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Sun makes slow light

The sun puts on a big flashy light show on the surface, but deep down the pace is much lazier, says Dr Karl.

Last time, I talked about how the Sun, the source of practically all life on our planet, was pretty slack with its nuclear burning. It did so little of it, that (volume for volume) it put out about as much energy as a backyard compost pile, not a hydrogen bomb.

But what's this about the Sun making slow light? Let me put it in a different way. A photon of light takes only eight minutes to get to the Earth from the surface of the Sun. But it can take 100,000 years from the core of the Sun to get to the surface — where it bursts out and flies at the speed of light. What's going on?

Well, the density of the core is incredibly high — 150 times greater than water. So all the atoms are jammed up against each other.

Let's look at a pair of hydrogen atoms that have fused together to make a helium atom — and released a lot of energy, in the form of gamma rays. The gamma rays can travel only a few millimetres, before they're absorbed by an atom, and then re-radiated. Over and over again, they are absorbed, and then re-radiated. So, very slowly, the gamma-rays that have been generated by nuclear burning work their way up from the dense core.

After thousands upon thousands of years, they make their way into what's called the 'radiative zone'. The radiative zone is huge — it stretches up from the outer core (that's at the 24 per cent mark) to about 70 per cent of the way to the surface. It's called the radiative zone, because here, energy travels by radiation. The gamma rays are still being absorbed and re-radiated, but each time they're being re-radiated at longer wavelengths. They gradually get converted from gamma rays down to visible light. The temperature in the radiative zone is around 7 million°C near the bottom, and about 2 million°C near the top.

Above the radiative zone, in the top 30 per cent of the Sun's radius, is the convective zone. It's called the 'convective zone', because energy is convected, or carried by moving matter, just like rising water in the pot on the stove carries heat energy.

Let's look at a photon, many thousands of years after it was first made at the core of the Sun, as it enters the convective zone. The gamma ray photon heats the 'gas' in the convective zone. The gas is very hot, but not quite hot enough to re-radiate the energy. Instead the hot gas rises, and it carries the energy with it. It carries energy (as I said earlier) in the same way that water in a pot on the stove carries heat — with the hot stuff rising and coming to the surface, spreading out to the side, cooling, and then sinking again.

With the right telescope, you see 'boiling' patterns on the Sun's surface.

Deeper below the surface, there are giant 'cells' called 'supergranules'. They carry this hot gas towards the surface, then sideways, and then bring it down again. These supergranules are around 30,000 kilometres across (about 2.5 times bigger than the Earth), about 10,000 kilometres deep, and last for about two days. The hot gas moves up, across, and down fairly "slowly" — only about half a kilometre per second.

But just under the surface, the supergranules break down into little baby granules. Once again, in these granules, the gas rises, spreads sideways and then sinks like water in your bubbling pot — but at around 2 kilometres per second. Granules are about 1,000 kilometres in diameter, and there are millions and millions of them covering the surface of the Sun. They're around 300 kilometres deep. These granules live for about an average of eight to 20 minutes. The granules are hotter in the centre, and cooler at the edges, but the average temperature at the surface is around 5,500°C.

The photons of energy have finally, after 100,000 years, come to the end of their journey inside the Sun. They have now reached a zone which is transparent to light. The photons escape into space, and travel at the classic speed of light — around 300,000 kilometres per second. The very surface of the Sun is called the 'photosphere', because that's where the photons have escaped from.

So while in eight minutes, a photon of light can travel the 150 million kilometres from the Sun to the Earth, in that same time, a gamma ray in the Sun's core will travel only about 13 centimetres. That's a pretty slow rate of delivery.

For all its flashy show on the surface, deep down the Sun would seem to be kind of lazy…

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