On the Shoulders of Giants…

… You can see forever!

Let’s talk about fusion, not metaphorically, but as a detailed look at exactly what made us and how it happened!

Look! A new Star!

On the 24th of August, 2011, the team at the Palomar Transient Factory discovered a brand-new supernova : PTF11kly, in the M101 galaxy, around 22 million light-years from Earth.

Here’s an animated GIF image, created by Thunderf00t, who photographed the before and after with his own equipment. You can see a beautiful rendition of what he saw and what it means on YouTube.

It’s now been confirmed that before the supernova (SN), at the same point in space there was a very, very difficult to see binary system – a white dwarf (much like what our sun – Sol – will turn into in a few billion years) orbiting a darker, colder companion star.

It seems that the white dwarf was ‘sucking’ ionised gases from its companion star, and probably has done for a few million years. All this extra ‘fuel’ finally allowed the white dwarf to begin converting the new fuel into carbon isotopes, before finally collapsing and expanding in a cataclysmic event.

How cool is that?

What’s a supernova?

OK, let’s go back to basics.

Most main sequence stars (like Sol) fuse (or ‘burn’ as it’s inaccurately known) hydrogen to form helium.

The fusion happens because all those trillions of tonnes of hydrogen gas gets squeezed together under the weight of it’s own gravity, until hydrogen nuclei (one proton plus one electron) get squished together so hard that they form helium – 2 protons and 2 electrons. This releases more energy than it takes to do the fusing – and the star starts to shine.

If you’d like details, see the end of this post, where I show the actual reactions taking place. For those of us more visually inclined, here’s a diagram of the first stage of the fusion process :

It can happen in a few seconds, literally – before there was a huge gob of hydrogen in a massive ball, getting smaller and smaller and smaller (this can take billions of years). Then all of a sudden, two nuclei are fused, which releases energy so that another two nuclei are fused, then another  pair, and another and another and another and voilá : you gotta sun!

Now, all that extra energy can’t just sit there, it has to go somewhere, so it eventually finds its way out to the surface of the newly-formed star – as light. And solar particles, and neutrinos, and a bunch of other weird and wonderful little strange things. But mainly energy, which emerges and is seen by us as light.

This light exerts an outwards pressure on all the nuclei between the fusion in the centre and the outside – which is why the big ball of gas doesn’t just explode instantaneously, fusing all the hydrogen in one vast destructive bang. So the fusion pressure balances the gravitational squeezing pressure just so – and this can last from a few million years (for really big motherfucker stars, like Betelgeuse started out as) to a few billion years (like Sol).

Stellar Structures, from the French CNES site

The Bigger They Are…

Most “typical” stars fuse hydrogen into helium for a few billion years, until the hydrogen is all used up. Then, since the energy output drops, the star begins to compress again under its own gravity, until the pressure and temperature rise enough to initiate helium fusion. This is stage two.

The helium fusion begins to create some heavier elements (Lithium, Beryllium, Boron, Carbon, Nitrogen, and our favourite, Oxygen).

But you can’t fuse these metals to make energy… can you?

Well, it turns out that you can. But it takes a LOT more effort than just fusing hydrogen!

Little Bang

For stars like Sol, hydrogen fusion is pretty much the only thing it can do really well. It’s just not big enough or badass enough to fuse metals, at least not in a useful way. So eventually, the Sun will start to bloat – expand – enlarge – giganticise, until the force of the internal reactions (by this stage, all the hydrogen’s used up and so is most of the helium)  suddenly stops dead – and the huge, bloated outer perimeter of the Sun (which will end up somewhere between us and Mars) will suddenly realise there’s no more pressure forcing them outwards, and will collapse suddenly (taking maybe a few minutes tops!), and rebound outwards in a massive explosion, called a nova.

NGC 7027 in IR by the HST imaging team

After the nova, the guts of the Sun will be a tiny, white dwarf, hundreds of times smaller than it is now, burning helium to sustain itself, until the helium runs out and the star finally dies down to a banked ember of what it formerly was – a brown dwarf. Then, a few more billion years later, the light will go out forever.

