What will happen when the sun dies?

Our Place in Space is an incredible collaboration between art and science. James Tytko spoke to Stephen Smartt, astronomer at Queen’s University Belfast who studies supernovae and was key to making the scaled-down solar system project happen...

Stephen - It was interesting to get together originally with the artist Oliver Jeffers. It was his idea to create a scale model of the solar system. And so my job was to make it scientifically accurate and also to try and create a scale that we can relate to. So I think we have done it and you can walk it, you can walk to Pluto. It's a long way, but it puts the size of the planets in terms of their distances into perspective, and then puts into perspective, the size of our solar system within the galaxy and the broader universe. I think the interesting idea from Oliver was not just to make it a scientific story, but to put humanity at the centre of this. And that's the idea of the big earth at the start, and then to walk the solar system and to see in reality how small earth is.

James - We heard from Matt a bit earlier that mind-blowingly the nearest star on the scale of the Our Place In Space walk would be about a lap and a half of the earth away. This is your area of expertise, isn't it? Can you tell us a bit about your research?

Stephen - I study stars mostly in other galaxies and stars that explode in other galaxies. So the nearest star to us in our backyard is Proxima Centauri. And as Matt said, that's one and a half laps around the earth. So the size of our galaxy, which contains a hundred billion stars, on this scale would be about the true distance between the centre of the sun in Cambridge and Jupiter and Saturn in the real solar system. Now, what I study are supernovae, the deaths of stars. They're quite rare, so we look at millions of galaxies every night to find these supernovae. And the distances to those on this scale are at least a thousand times the distance between the sun in Cambridge and the distance to Jupiter or Saturn. So they are quite immense distances we deal with.

James - Absolutely. You mentioned the death there of supernovae. That's quite dramatic terminology. What actually happens when a star dies?

Stephen - All stars greater than about eight to 10 times the mass of the sun will end their lives when the fuel runs out in the core. They survive by creating thermal pressure in the core through nuclear reactions, and they're trying to collapse under gravity. For most of their life, those two forces are exactly balanced and that's why stars are circular. At the end of their lives, when they've burnt through the nuclear fuel in the core, they can't support themselves. The core collapses and that releases a huge amount of energy and destroys the star. And these supernovae can as bright as single galaxies and may last for about a few weeks to a few months. That's what we search for on a nightly basis.

James - What about on a day to day basis? What are you looking for?

Stephen - We partner with several telescope surveys around the world. The ones we work with at the minute are mostly in Hawaii, and the nice thing about that is the time difference. So when Hawaii's observing 10 to 11 hours behind us, then it's our daytime and we can sift through the data while the telescopes are surveying the sky. There's some specialized telescopes over there that survey the whole sky every night. There are two telescopes in Hawaii, one in Chile and one in South Africa, and they're the same type of design. Probably , for the first time in history, we've been able to get to the sensitivity that we can achieve with these survey telescopes. And we basically look for everything that changes or moves in the night sky.

James - So all stars eventually meet their demise. What about our star, the sun? What what's in store for that one in its lifetime?

Stephen - That one will not create a supernova. A supernova is produced by a star, which is at least eight times the mass of the sun. So what happens with a star like the sun, these low mass stars, is that again, the core will run out of fuel. So the hydrogen will burn to helium. The helium will burn to carbon and oxygen. And at that stage, it will not be hot enough for the carbon and oxygen to burn any further. What happens in these stars, and we see it across the galaxy, is that they become what we call 'red giants' and the atmosphere swells up. So what will happen is that if you think of the sun on Midsummer common, it will expand by about a factor of a hundred and left over in its core will actually be something about the size of the Earth made of carbon and oxygen. That's what we call a 'white dwarf' and the atmosphere, which is very extended, will then just float away and will be heated by the radiation from the white dwarf. It'll form a nebular planet. So effectively, the sun will just swell up and eventually the other layers will just puff away and we'll be left with a very hot white dwarf, the size of this earth at the position of the sun. The timescales that we infer are very long, so although it sounds worrying that the earth will end up within a very tenuous, low density atmosphere of the sun and potentially be vaporized, this won't happen for another four and a half billion years. So the sun is about halfway through its lifetime, and the earth is about four and a half billion years old. This will last for another 4 billion years or so, until the sun becomes a red giant. We have to infer the timescales through looking at stars at different stages in their lives, and then apply sophisticated computer models. And that's mostly what astronomy is about; applying the physics that we know and understand on earth through sophisticated models to the observations that we take.