Stars
Posted January 30, 2001

I hope you've noticed the sun before, at one time or another. It's by far the brightest thing that anyone has ever seen, and it's also the largest and heaviest thing in our direct experience. The sun is on fire, or maybe I should say the sun is a fire, and the light and heat from that fire provide all of the energy needed to create and sustain life on this planet, with lots to spare for other planets, just in case. This is great, as long as the fire keeps burning; but what happens when the Sun runs out of fuel and stops burning? And what is the fuel that burns in the Sun? Where did it come from, how long will it last, and where will it go when the Sun burns out? And is "burn" really the right word?

These are the things that I wish to talk about later on in the show, but for now, I'll just say that "burn" is probably not the best word for what happens in the Sun. Normally, burning refers to oxidization, which is an oxygen atom attaching itself to another molecule and releasing some energy in the process. In terms of chemistry, burning and rusting are the same thing, except that they happen at different speeds. And the sun doesn't burn, or add oxygen, to anything; it creates oxygen. In fact, the fact that stars generate light and heat wouldn't be very useful to us if they didn't also create new elements, like oxygen and carbon. We couldn't very well have carbon-based life forms like plants and animals and amoebas, if we didn't have any carbon.

Another thing I should mention before we listen to some music is that the Sun is about 5 billion years old, and it's only used up about half of its fuel, so it is probably not going to burn out any time soon. But if you stick around, I'll give you a rough idea of what's going to happen 5 billion years from now when it does burn out.

You only need one thing to make a star, and that one thing is a lot of hydrogen. You might toss in a few other elements as well, if they're available, but all you really need is hydrogen. If you put enough hydrogen together in one big ball, or more to the point, if enough hydrogen happens to come together in one big ball, then it will form a sort of vapour planet, where the gravity of the ball is enough to keep any of the hydrogen from floating away. Keep adding hydrogen, and the vapour planet gets more and more dense, until the pressure on the hydrogen atoms, or protons, becomes greater than the electrical force that keeps them apart, and they start to get crushed together to form helium.

When you smush four protons together this way, you end up with one helium atom, which is to say two protons and two neutrons, all together in one nucleus. If you're looking at your periodic table, you'll notice that the atomic weight of a helium atom is less than four times the atomic weight of a hydrogen atom. If you've heard of the principle of conservation of mass, you might be wondering, "Where does the extra mass go?" Let's use the letter "m" to represent the difference between four hydrogens and one helium. And let's use the letter "c" to represent the speed of light, 300 million metres per second. And let's say "E = m c ²." A very small difference in mass, multiplied by 100 million billion, equals a fairly large amount of E. Which, of course, is energy.

The huge amount of energy created during this nuclear fusion reaction is what keeps the star from becoming more and more dense and eventually collapsing. There's gravity pushing all the hydrogen atoms together, and this eventually causes nuclear reactions, which push them apart. This goes on for about 10 billion years, until all of the hydrogen has been converted into helium.

But what happens to all those helium atoms? You've got a whole lot of helium atoms, which are four times as heavy as hydrogen atoms, and they all sink to the centre of the star and get smushed together by their own collective gravity. At this point the helium starts undergoing nuclear fusion, and this releases enough energy to push the outer layers outwards. When this happens to the Sun, it will grow enough that the planet Mercury will be inside it. At this point we have a red giant. The hydrogen atoms fuse together to form carbon and oxygen, which in turn fuse together to form neon, silicon, iron, and nickel. These all end up arranged in layers, like the star is a big onion. All the iron ends up in the centre, and when all the rest of the elements have fused into iron, the fusion process stops working. Fusing iron into heavier elements consumes energy rather than producing it. So the nuclear reactions stop, and the star becomes a ball of iron. It's got almost the same mass as it did 10 billion years ago, but now it's shrunken to about the size of the Earth.

This is much the same as the situation at the very beginning, where the hydrogen was under enough pressure to start nuclear reactions. The difference is that now it's many times smaller, and it's made of iron. Nuclear fusion reactions with iron consume energy rather than giving off energy, so this marks the end of the useful life of the star, which is called its main sequence. One of three things can happen to the core that's left behind, which is called a white dwarf, and what happens depends on how big the star is.

There are three kinds of stars: light stars, like the Sun; heavy stars, about 5 times as big as the Sun; and very heavy stars, about 40 times as big as the Sun. Light stars like the Sun just stick around as a red-hot core, called a red dwarf.

Heavy stars are a little more dramatic: the iron core collapses inward now that the nuclear reactions aren't causing any outward pressure, and the collapse can't be stopped by anything short of the strong nuclear force, and the whole thing turns into one gigantic atom called a neutron star. At this point it's so dense that one teaspoon of it would weigh thousands of tons. And the collapse happens so quickly that the rest of the star comes crashing down after it at a tenth the speed of light, and then stops.

The outer star rebounds from this collision, and in a few hours the resulting shock wave reaches the surface of the star and tears off its outer layers to expose the extremely hot interior. This is billions of times brighter than the star's normal brightness, and while it's in this condition, the star emits more light than all the rest of the stars in its galaxy combined. When the Chinese saw this happen in the 11th and 12th centuries, they called it a "guest star," but to avoid confusing them with Davey J's radio show, in the West we call it a supernova.

A supernova is the only place where elements heavier than iron can be created. Judging by the number of elements we have here on Earth, we're standing on the remains of a star that went supernova several billion years ago.

And if you think supernova sounds exciting, wait till you hear what happens to the very heavy stars.

Some stars are about 40 times as heavy as the Sun. That's enough mass to crush the neutron star beyond the point where a smaller one would form a red dwarf or a supernova. Even exploding doesn't help, because the gravitational field is strong enough to pull the debris from the explosion right back into the core. As it gets smaller and smaller, the gravitational field gets stronger and stronger, until all of the mass is concentrated at one point.

At this point, nothing can help the iron escape. Even photons, which is to say light, can't escape, because even though photons are not very heavy and they travel faster than anything else, they still get sucked back into that very heavy point that used to be a neutron star.

This point, which is so heavy that not even light can escape, is called a black hole. It's hard to learn much about a black hole, because everything that has direct experience with one is still inside it. All of the mass in a black hole is concentrated at one point, but it's surrounded by what you might call a sphere of influence, the border of which is called an "event horizon". Anything that crosses the event horizon has absolutely no chance of doing anything but falling into the black hole forever.

So there are three main things to know about dying stars. First, the Sun will most likely grow bigger and hotter until it uses up all of its fusible elements and slowly degenerates into a lump of iron. Of course, this won't happen for another 5 billion years, so we will probably find a new Sun by then, or at least invent a replacement. The second thing is that when you find gold, you're looking at atoms that were created inside a star when it went supernova billions of years ago. And the third thing is that you should always stay away from black holes.

Geoff, if you're listening, let me know whether I've answered your question. Everyone else, if you're listening, why not phone me in the studio and make a request? The number is 352-3706, and if you don't want to call during the show, you can call the office any time at 352-9600.