Go out into a big field with a football and plonk it down to represent the sun.
Then walk 25 metres away and drop a peppercorn to represent the Earth’s size and its distance from the sun.
The moon, to the same scale, would be a pinhead, and it would be only 5 centimetres away from the peppercorn.
But the nearest other star, Proxima Centauri, to the same scale, would be another (slightly smaller) football located about … wait for it … six and a half thousand kilometres away!
There may or may not be planets orbiting Proxima Centauri, but there certainly are planets orbiting other stars, maybe most stars. And the distance between each star and its planets is usually small compared to the distance between the stars themselves.
How stars work
The difference between a star (like the sun) and a planet (like Mars or Jupiter) is that stars are bright and hot, and we see them by their own light, whereas planets are relatively cold and we see them only by reflected light from a nearby star, which they are orbiting. And that difference, in turn, results from the difference in size. Here’s how.
The larger any object is, the stronger the gravitational pull towards its centre. Everything pulls everything by gravity. Even you and I exert a gravitational pull on each other. But the pull is too weak to notice unless at least one of the bodies concerned is large. The Earth is large, so we feel a strong pull towards it, and when we drop something it falls ‘downwards’ – that is, towards the centre of the Earth.
A star is much larger than a planet like Earth, so its gravitational pull is much stronger. The middle of a large star is under huge pressure because a gigantic gravitational force is pulling all the stuff in the star towards the centre. And the greater the pressure inside a star, the hotter it gets. When the temperature gets really high – much hotter than you or I can possibly imagine – the star starts to behave like a sort of slow-acting hydrogen bomb, giving out huge quantities of heat and light, and we see it shining brightly in the night sky. The intense heat tends to make the star swell up like a balloon, but at the same time gravity pulls it back in again. There is a balance between the outward push of the heat and the inward pull of gravity. The star acts as its own thermostat. The hotter it gets, the more it swells; and the bigger it gets, the less concentrated the mass of matter in the centre becomes, so it cools down a bit. This means it starts to shrink again, and that heats it up again, and so on. That sounds as though the star bounces in and out like a heart beating, but it isn’t like that. Instead, it settles into an intermediate size, which keeps the star at just the right temperature to stay that way.
I began by saying that the sun is just a star like many others, but actually there are lots of different kinds of stars, and they come in a great range of sizes. Our sun is not very big, as stars go. It is slightly bigger than Proxima Centauri, but much smaller than lots of other stars.
What is the largest star we know? That depends on how you measure them. The star that measures the greatest distance across is called VY Canis Majoris. From side to side (diameter), it is 2,000 times the size of the sun. And the sun’s diameter is 100 times that of the Earth. However, VY Canis Majoris is so wispy and light that, despite its huge size, its mass is only about 30 times that of the sun, instead of the billions of times it would be if its material were equally dense. Others, such as the Pistol Star, and more recently discovered stars such as Eta Carinae and R136a1 (not a very catchy name!), are 100 times as massive as the sun, or even more. And the sun is more than 300,000 times the mass of the Earth, which means that the mass of Eta Carinae is 30 million times that of the Earth.
If a giant star like R136a1 has planets, they must be very very far away from it, or they would be instantly burned to vapour. Its gravity is so huge (because of its vast mass) that its planets could indeed be a very long way away and still be held in orbit around it. If there is such a planet, and anybody lives on it, R136a1 would probably look about as big to them as our sun looks to us, because although it is much larger, it would also be much further away – just the right distance away, in fact, and just the right apparent size to sustain life, otherwise life wouldn’t be there!
The life story of a star
Actually, however, it is unlikely that there are any planets orbiting R136a1, let alone any life on them. The reason is that extremely large stars have a very short life. R136a1 is probably only about a million years old, which is less than a thousandth of the age of the sun so far: not enough time for life to evolve.
The sun is a smaller, more ‘mainstream’ star: the kind of star that has a life story lasting billions of years (not just millions), during which it proceeds through a series of drawn-out stages, rather like a child growing up, becoming an adult, passing through middle age, eventually getting old and dying. Mainstream stars mostly consist of hydrogen, the simplest of all the elements. The ‘slow-acting hydrogen bomb’ in the interior of a star converts hydrogen to helium, the second simplest element (something else named after the Greek sun god Helios), releasing a massive amount of energy in the form of heat, light and other kinds of radiation. You remember we said that the size of a star is a balance between the outward push of heat and the inward pull of gravity? Well, this balance stays roughly the same, keeping the star simmering away for several billions of years, until it starts to run out of fuel. What usually happens then is that the star collapses into itself under the unrestrained influence of gravity – at which point all hell breaks loose (if it’s possible to imagine anything more hellish than the interior of a star already is).
The life story of a star is too long for astronomers to see more than a tiny snapshot of it. Fortunately, as they scan the skies with their telescopes, astronomers can find a range of stars, each at a different stage of its development: some ‘infant’ stars caught in the act of being formed from clouds of gas and dust, as our sun was four and a half billion years ago; plenty of ‘middle-aged’ stars like our sun; and some old and dying stars, which give a foretaste of what will happen to our sun in another few billion years’ time. Astronomers have built up a rich ‘zoo’ of stars, of all different sizes and stages in their life cycles. Each member of the ‘zoo’ shows what others used to be like, or will be like.
An ordinary star like our sun eventually runs out of hydrogen and, as I’ve just described, starts ‘burning’ helium instead (I’ve put that in quotation marks because it isn’t really burning but doing something much hotter). At this stage it is called a ‘red giant’. The sun will become a red giant in about five billion years’ time, which means it is pretty much in the middle of its life cycle at the moment. Long before then, our poor little planet will have become much too hot to live on. In two billion years the sun will be 15 per cent brighter than it is now, which means that the Earth will be like Venus is today. Nobody could live on Venus: the temperature there is over 400 degrees Celsius. But two billion years is a pretty long time, and humans will almost certainly be extinct long before then, so that there will be nobody left to fry. Or maybe our technology will have advanced to the point where we can actually move the Earth out to a more comfortable orbit. Later, when the helium, too, runs out, the sun will mostly disappear in a cloud of dust and debris, leaving a tiny core called a white dwarf, which will cool and fade.
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, trappin
g 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.