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Authors: Richard Dawkins

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Supernovas and stardust

The story ends differently for stars that are much bigger and hotter than our sun, like the giant stars we were just talking about. These monsters ‘burn’ through their hydrogen much faster, and their ‘hydrogen bomb’ nuclear furnaces go further than just banging hydrogen nuclei together to make helium nuclei. The hotter furnaces of larger stars go on to bang helium nuclei together to make even heavier elements, and so on until they have produced a wide range of heavier atoms. These heavier elements include carbon, oxygen, nitrogen and iron (but so far nothing heavier than iron): elements that are abundant on Earth, and in all of us. After a relatively short time, a very large star like this eventually destroys itself in a gigantic explosion called a supernova, and it is in these explosions that elements heavier than iron are formed.

What if Eta Carinae were to explode as a supernova tomorrow? That would be the mother of all explosions. But don’t worry: we wouldn’t know about it for another 8,000 years, which is how long it takes light to travel the vast distance between Eta Carinae and us (and nothing travels faster than light). What, then, if Eta Carinae exploded 8,000 years ago? Well, in that case the light and other radiation from
the
explosion really could reach us any day now. The moment we see it, we’ll know that Eta Carinae blew up 8,000 years ago. Only about 20 supernovas have been seen in recorded history. The great German scientist Johannes Kepler saw one on 9 October 1604: the debris has expanded since he first saw it. The explosion itself actually occurred some 20,000 years earlier, roughly the time the Neanderthal people went extinct.

Supernovas, unlike ordinary stars, can create elements even heavier than iron: lead, for example, and uranium. The titanic explosion of a supernova scatters all the elements that the star, and then the supernova, have made, including the elements necessary for life, far and wide through space. Eventually the clouds of dust, rich in heavy elements, will start the cycle again, condensing to make new stars and planets. That is where the matter in our planet came from, and that is why our planet contains the elements that are needed to make us, the carbon, nitrogen, oxygen and so on: they come from the dust that remained after a long-gone supernova lit up the cosmos. That is the origin of the poetic phrase ‘We are stardust’. It is literally true. Without occasional (but very rare) supernova explosions, the elements necessary for life would not exist.

Going round and around

It is a fact we cannot ignore that the Earth and all the sun’s other planets orbit their star in the same ‘plane’. What does that mean? Theoretically, you might think that the orbit of one planet could be tilted at any angle to any other. But that is not the way things are. It is as though there is an invisible
flat
disc in the sky, with the sun at the centre, and all the planets moving on that disc, just at different distances from the centre. What’s more, the planets all go round the sun in the same direction.

Why? It is probably because of how they began. Let’s take the direction of spin first. The whole solar system, which means the sun and the planets, began as a slowly spinning cloud of gas and dust, probably the leftovers of a supernova explosion. Like almost every other free-floating object in the universe, the cloud was spinning on its own axis. And yes, you’ve guessed it: the direction of its spin was the same as the direction of the planets now orbiting the sun.

Now, why are all the planets ‘on the level’ on that flat ‘disc’? For complicated gravitational reasons that I won’t go into, but which scientists understand well, a big spinning cloud of gas and dust out in space tends to form itself into a revolving disc, with a massive lump in the middle. And that is what seems to have happened with our solar system. Dust and gas and small chunks of matter don’t stay as gas and dust. Gravitational attraction pulls them towards their neighbours, in the way I described earlier in this chapter. They join forces with those neighbours and form larger lumps of matter. The larger a lump, the greater its gravitational pulling power. So, what happened in our spinning disc was that the larger lumps became even larger, as they sucked in their smaller neighbours.

By far the largest lump became the sun in the centre. Other lumps, large enough to attract smaller lumps to them and far enough from the sun not to be sucked into it, became
the
planets. Reading from nearest the sun outwards, we now call them Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Old lists would put Pluto after Neptune, but nowadays it is regarded as too small to count as a planet.

