There are people walking around on planet earth who originally came from deep space. They look and act like normal human beings but their origins lie way beyond the outer reaches of our solar system.
Most of them go about their daily lives completely unaware of where they have come from.

This may sound like the script from a science fiction film but it is true.
If you would like to meet one of these strange beings, you won't have to go far....because you're one of them. In fact we all are.

All humans like every other living creature on this planet are made up of carbon.
Carbon is a heavy element that is only created under conditions of extreme heat and pressure. So where does it come from? Where does the very cloth that you are cut from get manufactured? The answer lies in a sequence of events that ends in the most explosive event in the universe, a supernova.

A distant star explodes - animation courtesy of NASA


A star of 25 solar masses (25 times bigger than the sun's mass) burns Hydrogen for roughly 7 000 000 years. (This is known as the Population I stage).

After this time, the mass of the star is large enough to start forcing the atoms in its core together, but not large enough to start expanding itself yet. So a core of Helium develops - when the star is known as a red giant - because two Hydrogen atoms joined together make Helium. It starts as a small ball inside the core of the star, while a 'shell' of hydrogen moves outwards.

This process of being a Helium burning star lasts around 700 000 years and is the Population II stage. The star is now on what it known as the Main Sequence - an escalator that will climb, relatively quickly, to supernova-hood.

Having gone through 7 million years of its life as a hydrogen burning star and the last 700 000 years as a helium burning star, things start to happen in a hurry.

The Carbon stage is when three Helium atoms join together to make a single Carbon atom. This is a tricky business and doesn't last long, only 600 years in fact.(Carbon being a fairly neurotic element). So at this point the very element you are made from is being cooked up in the middle of the star.
This is Population III stage. During this time we have a Carbon core, surrounded by a Helium shell, which is itself surrounded by a Hydrogen shell.

The next element to join the party is Neon, burning for a year.

Then Oxygen - for a mere six months.

Then the heat starts to turn up. We're talking about heavy elements here. Mucking about with them is nothing short of toying about with spanners inside a nuclear reactor, which is exactly what a star is.

So then we get Silicon for a day,

and then Iron for just a few hours. Think about it: all the iron in the universe was made this way. Everything from the Sutton Hoo burial swords to the railings outside your house, all of it was made in a few hours inside some dying star out on the edge of the galaxy.

So how did the Iron and all the other elements get to be here on earth?

After burning Iron, the star is really really hot. It's too hot to start welding atoms together anymore. They want some space, so they start pushing against each other.
Volunteers at a punk rock gig demonstrate the atomic meltdown in a dying star.
It gets like the mosh pit at a punk gig: everyone's forced together tightly but they're also slamming hard against one another.

What happens? The biggest nuclear meltdown you've ever seen, that's what! Or to give it its scientific name, supernova. After over seven million years of life, the star suddenly collapses in its core for nothing longer than a quarter of a second.It happens at 15 % the speed of light. What's going on here is that the pressure from outside finally gets too much and the atoms are smashed together by the weight of all the shells around them in the outer parts of the star. But then we cross over the line at which atoms are able to sit alongside each other and the core 'bounces'. This lasts a matter of milliseconds.

The star now explodes, for about 10 seconds, ejecting 24 of its 25 solar masses far out into space: this is how earth got its various elements.
The expanding cloud of material expelled by a supernova is captured by a radio telescope.

There is just a sick corpse of a star left, about the size of earth's sun - a shadow of its former self. This is known as a supernova remnant, which will now keep 'cooling' until it becomes a white dwarf. If it keeps on collapsing, it becomes a neutron star, and if it doesn't stop there it goes on to be a black hole.

So there you have it. From stellar giant to supernova and possibly on to black hole and in the middle of it all, carbon.

In what distant deeps or skies
Burned the fire of thine eyes?

(from The Tyger by William Blake)

Thank you to Gareth for all his help with this article.
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