Hot and Heavy — The story of Stellar Formation

This was originally the script for the first episodes of my podcast, Physical Attraction. You can find it online on iTunes or via; and tweet us @physicspod

Intro / Hot and Heavy: Stellar Formation

Artist’s impression of a protostar, via Wikimedia Commons. ESO/L. Calçada

Background and hideous biographical detail (skip to the physics if you like)

Hi, and welcome to Physical Attraction; the show that explains physics, one chat-up line at a time.

So I want to start off by giving you a little bit of background. I studied physics for four years, during which time I did a lot of problem sets and almost no chatting-up. Which might be familiar to any other physics students out there. But one thing I did notice was how the subject is just filled to the brim with innuendoes. It seemed that every way I looked, there were hot, dense bodies, fully degenerate limits, or just… references to wet and dry friction. So I wrote a whole bunch of physics based chat-up lines… more as a kind of ironic “Haha I’m going to die alone” type exercise than as a genuine attempt to attract people. I mean, in all honesty, I’m kind of a prude. And anyone who uses a chat-up line in real life is probably quite sad and/or unoriginal. At least, that’s what I thought, until I was elected president of my college’s physics society, and suddenly found that I had to make a speech. And so it was that I ended up making jokes about spontaneous emission and Pouseille Flow in front of an audience of educated and venerable professors. In black tie that didn’t really fit. They were dark times.

Anyway, this would have been just another side-note in a life filled with tragicomic events. Except I still had all the chat-up lines, lurking in a dark and musty corner of my computer. And I still had a whole bunch of physics knowledge, lurking in a dark and musty corner of my brain. And I wanted to demonstrate to the world that I’ve got the moves, as well as the looks. And I thought, maybe, just maybe, some of you out there are trying to seduce physicists, and you could use a little help, or you’re trying to learn physics, and you could use a little light relief. So over the next few episodes, I’m going to explain some ideas about physics using my chat-up lines. This will combine my four great passions in life: awful puns, awful flirting, awful physics, and blathering endlessly on about stuff that no-one could possibly be interested in.

Merton College, Oxford, where the idiotic speech was written and made

So there are two massive disclaimers here. I am going to try and be as correct as possible. But I have to balance that with the desire to actually communicate something. Physicists among you will know that our field is a set of approximations to reality; they get better, and better, and more and more detailed and complicated, but there is always value in explaining things in a simplified form. After all, when all of you went to school, first you learned Newton’s Laws, and then later on they explained general relativity. So if I’m ever guilty of being only approximately true, I justify it by saying it’s the only way we can ever learn. And, also, I’m just a lowly graduate, so there is always a chance I make some mistake, or say something you might disagree with. In that case, email me, @ me, and we can sort it out and argue about the best ways of doing physics / flirting. Unless you’re one of my tutors or fellow students in which case you’d better not talk physics or flirt with me because you actually know what I’m like.

The other one is; don’t rely on chat-up lines! Tell them what’s in your heart. Your horrendous, festering, lonely, lonely heart. And if they don’t like it, you pick yourself up and carry on until someone does. Be as kind as you possibly can be; and, if it doesn’t work out, at least you made the world a better place.

Thankfully we will now talk about physics and not the author’s wretched personal life

Okay, so, without further ado. The first chat-up line:

“Are you in the early stages of stellar formation? Because things are about to get…. hot and heavy.”

