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Lazy Sun is less energetic than compost

It might be the major source of life and energy for our planet, but the sun is a lightweight in the energy stakes, says Dr Karl.

The Sun is the major source of life and energy for our planet, and it's also the source of a few mythconceptions. Here's two of them.

First, most of us know that the Sun does nuclear burning to generate its energy — and we also know that nuclear bombs get a huge amount of energy from a tiny amount of mass. So how come that, weight for weight, the Sun puts out less energy than the compost pile in your backyard?

Second, most of us know that light travels really quickly. We might not know that light travels at close to 300,000 kilometres in each second, but most of us know that it's really, really fast. So how come, once light has been generated deep inside the Sun, it crawls to the surface at about one quarter of a millimetre each second?

First, let's deal with the Sun being less energetic than a compost pile.

The Sun is a seething ball of hot gas about 150 million kilometres away. The Sun is such a mighty beast, that in 11.5 seconds, it dumps enough energy on our little planet to satisfy all our Earthly energy needs for one day. Every second, the Sun burns 620 million tonnes of hydrogen and turns it into about 616 million tonnes of helium. That leaves about four million tonnes missing. Three million of those tonnes are turned into energy (heat, light etc), obeying Einstein's famous 'E=mc2' equation. And one million tonnes are spewed out from that throbbing ball of hot gas as high-energy charged particles — such as protons and electrons.

Let me give you a handle on how much mass the Sun has 'burnt' so far. The Sun is about 4.7 billion years old — about half-way through its 'life span'. So far, it has burnt the equivalent of about 100 times the mass of the Earth. But the Sun is massive — its mass is about 330,000 times the mass of the Earth. So 100 times the mass of the Earth hardly makes a dent.

This nuclear burning of hydrogen doesn't happen everywhere in the Sun — no, about 99 per cent of it happens down in the central quarter of the Sun. This central core has a temperature around 16 million° C, and is about 10 times more dense than gold or lead. This core is roughly 25 times bigger than the Earth. Under these extreme conditions of temperature and density, a hydrogen atom collides with another hydrogen atom to make a single helium atom — and a release a huge amount of energy.

The power output of the core of the Sun is about 276.5 watts per cubic metre — that's almost three of the old 100W light bulbs. On a power/volume basis, it's a lot less than your body emits (about 100 W) and around the same as a compost pile.

How can it be so low? After all, a relatively small hydrogen bomb such as the American W88 has the explosive power of just under half-a-million tonnes of TNT — and fits into a volume smaller than a rollaway wheelie bin. So, if the Sun does nuclear burning, how come it generates less power (volume for volume) than a hydrogen bomb?

The answer is surprising. The Sun does do nuclear burning of hydrogen atoms, but only very occasionally. How occasionally? On average, any given hydrogen atom will run into another hydrogen atom only once every five billion years. The Sun's core does do nuclear burning, but very sparsely, with the reactions few and far between. Luckily, the core of the Sun is enormous, so even though it's only as powerful as a compost pile, there's enough energy given off to keep our planet warm.

So now that we know how lazy the Sun is, let's see how slow is the light that it makes for us, next time …

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