutes it takes light to cross the distance from the Sun to the Earth, but unlike sunlight, it has not spent an immense time diffusing through the solar interior first.

So when neutrino detectors see neutrinos coming from the Sun, they are giving us immediate evidence that nuclear fusion is still taking place there now.

If the core of the Sun were somehow to stop fusing today, the neutrino signal would fade almost immediately, on the timescale of minutes. The sunlight, however, would not suddenly disappear eight minutes later. The energy already working its way outward through the body of the Sun would continue to feed the surface for a very long time.

This is somewhat similar to the delayed afterglow of a light bulb after we hit the switch. The bulb does not go dark at once, because it takes time to cool off. For the Sun, this would not be a mere dim afterglow. It would continue shining at nearly its usual brightness for hundreds of thousands of years.