One of my favourite parts of physics — a lot of people’s favourite part, really — is the life-cycle of stars. The life-cycle of humans is pretty boring by comparison, I mean; your folks meet, maybe one of them forgets to use protection, you get born, go to school, get a job you don’t like, meet someone else and maybe have a few kids of your own, and then you’re dead. And, after you die, everyone eventually turns into the same thing; a skeleton, or maybe some ashes. Which is not the case with stars. They can die in such dramatic ways as to tear holes in the fabric of SPACE AND TIME ITSELF. They can turn into objects with magnetic fields strong enough to kill you from 1000km away by bending every single one of your atoms out of shape and ceasing all chemical reactions. Magnetic fields strong enough that, if we had one on Earth, it would wipe credit cards on the moon. They can collapse into objects so dense, that a single teaspoon of their matter weighs as much as a mountain. In other words, they can die in such over-the-top ways that Nicholas Cage would meekly hand over his bad-acting crown. While they live, stars are awesome in scale and magnitude. And, when they die, they flood the Universe with the building-blocks that make up everything else, including most of the elements in your body. Plus, we think about people as the natural building-blocks of the Universe, because we’re so self-centred. Really, what we are, is a tiny tiny weird bizarre and annoying self-aware fraction of the matter in the Universe. But matter doesn’t really like turning into humans, which is why there’s so few of us. If you leave matter to its own devices, it turns into stars. They are the building-blocks, the natural units, the shimmering, glorious and beautiful products of the laws of physics. We’re just some weird, complicated, cosmically irrelevant offshoot. So, yes; stars are better than people. Luckily there are enough in our galaxy alone for us all to have fourteen each. Dibs on The Sun.

Physics works in outer space…

What’s really amazing is that physics actually works pretty well in outer space. I say this because a lot of fundamental physics is really difficult to see here on Earth. For example, you probably all know Newton’s First Law, which says that if you give something a kick and it flies off with a certain speed, it will carry on going forever at that speed until something else stops it. Which is amazingly counterintuitive, because that’s not how our world works; we kick things, and they either complain, or move for a little bit and then stop. The Ancient Greeks used to think that the natural path for everything to travel was a parabola — things want to arc downwards — and it makes sense, because if you throw a javelin, or a wheel of cheese, that’s what it will do. They didn’t know that air resistance was acting on the objects. They didn’t realize that the atmosphere makes things more complicated; that there’s so much friction and drag, and so we can’t really see how physical objects “naturally” behave. Nowadays, we know that in everything that happens, energy and momentum — that is, a measure of what direction stuff is going in and how much stuff is going that way — are conserved. But it’s not at all obvious to see that on Earth. But waaaay out there in space, things are, in a way, less complicated. ‘Stuff’ is free to fly about, being pulled in all various different kinds of direction by gravity and electromagnetic forces, but in ways that we can understand. So we can get a pretty good idea of how stars might form by just saying: “OK, computer, what happens if we put all of this gas and dust in this one location and just let it do its thing?”

So for stellar formation, we have a pretty good model. What we think happens is something like this — in some regions in galaxies, there’s more dust and matter than in other regions. The nature of gravity is such that everything is attracting everything else — so if a region starts off dense, it gets more and more dense and collapses over time. These regions are incredibly cold — maybe around 10 Kelvin. The Kelvin scale of temperature is defined so that zero is as cold as anything can possibly get, and 1 degree is the same as 1 degree Celsius. So 10K is -263 degrees Celsius. The “background temperature of space”, although it’s tricky to define, is around 2.73 kelvin. So, not hot and heavy. The seeds of this gas and dust could be former stars — although it’s worth pointing out that the Universe is fairly young compared to the lifespans of some stars. The Sun is stable for around 10 billion years, and the Universe is around 14 billion years old — so it’s not like the air you breathe that may have been inhaled and exhaled many millions of times.
The thing to understand about stars is that they’re stories of force balance, and force imbalance. When the forces are balanced, things can stay exactly like they are, but as soon as they become imbalanced, something about the star — usually its size — has to change to compensate. We will see this time and time again.

via Henry Norman (Quora), Hydrostatic equilibrium explained

The gas and dust collapses towards its centre of mass, and we get a dense cloud. For a while, this situation is fairly stable, because the gas heats up and therefore has a certain pressure. This pressure works in the same kind of way as air pressure in a balloon does — all the molecules of gas are whizzing around at speeds due to their internal temperature, colliding with the walls and pushing out on them. So for a while, there’s a balance; new gas particles get dragged in by the gravity of the cloud, but pushed back by these collisions. Eventually, though, the cloud gets too heavy — at a mass called the Jeans Mass — and gravity can overcome this pressure. There are loads of theories as to how this happens; a nearby supernova might throw a whole bunch of hot matter at the cloud, which can disrupt the balance. It’s even possible that the gravitational effects of distant galaxies colliding with each other could disrupt the gravitational equilibrium.