Ohhhhhh. Isn’t that sad – the death of a star? What an opera could be made of this!

But for bigger, badder stars – stars more than about 30 times the mass of the Sun – there’s a much, much more spectacular end in store.

Big Badda Boom!

So you’re a massive star, let’s say. You were originally composed of hydrogen, helium, and the metals created by smaller stars; but you’re so big, and so bad, that even when you run out of hydrogen, and then helium,  even the ashes you’ve created – the lithium, beryllium, boron, and so on – will start to fall towards the centre, compressing and heating up as they go.

In fact, the gravity is now so strong (remember, we’re now talking about quite large nuclei, much bigger than hydrogen and helium, squishing into roughly the same amount of space as before) that these new metallic elements begin to fuse together, again releasing energy!

It would be like running out of petrol in your car, and being able to fill the tank with sand and still run the engine – although the exhaust would be much worse than carbon monoxide and water vapour – it would be metal particles and more.

So the process continues, fusing heavier and heavier elements, running out of the lighter elements, expanding then recompressing under gravity, until finally, the temperatures and pressures are high enough to create iron. And this is the end of the road for you, because after iron, creating heavier elements requires monumentally more energy (instead of releasing energy, as fusion has done up until now).

Now you’ve got the ingredients for some of the biggest explosions in the modern universe : a SUPERnova!

A supernova is to a nova, what an atomic bomb is to a firecracker. We’re talking major destruction here. But even in the destruction of these massive stars, something fantastic happens…

Not only are all the remnants of the elements created as fusion’s by-product scattered out into space (where they can form clouds and eventually planets!), but the force of the explosion itself creates heavier elements – gold, lead, tin, copper, all the stuff we take for granted.

In fact, you can even see the process happening – right in the middle of the Eagle nebula. You may have seen these images before, but hopefully now they make more sense…

So that’s how all the elements you see around you – rocks, trees, bumblebees, your moronic brother-in-law, the oceans’ water, your wedding ring, your teeth – are created from the simplest thing of all – hydrogen atoms.

I use the word cool a lot, but to me, that’s the coolest thing of all.

And there’s no need to talk about Gods, or invisible creators, or intelligent design, or any of that bronze-age mumbo jumbo. It just happened this way, and we’ve been smart enough to figure it out without outside help. Now THAT’s cool!

I hope you enjoyed this little explanation.

The Equations :

The equations aren’t really that frightening, and we’ll break them down step-by-step. If you remember any chemistry at all from high school, it will be a piece of cake!

Stage One : Hydrogen to Helium

First, two hydrogen nuclei (which are really just a proton with an electron) fuse together to form deuterium (H2), with a positron (e+) and an electron neutrino (ν) released :

H1 + H1→H2 + e+ + ν

So far, so good – we have deuterium (which is still hydrogen!) plus some “bits” ejected. There’s not a lot of energy made, as the positron hasn’t reacted with anything yet, and the neutrino doesn’t really react with anything at all!

Now, the deuterium nucleus that was just formed fuses with a single hydrogen nucleus to form Helium3, and emits a gamma ray (γ). Now, a gamma ray is (as you may have heard) a pretty nasty little doohickey. It’s basically a proton with a LOT of energy – enough to pass through lead! So there’s the first bit of real energy available.

H2 + H1→He3 + γ

Finally, as the last part of this stage of fusion, two He3 nuclei fuse to create a helium nucleus, plus two hydrogen nuclei :

He3 + He3→He4 + 2 H1

The hydrogen nuclei then become available for further reactions, sustaining the fusion process and emitting gamma rays, positrons, and electron neutrinos like there was no tomorrow.

But that’s not the end of things. Not by a long shot!

Stage Two : Helium to Lithium and Beryllium

Stage Three : Li and Be to Boron

Phase Two : Making Us

Now the star is creating an enormous amount of energy and heavier nuclei. This allows the creation of even more useful things, like Carbon, Oxygen, and Nitrogen

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