Asteroids and shooting stars

Under different circumstances another planet could have formed too, between the orbits of Mars and Jupiter. But the small bits that could otherwise have joined together to make this extra planet were prevented from doing so, probably by the brooding gravitational presence of Jupiter, and they have remained as an orbiting ring of debris called the asteroid belt. These asteroids swarm in a ring between the orbits of Mars and Jupiter, which is where the extra planet would have been if they had managed to get together. The famous rings around the planet Saturn are there for a similar reason. They could have condensed together to make another moon (Saturn already has 62 moons, so this would have been the 63rd), but they actually stayed separate as a ring of rocks and dust. In the asteroid belt – the sun’s equivalent of Saturn’s rings – some of the bits of debris are large enough to be called planetesimals (sort of ‘not quite planets’). The largest of them, called Ceres, is nearly 1,000 kilometres across, large enough to be roughly spherical like a planet, but most of them are just misshapen rocks and bits of dust. They collide with each other from time to time, like billiard balls, and sometimes one of them gets kicked out of the asteroid belt and may even come close to another planet such as Earth. We see them, quite commonly,
burning
in the upper atmosphere as ‘shooting stars’ or ‘meteors’.

Less commonly, a meteor may be large enough to survive the ordeal of passing through the atmosphere and actually make a crash landing. On 9 October 1992, a meteor broke up in the atmosphere and a fragment about the size of a large brick hit a car in Peekskill, New York State. A much larger meteor, the size of a house, exploded above Siberia on 30 June 1908, setting fire to large areas of forest.

Scientists now have evidence that an even larger meteor hit Yucatán, in what is now Central America, 65 million years ago, causing a global disaster, which is probably what killed off the dinosaurs. It has been calculated that the energy released by this catastrophic collision was hundreds of times greater than would be released if all the nuclear weapons in the world were simultaneously exploded in Yucatán. There would have been shattering earthquakes, epic tsunamis and worldwide forest fires, and a dense cloud of dust and smoke would have darkened the Earth’s surface for years.

This would have starved the plants, which need sunlight, and starved the animals, which need plants. The wonder is not that the dinosaurs died but that our mammal ancestors survived. Perhaps a tiny population survived by hibernating underground.

Light of our lives

I want to end this chapter by talking about the importance of the sun for life. We don’t know whether there is life elsewhere in the universe (I’ll discuss that question in a later chapter), but we do know that, if there is life out there, it is almost
certainly
near a star. We can also say that, if it is anything like our kind of life, at least, it will probably be on a planet about the same apparent distance from its star as we are from our sun. By ‘apparent distance’ I mean distance as perceived by the life form itself. The absolute distance could be very much greater, as we saw in the example of the super-giant star R136a1. But if the apparent distance were the same, their sun would look about the same size to them as ours does to us, which would mean that the amount of heat and light received from it would be about the same.

Why does life have to be close to a star? Because all life needs energy, and the obvious source of energy is starlight. On Earth, plants gather sunlight and make its energy available to all other living creatures. Plants could be said to feed off sunlight. They need other things too, such as carbon dioxide from the air, and water and minerals from the ground. But they get their energy from sunlight, and they use it to make sugars, which are a kind of fuel that drives everything else that they need to do.

You can’t make sugar without energy. And once you have sugar, you can then ‘burn’ it to get the energy back out again – though you never get all of the energy back; there is always some lost in the process. And when we say ‘burn’, that doesn’t mean it goes up in smoke. Literally burning it is only one way to release the energy in a fuel. There are more controlled ways to let the energy trickle out, slowly and usefully.

You can think of a green leaf as a low, spread-out factory whose entire flat roof is one great solar panel, trapping sunlight and using it to drive the wheels of the assembly lines
under
the roof. That is why leaves are thin and flat – to give them a large surface area for sunlight to fall on. The end products of the factory are sugars of various kinds. These are then piped through the veins in the leaf to the rest of the plant, where they are used to make other things, like starch, which is a more convenient way to store energy than sugar. Eventually, the energy is released from the starch or sugar to make all the other parts of the plant.