TANGENT ALERT: James Jeans and his philosophy

Yeah, you could easily get a chat-up line out of this — “you’re hot enough to push me over the Jeans mass threshold” or something like that. But of course, it’s not named after denim, but a scientist, James Jeans. As well as his considerable contributions to stellar physics, astronomy, cosmology and quantum physics, he also had an interesting philosophical perspective on science. And I’m including it here because it totally contradicts something I said earlier. That’s what we call fair and balanced.

James Jeans, looking idealistic. (Public Domain)

Jeans, in his 1930 book The Mysterious Universe, wrote:

“The stream of knowledge is heading towards a non-mechanical reality; the Universe begins to look more like a great thought than like a great machine. Mind no longer appears to be an accidental intruder into the realm of matter… we ought rather hail it as the creator and governor of the realm of matter.”

This is, literally, idealism — his philosophy, probably inspired by the bourgeoning field of quantum physics, that consciousness is somehow a necessary part of the Universe — that it, in a sense, defines the Universe. He loved to put things in these poetic terms — he once said: It may well be, it seems to me, that each individual consciousness ought to be compared to a brain-cell in a universal mind.

I can’t quite subscribe to this philosophy all the way. Maybe it’s just the case that I’d rather think of humanity, and our consciousness, as a sort of bizarre, weird, cosmic accident — for my own personal reasons. Maybe it’s too tempting to think of physics, and the laws of physics, as “what continues to occur even without the presence of humans or any life…” but quantum mechanics might contradict that. Of which more later.

One thing I will get out of the way early, while I’m on this tangent, is to talk about how our understanding of science relates to our philosophical beliefs, and even our religious beliefs. I think this issue is wildly overplayed. I know plenty of physicists, far smarter than I can ever hope to be, who are religious. I’m not, but I can see how one can, with a religious perspective, look at aspects of physics and ooze your confirmation bias all over them in just the same way as I can look at others and use them to back up my own beliefs. You “see” what you want to “see”, and, unless heaven is discovered in the heart of some active galactic nucleus, or we discover that the Universe has a copyright logo somewhere, we’re not going to answer this kind of question with science. I think the human brain is too good at compartmentalising these things. And thank goodness, because most of us need a little bit of irrational belief — or, at the very least, beliefs for which we cannot find conclusive evidence — in our lives to find the will to get out of bed in the morning.

— — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — — —

Okay, so; thanks Jeans, for that tangent, and for working out when hot clouds of gas are going to collapse. Things are starting to get… ‘hot and heavy’. That mass is actually thousands and thousands of times heavier than the Sun — so what happens is that the cloud breaks up and fragments into lots of various sub-clouds that continue to collapse. Eventually, each of these little cloudlets become rotating spheres of gas.

Angular momentum and stellar formation

Wait — why rotating? Well, one of the most universal laws of physics that we know and understand now is that rotating things like to keep rotating. It’s called “conservation of angular momentum”. Whenever physicists don’t understand what’s going on in a system, they usually say: okay, energy, momentum, and angular momentum have to be conserved. This happens in pretty much every process we know about, so it’s a good bet.

One great analogy people like to use for conservation of angular momentum is an ice-skater spinning. (Every time I go ice-skating I usually end up falling on my bum, which, I argue, is still better than falling on my face.) As the ice-skater pulls in their arms, they’ll speed up and spin faster. This is because things have more angular momentum if they are further away from the centre of rotation, or if they’re spinning faster. So, pulling the arms in means they’re closer to the centre of rotation. For angular momentum to be conserved overall, the skater has to spin faster.

Yuko Kawaguti, in the 2010 Cup of Russia. By pulling in her arms, the skater reduces her moment of inertia (which measures the distance of the mass from her central axis) and thus spins faster to compensate. Yuko won this competition. This was all before the Yuri on Ice craze (remember that?)

Because of angular momentum conservation, almost everything in space is rotating about something or other — usually lots of things at once. Of course, we’re familiar with our moon rotating around us, and the planets rotating around the sun, but the Sun — and most stars in the galaxy — are also rotating about the galaxy’s centre. This is true for clouds of gas, as well. Since clouds that are closer to the centre are moving more quickly, in a big cloud, some of the gas is moving around the centre faster than others.

