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Dublin: 16 °C Wednesday 27 August, 2014

Column: Ashes to Ashes, Stardust to Stardust

It’s been said that we are all made of ‘starstuff’. It’s an awe-inspirining statement, but how accurate is it? Conor Farrell explains.

Conor Farrell

“The nitrogen in our DNA, the calcium in our teeth, the iron in our blood, the carbon in our apple pies were made in the interiors of collapsing stars. We are made of starstuff.”

– Carl Sagan

WE’RE ALL MADE of ‘stuff’. We were formed from stuff, we live as stuff, and when we die we will still be stuff. Where does this stuff come from?

You might have heard scientists say that we’re made of stars. While this gives people a sense of interconnectedness and thoughtfulness, it isn’t quite meant to only stir up feelings of emotion and wonder.

When scientists say it, they mean it: we are, very literally, made of stardust.

To understand how, we need to take a journey back in time, around 13.7 billion years, to what’s known as the hadron epoch.

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“Pillars of Creation” is a photograph taken by the Hubble Telescope of elephant trunks of interstellar gas and dust in the Eagle Nebula, some 7,000 light years from Earth.

The ‘hadron epoch’

The ‘hadron epoch’ is a rather grandiose term, as though the ‘epoch’ in question was a vast expanse of time, lasting for millions of years. In reality, it began when the Universe was only a millionth of a second old, and ended in less than a second. But it was an extremely important time in the early universe, where matter began to form. The type of matter I’ll talk about now is known as hadronic matter, and the hadrons created were protons and neutrons.

(Don’t worry, you don’t have to know all about astrophysics or cosmology to follow this piece: I’ve tried to briefly explain the science throughout, but if it’s not making sense to you, just skip over and keep reading!)

As the Universe expanded, it cooled enough to allow nuclear fusion to occur. At this point, when the Universe was 3 minutes old, nucleosynthesis began. During a period of about 17 minutes, all protons and neutrons joined together in various forms to create hydrogen and helium nuclei (the things that would later on join with electrons to form atoms). After 17 minutes, when the Universe had cooled even further, it was too cold to continue nuclear fusion and nucleosynthesis stopped. All the matter in the Universe was now created, only 20 minutes after the Big Bang.

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How hydrogen forms helium and produces energy

Nuclear fusion inside a star

Over time, various other epochs took place, where atoms formed and where light separated from matter. But before stars formed, all the matter in the Universe was mostly made of hydrogen and a little helium. Nothing else.

So where did the oxygen and nitrogen we breathe come from? What about the carbon that makes up our bodies? This is where stuff gets cool.

The density of matter in the early Universe was irregular, so there were regions that had more matter than others. Anything that has mass has a gravitational field, so these clumps of hydrogen pulled themselves together under gravity to become even denser. As the density increased so did the temperature until, boom!, they were hot enough to trigger nuclear fusion and form stars.

The process of nuclear fusion inside a star (such as our Sun) is what keeps it glowing brightly. In the core of a star, hydrogen nuclei are fused together to form helium. The ‘energy pressure’ resulting from the fusion process keeps the star at a certain size. In the case of a massive star, when the hydrogen begins to run out, this pressure reduces, and the star’s core contracts. But as it contracts, it also forces the newly-formed helium to become even denser, thereby increasing the temperature yet again. What happens then? The helium becomes hot enough to undergo nuclear fusion, and ultimately carbon is formed from that process.

This cycle is repeated over time and more and more elements form inside the star until it’s something of a mixed soup of hydrogen, helium, carbon, nitrogen, oxygen, silicon, iron, and a fair smattering of other byproducts of the nuclear fusion process.

But the process stops at iron. At this point, there is simply not enough energy for the core to fuse iron into a heavier element. In a matter of seconds, the outer shells of the star fall inwards, hit the iron core, bounce back out, and blow the star to pieces. This is a supernova, and it spreads all the elements created inside out into space. During the supernova itself, even more elements – heavier than iron that can’t form in the core of a star –  are created, such as gold and plutonium, and are also blown out into deep space.

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Remnant of Kepler’s Supernova as seen by the Hubble Space Telescope

Nebula

The expanding supernova remnant forms a vast cloud of elements and compounds called a nebula. There are different types of nebula: some are the remnants of dead stars, full of a cocktail of elements, whereas others are mostly hydrogen, and are a nursery for new stars. These nebulae can be seen with telescopes, binoculars, and even with the naked eye.

A nebula can be disturbed, or might have irregular densities, meaning parts of it will undergo gravitational contraction yet again. As before, this leads to increased temperatures, nuclear fusion, and new stars. Each time the cycle repeats itself, a new star will contain the elements created from the star before it. Indeed, by measuring how much ‘metal’ is in a star (we call anything that isn’t hydrogen or helium metal in astronomy) an astronomer can work out what generation the star is.

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New stars forming inside the Orion Nebula, seen by the Hubble Space Telescope

Our Sun

Now, think about this following statement:

Our Sun is of the newest generation of stars, called Population I, and has a lot of metals in it (that is, elements that are not hydrogen or helium).

Do you see where this is going? Because the Sun has metals in it, we know that it formed from the cloud of gas left over from a previous star. As well as this, we find other elements created in supernova explosions right here on Earth, meaning the stuff that forms our solar system – including our planet and everything on it – was borne from the life and death of a huge star that existed billions of years ago, before the Solar System and the Sun formed. If you are wearing a gold ring, the gold inside it was probably created during a cataclysmic explosion of a dying star.

And what if it wasn’t created in a supernova? New science is showing that metals like the gold in your ring, or the trace metals in our bodies, can be produced during a particular astrophysical event: the collision of two neutron stars could form the precious metal. A neutron star is a very exotic object that is as heavy as the Sun, is the size of Dublin, and bends space and time around itself. A teaspoon of neutron star material weighs as much as a mountain. A neutron star is a sphere, but if you were too look at it straight on you wouldn’t just see one side of it, you’d also be able to see some of the ‘back’ of the object.

Your jewellery and the trace elements inside your body come from these mind-bending things.

But let’s not get too excited. The oxygen, carbon, and other elements inside your body were once inside a star. Even consider the most abundant chemical in your body: water, H2O, hydrogen and oxygen. We know that oxygen was formed inside a star as it began to die, and hydrogen was formed during the creation of the Universe itself.

In billions of years when the Sun’s life ends, long after we have died, the stardust that was once inside us will form part of a new nebula, from which a new star may form.

You are literally made of stardust, and in the future, new stars will be made of you.

Conor Farrell is an avid science enthusiast and studied physics with astronomy at Dublin City University. He now works with Astronomy Ireland to promote all things space-related to a wider audience. In his spare time he writes about science and current affairs, and can be followed on Twitter at @conorsthoughts.

Read more of Conor’s columns here.

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