Supernovae are amongst the most spectacular of all known physical phenomena in the known universe. Pretty much everyone who gets into physics or astronomy in some way, soon takes an interest in these amazing happenings. Essentially, a supernova is a humungous explosion. When I say humungous, I mean really big, in ways it’s hard to wrap our heads around.
Many stars, at the end of their normal life, reach a point where the fuel burning processes within the star can no longer maintain equilibrium with the other forces occurring within the star. As you can imagine, when a giant ball of incandescent gas (i.e. a star) gets to the point where it’s out of balance – something cataclysmic happens.
I’m going to talk about just one of the several types of Supernovae, as we haven’t much time. I’m going to describe for you the before, during and after of a Type II supernova – also called by a much better name: A “Core Bounce Supernova”. To understand why they happen, we need to understand what’s going on in the star before the supernova, and to understand that, we’ll need a little physics – but DON’T PANIC (or indeed switch off)! It’s really quite straight forward.
So let’s start with a star that is at least 8 times the mass of our sun – they’re not hard to find out there, as our sun is quite a modestly sized star really. Now, the reason why stars shine is that they are hot – simple as that. Anything that is a few thousand degrees Kelvin gives off photons that we can see. And the reason why stars are hot is because they are gigantic nuclear reactors. So why do stars burn in this way? When a star is born, it is formed out of a huge, spread out cloud of Hydrogen gas (and maybe a few other elements that are hanging around, but basically Hydrogen). Sometimes these clouds of gas are perturbed by some nearby event (think of wisps of wood smoke rising from a camp fire disturbed by drafts, then scale that thought up billions of times). This disturbance can result in gravity getting a hold, and the giant gas cloud starts to collapse in on itself, shrinking in size until all those Hydrogen atoms start coming into much closer contact with each other. And when that happens, they heat up – the more atoms you have in a certain space, the more those atoms collide with each other and interact. Eventually the pressure and density in the (now spherical) gas cloud reaches a point where the Hydrogen atoms are coming into contact with each other so often, and with so much force, that they actually fuse together – a fusion reaction has started.
Atomic physics is a strange creature indeed if you are not used to it (and when you do get used to it it seems even stranger!). Without going into lots of detail, when two Hydrogen atoms fuse together, they create a Helium atom. But, because one Helium atom has less mass than the two Hydrogen atoms, this missing mass is released as energy. Energy is mass and mass is energy, so said the world’s most famous Swiss Patent Clerk. So every time we fuse two Hydrogen atoms to make one Helium atom, we also liberate energy which is given off as photons.
So, to recap, our young star is burning Hydrogen, giving out large amounts of energy as photons and leaving behind ashes – the Helium atoms. The energy liberated by nuclear fusion heats up the core of the young star and this produces an outward pressure in the core of the star which holds up against the inward force of gravity, so our young star is in a delicate balance, keeping its size and spheroidal shape precisely because of these balancing forces. So, what next? Well, after a long while, when most of the Hydrogen has been burnt into Helium ashes, the star wants to contract because there is less Hydrogen burning going on to keep the pressure up in the core. So the star squeezes down a bit – and guess what, this causes the internal temperature to rise to the point where the Helium atoms, left over from the first stage of burning, can themselves fuse together to give Carbon & Oxygen atoms and, you guessed it, lots of energy output which, yep you guessed it again, serves to hold the star up against it’s desire to collapse in on itself due to gravity, and we’re back in the balancing business. This sort of burning and rebalancing process goes on several more times, with each cycle burning through its fuel faster than the previous cycle, producing ever heavier atoms as the ashes – Neon, Sodium, Magnesium, Aluminium, Silicon, Sulphur, Argon, Calcium, Nickel and Iron. Physicists call this balancing act hydrostatic equilibrium, but don’t be put off by the jargon – it’s just a balancing act between the pressure in the core wanting to bloat the star outwards, and the gravitational attraction of all the stuff in the star which wants to collapse it.
OK, back to our dying star. So, when the star has contracted enough, after a few billion years of burning through the ashes of the previous fusion cycles, the star’s core pressure is immense and the core temperature reaches about 3 billion degrees or so, at which point it can start burning Silicon. This is a very fast process, lasting only about 1 day, and it produces Nickel and Iron. So, if you’re guessing that this process just goes on and on, producing ever heavier elements with each new stage of burning then that would be a good guess, but for one thing: The Iron and Nickel atoms cannot fuse together to produce even heavier elements and liberate energy. Fusion has stopped. The internal furnace of the star has been switched off.
Now, something really wild is about to happen. Remember from earlier that the only thing holding the star from gravitationally collapsing in on itself was the pressure of the internal furnace at the core? Well, the furnace has just been turned off so the star starts to collapse. Eventually the the temperature in the core reaches about 10 billion Kelvin, at which point neutrinos start streaming out of the star, carrying away energy really quickly. One of the defining characteristics of neutrinos is that they carry energy but cannot be physically stopped by almost anything, so not only has the star’s internal furnace been switched off, but also the star starts radiating away its internal energy. This accelerates the collapse of the star even further, and it contracts from the size of the earth to the size of a city in about one second. Now we have a star that is about 1.5 times more massive than the sun but is only the size of a city. That’s a phenomenal density. It’s so dense, in fact, that it cannot collapse any further because it’s already a tightly packed sphere of neutrons and protons. The collapse stops. Stone dead. Don’t forget the neutrinos, though, they’re still being generated in the core, but now the core is so dense that even neutrinos can’t get through – the temperature therefore rises in the outer part of the core and nuclear fusion kicks off again in a very big way – such a big way, in fact that no amount of gravity can contain it and the outer envelope of the star explodes away at 10,000 km per second. So the core is left behind as a hyper-dense object (a black hole or a neutron star), and the outer layers detonate, giving off huge amounts of energy, much of it in the visible spectrum and thus the supernova shines many thousands of times brighter than the star.
It’s worth a pause at this point to reflect on this star that is a giant factory for producing elements. If you are listening to this podcast on an aeroplane, then many parts of your plane will be made of Magnesium and Aluminium alloys. The actual atoms around you and of you were made in the nuclear furnaces of stars aeons ago. I think that fact alone is amazing. To mangle a famous quote from Dr Johnson - “If you are tired of knowing you came from stardust, you’re tired of life!”.
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