Imagine you’re a gas molecule in the centre of the cloud. Looking towards the centre of the galaxy, all those guys are whizzing around quicker than you; looking backwards, away from the centre of the galaxy, they’re moving more slowly. Relative to you, you can see them rotating around you, and this is what means the collapsing cloud has angular momentum.

The diagram illustrates angular momentum due to linear flow. As you move up the image, particles are moving faster to the left. Relative to the central point, the particles lower down are moving more slowly in the left direction — particles higher up are moving more quickly to the left. So the overall effect is a ‘twisting’ about the centre.

But nothing can destroy angular momentum. What this means is that the cloud can’t collapse directly onto its central point, at least not straight away. So what actually happens is that an “accretion disk” forms. This disk lets matter flow to the centre more slowly, conserving angular momentum; spiralling in, rather than falling directly to the centre. When our star forms, it will be spinning in some direction (set by the line to the centre of the galaxy.) And this is super important for what will happen to the star when it dies.

So our big cloud is collapsing and getting hotter and denser. The dust around the star is radiating away most of the energy released from the collapse, and it’s starting to get a little warmer — maybe 80–100K. At each stage, the star will accrue a bit more matter, collapse a bit further, and heat up a little more. The central part of the star starts to get dense enough that it becomes “optically thick” — radiation can’t just freely flow out without hitting other molecules and atoms, so the radiation has to come from the outer layers of the star. And this process continues for around a million years: the core heats up so that its pressure is high enough to support the star, but more matter accrues through gravity so the core heats up more, and so on. Things are getting hotter, and hotter, and heavier, and heavier. Eventually, the star is producing enough radiation, streaming outwards, to push away its envelope of gas and dust. And, if there were no other processes, things would carry on like this: the star collapsing and heating up until it couldn’t collapse any more, and just became a brown dwarf; that’s like a planet, but without orbiting any other star. Jupiter, for example, is still collapsing and heating up — just very, very slowly. It’s hotter than other planets partly because it’s shrinking. But luckily for us, there are other processes. Namely, nuclear fusion.

Nuclear Fusion: the reaction that drives the Sun

Time for some basic nuclear physics!

So a nucleus is around 10^-15 metres across — that’s 1 divided by 1 with 15 zeros after it. Scientists call this a femtometer. But the nuclei contain protons and neutrons, and the protons are positively, electrically charged. Like charges repel; and, like gravity, the electrical force is stronger the closer two objects get together. So the protons, squeezed together across tiny distances in the nucleus, are being pushed apart by an enormous force. How do they stay together? Gravity won’t cut it — not even close. The strong nuclear force is required. But it can’t act over very long distances, or else all the protons and neutrons would eventually clump together in a massive nucleus. So the strong nuclear force can only be important on really small length-scales, around a femometre, the size of a nucleus. It’s just like me, in a way: from a long distance, I can basically be ignored, but up close, I’m irresistibly attractive. But, if a nucleus gets too big or has too many protons, the strong force won’t be enough to keep it together. That’s why we only have stable elements up to a certain atomic number: beyond this, the strong force won’t cut it either.

If temperatures are hot enough, electrons will be freed from atoms, giving just bare nuclei. And if we can push them close enough together, against their electrical repulsion, the strong force will take over, and — under certain circumstances — they can combine. Generally, in physics, processes that release energy are always allowed, and tend to happen. Those which require energy will need some input from the surroundings. So, if the nuclei combining causes them to rearrange in a way that releases energy, that can happen, and energy is released; this is nuclear fusion.

This image is of a tokamak — one of the experiments aiming to get nuclear fusion working on earth. If they can succeed, a huge source of energy is opened up to us that’s clean and safe. So far, all experiments require more energy to run than they generate via fusion.