When plants are eaten by herbivores (which means just that: ‘plant-eaters’), such as antelopes or rabbits, the energy is passed to the herbivores – and again, some of it is lost in the process. The herbivores use it to build up their bodies and fuel their muscles as they go about their business. Their business includes, of course, grazing or browsing on lots more plants. The energy that powers the muscles of the herbivores as they walk and munch and fight and mate comes ultimately from the sun, via plants.

Then other animals – meat-eaters or ‘carnivores’ – come along and eat the herbivores. The energy is passed on yet again (and yet again some of it is lost in the transition), and it powers the muscles of the carnivores as they go about their business. In this case, their business includes hunting down yet more herbivores to eat, as well as all the other things they do, like mating and fighting and climbing trees and, in the case of mammals, making milk for their babies. Still, it is the sun that ultimately provides the energy, even though by now that energy has reached them by a very indirect route. And at every stage of that indirect route, a good fraction of the energy is lost – lost as heat, which contributes to the useless
task
of heating up the rest of the universe.

Other animals, parasites, feed on the living bodies of both herbivores and carnivores. Once again, the energy that powers the parasites comes ultimately from the sun, and once again not all of it is used because some of it is wasted as heat.

Finally, when anything dies, whether plant or herbivore or carnivore or parasite, it may be eaten by scavengers like burying beetles, or it may decay – eaten by bacteria and fungi, which are just a different kind of scavenger. Yet again, the energy from the sun is handed on, and yet again some of it leaks away as heat. That’s why compost heaps are hot. All the heat in a compost heap comes ultimately from the sun, trapped by leafy solar panels the year before. There are fascinating Australasian birds called megapodes that use the heat of a compost heap to incubate their eggs. Unlike other birds, which sit on their eggs and warm them with their body heat, megapodes build a big compost heap in which they lay their eggs. They regulate the temperature of the heap by piling more compost on the top to make it hotter, or removing compost to make it cooler. But all birds ultimately use solar energy to incubate their eggs, whether through their body heat or through a compost heap.

Sometimes plants are not eaten but sink into peat bogs. Over centuries, they become compressed into layers of peat by new layers added above them. People in western Ireland or the Scottish isles dig up the peat and cut it into brick-sized chunks, which they burn as fuel, to keep their houses warm in winter. Once again, it is trapped sunlight – in this case trapped centuries earlier – whose energy is being released in
the
fires and cooking ranges of Galway and the Hebrides.

Under the right conditions, and over millions of years, peat can become compacted and transformed, so that it eventually becomes coal. Weight for weight, coal is a more efficient fuel than peat and burns at a much higher temperature, and it was coal fires and furnaces that powered the industrial revolution of the eighteenth and nineteenth centuries.

The intense heat of a steel mill or a blast furnace, the glowing fireboxes that sent the Victorians’ steam engines thundering along iron rails or their ships pounding through the sea: all that heat came originally from the sun, via the green leaves of plants that lived 300 million years ago.

Some of the ‘dark Satanic mills’ of the industrial revolution were driven by steam power, but many of the earlier cotton mills were powered by water wheels. The mill was built near a fast-running river, which was ducted to flow over a wheel. This water wheel turned a great axle or drive shaft, which ran the length of the factory. All along the drive shaft, belts and cogwheels drove the various spinning machines and carding machines and looms. Even those machines were ultimately driven by the sun. Here’s how.

The water wheels were driven by water, being pulled downhill by gravity. But that works only because there is a continuous supply of water up on the high ground, from where it can run downhill. That water is supplied in the form of rain, from clouds, falling on the hills and mountains. And the clouds get their water through the evaporation of seas, lakes, rivers and puddles on Earth. Evaporation requires
energy
, and that energy comes from the sun. So ultimately the energy that drove the water wheels that turned the belts and cogwheels of the spinning machines and looms all came from the sun.

BOOK: The Magic of Reality
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