Children of starlight

How do stars make new elements?
28 March 2017

Interview with 

Professor Marialuisa Aliotta, Edinburgh University.


Pushing atoms together and fusing them, is an ancient process that has gone on for billions of years. In fact, it’s what keeps stars, like our sun, burning. And this is where the atoms that we’re made of came from… Georgia Mills has been finding out how…

Georgia - There are plenty of elements that build our world. You and I are mainly hydrogen, oxygen and carbon. But where did these elements that make us, along with our entire world come from? And what makes an atom of silicon different from an atom of silver?

Marialuisa - A chemical element is defined by the number of protons it contains in its nucleus. So every time you increase the number of protons by one unit you are producing a new element.

Georgia - So hydrogen has one proton, helium has two, lithium has three, and so on. This is Marialuisa Aliotta; she’s an experimental nuclear astrophysicist from Edinburgh University and she’s here to explain how to build the universe…

Marialuisa - The first particles were created in the big bang so originally there were, essentially, protons, neutron, electrons, and there was a lot of gamma rays (high energy photons). Then, eventually, gravity starts kicking meaning that all of these particles and kinds of gas and dust starts contracting, and gradually you keep increasing the temperature of these gas clouds until, eventually, it is possible to reach temperatures that are so high that nuclear fusion can start. This is when we say that a star is born.

Georgia - Stars like our Sun are mostly made up of this basic atom hydrogen, which only has one proton. But stars can convert hydrogen into larger atoms through this process called nuclear fusion…

Marialuisa - These protons, which are nuclei of hydrogen, they kind of scatter off each other because you have to imagine this gas has a certain temperature and so all these particles are moving around, so they have thermal energy as we call it. And so they collide with one another and most of the time nothing really happens. This is like colliding billiard balls; they scatter off each other and nothing much happens. But, occasionally, some of these collisions may lead to the formation of a heavier particle and in doing so some energy is liberated. So this is the fusion process that occurs.

Georgia - So thinking about this snooker table analogy: if you’re playing snooker that balls are going to bounce off each other because of their tough outer shells. But, if for some misguided reason you were to play snooker in the core of the Sun, there is so much energy there that every now and then these turbo-charged snooker balls would smash into each other and stick together. This means you can make a heavier atom, like helium, from hydrogen…

Marialuisa - This is what powers our Sun. The sun has converted the hydrogen into helium for the last five billion years and will continue to do so for another five billion years.

Georgia - This process also releases energy which we eventually see as starlight or sunlight. But what happens next?

Marialuisa - After hydrogen has been converted into helium in the core of the star, the star has, eventually, no longer a source of energy that can sustain the gravity of the outer layers and so the star starts contracting. As it contracts it heats up and then eventually a new phase of nuclear burning can begin. That’s then the next stage which is called helium burning, when helium particles can fuse together to form, for example, carbon. Carbon can also interact with another helium nucleus and form oxygen and so this is how heavier elements are then produced.

Georgia - These heavier elements can only be fused in a much hotter star, as the more protons in an atom, the more energy you need smash through that tough snooker ball exterior and, after a certain point, nuclear fusion just can’t cut it anymore…

Marialuisa - Iron is the last element that can be created through fusion of charged particles. Once iron has been created it’s no longer possible to obtain energy by fusing together two hydrogen nuclei. And yet, elements heavier than iron we know they exist so the process that occurs then is a capture of neutrons on pre-existing heavy elements up to iron that then form heavier species. This is how elements heavier than iron have been created through processes of neutron captures.

Georgia - So imagine our snooker ball atoms. The biggest ones can’t stick with any more protons, they just bounce off their tough snooker ball shells but they can stick to something else - golf balls a.k.a. neutrons. And if they stick to enough neutrons they start to become unstable and then one of these neutrons actually turns into a proton, therefore building a heavier element. This process can happen during a star’s violent death throes, a supernova explosion, so all these different elements get thrown out across the universe and, eventually, find themselves in you and me…

Marialuisa - When I was a child there was a song by an Italian singer Alan Sorrenti, and the literal translation of the title of the song was “We are all children of stars.” I remember thinking this is so silly - what have we got to do with stars? But, in fact, I realise now how very accurate that picture was, Yes we are indeed, stardust!


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