Nuclear fusion is responsible for almost all types of energy on the Earth. Seriously. It powers the Sun. Coal, oil, and natural gas are all formed by dead creatures that absorbed energy from the sun, or ate plants that absorbed energy from the Sun. That’s where we get all of our energy from as humans. Even the wind is driven by temperature differences that are caused ultimately by the Sun. The nuclear elements in the Earth’s crust — that decay to keep it warm, and provide energy in hot springs, or that we dig up and decay for ourselves in nuclear fission reactors — they are also formed by fusion in supernovae. Hydroelectric power generates electricity by letting water run down a hill. How does the water get to the top of the hill? If it’s not pumped up by humans, it’ll be because of rain — and the Sun evaporated that water in the first place! So it’s good job fusion is a big deal, otherwise, I guess, our main source of power would be rolling rocks down hills. Sisyphus has us covered already.

Nuclear fusion requires things to be really hot, and really dense. Returning to our star, the core temperature has to be around ten million Kelvin for fusion to start to occur. So this translates to a certain amount of mass that has to collapse, in order to get the densities and temperatures up high enough. But the stars can be a range of different masses at this point — from around a tenth the mass of our sun, up to maybe around 100–200x the mass of our sun. If the star is too small, it just becomes a planet-like brown dwarf. If it’s too large, radiation pressure tends to blow away lots of that excessive mass. When hydrogen fusion begins, it causes a vast amount of heat energy and radiation to be released, and this blasts away the outer layers of gas and dust surrounding the star. The star usually expands during this period. Now, finally, we have what’s called a main sequence star. By the way, that means the phase before is called a “pre-main-sequence star”, or sometimes a PMS object, which I find hilarious because I am immature. Our Sun is a main sequence star; and the main sequence is pretty stable, because there’s a lot of hydrogen to fuse. The Sun is estimated to live for around 12 billion years in its hydrogen-burning phase, so there’s plenty of time left if you still want to learn the piano.

Next episode, we’ll talk about how stars evolve, and how they die. Until then; talk nerdy to me.

Episode Two: Stellar Formation — Hot and Heavy

Hi, and welcome to Physical Attraction, the show that tries to explain physics… one chat-up line at a time. As part of our two-part opening episode, we’re looking at the life cycles of stars: so our chat-up line is:

“Are you in the early stages of stellar formation? Because things are about to get… hot and heavy.”

Well, at this stage, things have already got hot and heavy. In fact, they’re alarmingly dense and producing heat at a rapid rate. Hydrogen nuclei are flying around and hitting on each other, then uploading meetcute stories to subatomic snapchat. Our star, depending on how heavy it is, is sitting somewhere on the Hertzsprung-Russell diagram of stellar evolution in the main sequence, that big band across the middle. But what’s actually going on in the star? Let’s talk about that.

Richard Powell’s wonderful Hertzsprung-Russell diagram. The diagram plots the magnitude (brightness) of stars against their colour which is determined by their surface temperatures. Note the main sequence stars across the middle, providing the band that includes the sun, and the white dwarves:

Hydrostatic equilbrium

The main process here is that the force of gravitational collapse, with the star’s mass pulling inwards, is balanced by the energy of the nuclear fusion in the core, pushing the outer layers… outwards. This results in lots of different regions of the star. The star is made of plasma — that’s the fourth state of matter, which is atoms with their electrons stripped away. There’s the hot dense core of plasma, where the fusion is taking place, and all of the radiation is produced. For a layer outside this, the radiation pushing outwards dominates, and heat is transported outwards from the centre by the radiation. Quickly, though, this stops and the heat starts to be transported by convection in the outer layers of the plasma. Imagine it’s like a radiator: the central heat engine is the fusion core, then the radiation zone is like the hot air straight above the radiator. The hot air cycling around the rest of the room is like the outer layers of the star. Some of the outermost layers are constantly being pushed away by the radiation pressure from inside the star; this means there’s a stream of particles flying from a star, which we call the stellar wind.

