The death of stars

What happens when a star reaches the end of its life? Graihagh Jackson askedAndrew Norton about the evolution of stars like our Sun...

安德鲁-太阳走过了一半的生命now and deep in the core of the Sun it's busy converting hydrogen into helium through nuclear fusion reactions. Now, in about 5 billion years time (it's just a round number), the amount of hydrogen in the core of the Sun will mostly have been used up. At that point, the Sun will expand into what we call a red giant star. At that point, it will engulf the planet Mercury, it will engulf the planet Venus. It probably won't quite engulf the planet Earth at that point because the Earth's orbit will probably move out a little bit as the Sun readjusts. But even so, that will strip away the Earth's atmosphere, boil away the oceans, and make Earth completely uninhabitable for life as we know it.

But, at that point when the Sun has become a red giant, it will readjust its internal structure, the core will become rather hotter and, at that point, it will start a new set of nuclear fusion reactions, this time converting helium into carbon. It's a nuclear reaction called the 'triple alpha process.' It's called that because an alpha particle is the nucleus of a helium atom, and if you combine together three of those, hence triple alpha, you get a nucleus of carbon. Regular carbon that we know here on Earth as coal or charcoal or something like that.

So, for a few more million years, maybe a 100 million years, something like that, the Sun will convert helium into carbon. It might also generate a little bit of oxygen in the core as well by adding on another helium nucleus to the carbon to make oxygen but, at the point, the Sun is going to run out of fuel. The outer layers of the Sun will just drift off, relatively gently into space, over the space of a few 10,000 years, something like that. And what's left behind, the remaining core of the Sun, will collapse down into a dead star really. A type of object we call a 'white dwarf'. That's what fate has in store for our Sun, as I say, in about another 5 billion years time, something like that.

Graihagh - So we've not got to worry just yet then?

Andrew - Well not just yet but, hopefully, we might have found our way to another planet by then and be living somewhere else, who knows?

Graihagh - I hope so!. White dwarfs though, I mean, what are these are they just cold balls of what, carbon?

Andrew - Yes. A star like the Sun when it sheds those outer layers, that will remove about half its mass , but the half of the mass it's left behind in the core will collapse down to something about the size of the earth. So, if you can imagine half the mass of the Sun contained in something about the size of the Earth, you can probably imagine that's pretty dense.

Graihagh - It's time for me to impress you, Andrew, with my fact because I read that when it gets to this white dwarf stage, apparently one teaspoon of that stuff is the same as one ton - it would weigh one ton.

Andrew - That sounds about right, yes. It really is going to be very dense indeed, and that's what these white dwarfs are. They're super dense object held up by something that's called 'electron degeneracy pressure,' which is a neat feature of quantum mechanics when you try to force atoms very close together. This pressure develops which holds them up and stops them collapsing and further and that's what will happen to our Sun. When it become a white dwarf, it will be this really dense ball of essentially carbon and oxygen and, when it's formed, it will be very hot because it's formed from the core of the Sun so it will initially be at temperatures millions, or at least hundreds of thousands of degrees. And then, over the course of billions of years, it will just cool and fade away ultimately becoming something we call a 'black dwarf', when it's no longer radiating any energy or heat.

Graihagh - What about stars that are bigger than our Sun? Because I understand actually our Sun is not... I mean it may seem very big given that a million Earths would fit into it, but it's actually not very big if we look at other stars around us.

Andrew - That's right. So, anything up to about 8 or 10 times the mass of the Sun, will go through pretty much the same sort of lifecycle that I've previously described ending up as this carbon/oxygen rich white dwarf. But if you've got a star that's say more than 10 times the mass of the Sun, because it's more massive, its core it hotter, and so after it's converted the helium into carbon and oxygen it can start undergoing further nuclear fusion reactions to make yet heavier and heavier elements.

So it can build up various elements through the periodic table until you get a core of that star that's composed of atoms around about iron in the periodic table. Atoms like cobalt and nickel and iron, they all sit at a particular part of the periodic table. And the thing is when you're doing nuclear fusion inside a star, as you build up towards iron, you get more energy out than you put in and that's good because that means the star can continue burning. But to go beyond iron to create yet heavier elements, you have to put in more energy than you get out and that's not a viable way of life for a star.

So, when the core of one of these massive stars is composed principally of iron it really has run out of fuel, and at that point the outer layers will fall in very rapidly, hit this sort of incompressible core, bounce out again, and that's what we call a 'supernova explosion.' When all of this outer material in the star is thrown out into space in a hugely energetic explosion and the core of the star that's left behind, this iron core, is now so dense that it collapses in on itself to become what we call a 'neutron star,' and that really is essentially is just a ball of neutrons.

Graihagh - Where the heck have all the protons and electrons gone?

Andrew - Ah, that's a good question. Essentially what happens is that because of the high densities, the high pressures there, the electrons and protons get forced together. And if you combine an electron with a proton, then you get a neutron.

Graihagh - Mmm, pretty cool. Often people talk about black holes though when stars die, so how does that happen?

Andrew - Well, we don't really know it's fair to say. But just as there's an upper limit for the mass of a white dwarf, there must be an upper limit to the mass of a neutron star. It's the so-called 'oppenheimer volkoff limit.' The trouble is, nobody knows exactly what that upper limit mass is but if you've got a core of a star remaining, then even the so-called 'neutron degeneracy pressure' that hold up a neutron star, even that is overcome. And then the material will collapse further still and there's nothing that can hold up a stellar core that's than dense and so, at that point it will, presumable, collapse into this object that we call 'a black hole,' which is as dense as we can get... an infinitely dense object left behind.

Graihagh - But it's all over just yet. Because these massive explosions, the supernovae, cause news stars to form, which brings us back to this...

Brendan - Something has to trigger the collapse of the gas. One cause often is a supernova explosion from another star nearby which trigger the collapse of the gas, gravity pulls in clumps of gas together, and that starts the...

Graihagh - And this is happening an infinite number of times, so I suggest to truly understand the nature of a star, you should listen to this podcast on repeat (it would also do wonders for my ratings!)