What does life look like for a photon produced by a fusion reaction in the heart of our sun? It’s a lot like being a really slow walker in a busy Underground station; there are a lot of collisions involved. The photons are constantly being absorbed, re-emitted, broken up, recombined. In a star, a photon can travel around 1cm before colliding or interacting in some way. That means that, for the energy from our fusion reaction to make it to the surface of the star — that takes a million years. Once it’s escaped the star entirely, it only takes a photon eight minutes to reach the Earth! Which might be familiar as the joyful speed of anyone who’s ever *finally* managed to overtake those slow, slow, slow elderly road-hoggers. Or couples holding hands. Yech.

Lots of things can happen to change a star while it’s still on the main sequence. For a start, most stars aren’t alone; our Sun is fairly uncommon because it has no companion. 80% of all stars are estimated to be in binary systems — two stars orbiting each other. So while the Sun is the equivalent of Miss Havisham, or me, lots of stars are happily paired up, gravitationally bound together in marriages that last for billions of years. Remember last episode, we talked about the angular momentum problem? The clouds that form stars have lots of angular momentum, which stops stars from collapsing directly. Forming two stars that orbit each other, or a star with a planetary system, can help you to solve that problem, because the orbiting objects carry a lot of angular momentum. So most stars are locked in binary orbits, constantly circling around their companion. It’s kind of like being shy at the Year Eleven disco (back when I was in Year Eleven, they won’t let me in now.) You circle around your companion, wondering who’s going to make the first move, for billions of years. Then, eventually, you either spiral in and merge, or one of you explodes. (Okay, maybe it’s not that much like the Year Eleven disco after all.)

via Learn Something New Every Day ( Binary stars regularly transfer mass to each other and often have common stellar envelopes.

The lifetime of a star depends on its initial mass. The heavier stars live for a much shorter time — the heaviest class of stars is only on the main sequence for three million years, compared to ten billion for our Sun. This is because, although the heavier stars have more hydrogen fuel, they use it up even faster.

When the hydrogen runs out… end-times for a star

The really exciting stuff in stellar evolution starts to happen when the star runs out of hydrogen to burn in its core. What happens next depends completely on how heavy the star is, for reasons we’ll explain. First, we consider stars that are less than eight times heavier than the Sun — so this will of course be the fate for our star. When the core runs out of hydrogen to burn, so is now made of helium, the star will begin to collapse. As layers and layers of outer hydrogen collapse into the core, it will be hot and dense enough for these hydrogen layers to start undergoing fusion again. The star then expands rapidly into a “red giant” driven by the fusion in these layers. By the way, when this happens, the Sun will probably engulf the Earth and all of our leftover plastic bags, nuclear waste, and copies of Shrek on DVD. It’s red because the surface temperature is much lower than, for example, the Sun. The colour of light is determined by the temperature, and red corresponds to lower temperatures and energies. The core continues to collapse, with no fusion to support it. For stars like our Sun, it will get dense and hot enough — around 100 million Kelvin — for the Helium in the core to suddenly start to fuse, in what’s called a “helium flash”. This helium flash is over in a few seconds, and it’s like a miniature explosion. For a few seconds, the star produces energy 100 billion times faster than normal. You might be concerned about this happening in the Sun. Well, you’ll be dead long before that does happen, so don’t panic. But also, all of this energy goes into expanding the core, so it can’t escape and kill us all necessary. It’s like a violent, internal eruption, familiar to anyone who’s ever had dodgy chicken.

Stars will continue up this chain for a little while, depending on their initial masses. But, eventually, they run out of fuel. What’s left over is usually a very dense core, typically made of carbon and oxygen that have fused up from Helium, and outer layers containing various elements. The outer layers eventually get blown away by radiation pressure from the dense core, and all that’s left is that core — a white dwarf star. Again — it’s white because it’s at such a high temperature. And, unless anything interesting happens, the white dwarf will just cool down and cool down forever, radiating away all of its energy. It will take billions and billions of years for a white dwarf to cool down. The Universe is 14 billion years old; the oldest white dwarfs we’ve seen are 12 billion years old, and they’re still 3700K hot. The Universe is just too young for them to have cooled down.

But what is a white dwarf? It’s the leftover core of a star, but if fusion isn’t occurring, what pushes out against gravitational collapse? Well, the answer might wind up being similar to “what stops me from falling through the ground when I step on it.” Electrons hate being squished together because of quantum mechanics — we’ll come onto this in a future episode. If matter gets too dense, it feels an “electron degeneracy pressure” which pushes outwards against the gravitational collapse. But the density has to be really high — the average white dwarf has the mass of the Sun, but it’s around the size of the Earth. The Sun weighs a million times more than the Earth, to give you an idea. So a white dwarf is made of this “degenerate matter”, where only this force stops further collapse.

Hubble Space Telescope image of Sirius A and Sirius B. The White Dwarf star, Sirius B, can be seen as a tiny speck compared to its much larger sibling — but it’s very bright.

What happens if the star’s initial mass is higher than 8 solar masses? The answer is, EXCITING THINGS. With these heavier masses, the gravitational collapse is awfully strong. In the lighter stars, it can be halted by electron degeneracy pressure. The heavy stars might not know it yet, but electron degeneracy pressure will not be enough to stop their core-collapse. So, first, the core collapses and starts fusing helium — then carbon, then neon, then oxygen, then silicon… moving to heavier and heavier elements. There are all kinds of wild and crazy reactions going on in the star at this point — and everything’s getting faster, and faster too, as the core collapse starts to run away. So, the hydrogen burning phase of such a star might last for hundreds of millions of years. But it will burn all its helium in a few hundred thousand years, and it’ll burn all of its carbon in six hundred years, all of its oxygen in a few years… and all of its silicon in a day or so. At each phase, the new fusion of the new elements is really just enough to slow down the collapse, and the star is changing rapidly in terms of its size and shape.
Then the star runs out of silicon. And things really hit the fan.

The issue is that once the star has fused silicon, that means its core is made of iron. And that’s a problem, because, while all the lighter elements release energy when they fuse together, iron doesn’t. Iron requires energy to fuse together. Fusing iron is not going to help you. There is now nothing, no fusion process, that can stop the core’s collapse. Even electron degeneracy pressure isn’t enough to save the core. So what happens?

The answer is: it collapses. On a timescale of a few seconds, with the catastrophically infalling core reaching speeds of up to a quarter the speed of light, it collapses. Fast enough to force together all of the protons and electrons to make neutrons, it collapses; this releases billions upon billions of high-energy neutrinos (subatomic particles produced when electrons and protons combine.) And these fly outwards, carrying a phenomenal amount of energy that’s released by this violent collapse; and they transfer their energy to the outer layers of the star, which explodes in a massive supernova.

Let’s talk about supernovae. A supernova releases 10⁴⁴ joules of energy. That’s about enough to power the entire energy consumption of every human on Earth for a trillion trillion years — that’s far longer than the age of the Universe. The biggest bomb ever detonated on Earth was Tsar Bomba. It had a yield of a few petajoules. If you wanted to get the same power as a supernova, you would need 10 octillion Tsar Bombas. That’s enough nuclear bombs that they would actually weigh more than a hundred Suns. Supernovae, at peak power, can outshine the entire galaxies that contain them, even when the galaxies contain a hundred billion stars. Over the course of a few hundred years a supernova puts out more energy than the Sun will in its entire life. Am I getting the message across?

And, of course, supernovae are absolutely vital. When the Universe formed, we think that it was mostly hydrogen, around a quarter helium, and with just traces of the other elements that exist. It would be a boring world if everything was just made of hydrogen and helium. So, yes, the supernovae are responsible for populating the Universe with heavier elements — including the ones that make up you and me. Whenever you look at a lump of gold, you’re looking at a supernova remnant. In fact, there’s some evidence that a nearby supernova was the trigger to the formation of our Sun from one of those clouds we started with back in the first episode. The stars, indeed, died so that you could be here today, listening to my voice. Aren’t we all lucky?

With the outer layers of the star blasted off to space in a cataclysmic supernova we produce, by the way, some of the most beautiful images in astronomy — please Google planetary nebula or supernova remnant if you’d like to drool.

The Crab Nebula, one of the most famous supernovae remnants. I know, I know, I should have tagged it NSFW.

But what becomes of the core?
The ever-collapsing core?

Stellar remnants

It will not surprise you to hear me say that… it depends on the mass.
For cores between 8 and 15 solar masses, the core collapses and becomes incredibly dense; so dense that all of the protons and electrons combine into neutrons. Only the neutron degeneracy pressure, which stops them from being squeezed any tighter, is strong enough to prevent the star from further collapsing, and we get a neutron star. These objects, in terms of density, are just insane: mindbogglingly insane. A teaspoon full of neutron star material weighs ten million tonnes. That’s as much as a mountain. Of course, if you somehow managed to remove a teaspoon full of neutron star material from the star (where gravity prevents this), it would explode with the energy of a trillion atomic bombs, so lifting it up would be the least of your problems. A neutron star might be three or four times heavier than the Sun, but only 20km across — you could fit one pretty comfortably in a decent-sized city. Although, for reasons similar to those mentioned above, this is a very. Bad. Idea.

The gravitational field of a neutron star is very strong. How strong? Hundreds of billions of times stronger than Earth gravity. In fact, let’s imagine you stood somewhere near a neutron star. Let’s say you were one Earth-length away from it, that’s pretty close. Not only is the gravity a million times stronger than that on Earth, but the difference in gravitational pull between your head and your feet is about as strong as Earth’s gravity. You would be quickly pulled towards the star, and slowly pulled apart. Which is no-one’s idea of a good first date.

Neutron stars are also rotating rapidly, because of conservation of angular momentum, like we said before. Quick enough to whip up incredibly strong magnetic fields, which is what you get when charged particles (of which there are still some) are moving around at speed. Recently, a distant neutron star suffered a sort of “Earthquake”, or, to be more accurate, a starquake, and we could detect the pulse on Earth because of how the magnetic field changed.

Artist’s impression of the Neutron Star compares its size with Vancouver. Of course, if the neutron star were really this close to Earth, the planet would be torn asunder in milliseconds.

Stellar remnants: greater than fifteen solar masses.

So now there’s really only one question left for you guys to ask me. Which is: what happens if the initial mass of the star is greater than 15 solar masses?

Then, not even neutron degeneracy pressure can prevent the core from collapsing further. The core will collapse into an object so dense that nothing — not even light — can escape from it. This is how black holes are formed.

There are so many amazing, fascinating facts about black holes that I just know I’m going to have to come back and do another episode about them to do them justice. So I won’t go into too much detail here about them.

And this is how the Universe reacts to matter; it converts it into these weird, inexplicable objects; white dwarves, which cool and fade to nothing, and neutron stars, black holes: bizarre, dense cores. As the Universe expands and cools, we might expect stellar formation rates to go down, until eventually no new stars are forming at all. But, as far as we know, these final-state objects are stable forever. The Universe may be getting bigger, but it’s also getting colder, and more degenerate.

It’s amazing to think about how these processes, on these ridiculous scales of stars and neutron stars and black holes, are dictated by the processes that occur on the very smallest scales — between nuclei, electrons, and protons. The very small linking with the cataclysmically massive. We find it all in physics.

Artist’s concept of a black hole via the NASA Goddard Space Flight Centre. Notice the accretion disc of matter around the outside, spiralling into the black hole, and the jets of Hawking radiation that emerge. Further from the black hole, charged particles that are accelerated and rotating around the black hole emit synchotron radiation, producing an eerie glow similar to the aurora borealis. (AT THIS TIME OF DAY, IN THIS PART OF THE WORLD, ENTIRELY LOCALISED TO YOUR KITCHEN?)

And that, broadly, is how stars are born, how they live, and how they die. There are plenty of fascinating, intricate little details I left out; there are loads of weird objects that can form in all kinds of bizarre ways through different processes, and lots of caveats I left aside. But this picture, I hope, is broadly accurate. And it’s really amazing that we’ve been able to piece it together just by observing space, working out the laws of nuclear physics and gravity and electromagnetism on Earth, and using logic + computer simulations to smooth out the rough edges. This theory took hundreds upon hundreds of geniuses to construct, and I think we’re all indebted to all of them for helping us see just how beautiful and just how bizarre the processes that are going on in the Universe around us all the time really are.

Thanks for listening to Physical Attraction!