We’ll start with haemoglobin – the complex molecule in your red blood cells that carries oxygen to your cells. Pretty obviously, without it none of us would be here. Haemoglobin’s crucial bit is a central area with four iron atoms that binds to the oxygen… and iron is the interesting thing. Iron is element number 26, which makes it a heavy(ish) element. Where did it come from? To explain that we have to go back a loooong way.

About 13.8 billion years ago, there was no space, time, matter or energy in this universe. We can’t say much about what existed elsewhere, if indeed there is any ‘elsewhere’. The event that tends to be called – perhaps a little misleadingly – the ‘Big Bang’ occurred. It doesn’t make a lot of sense to think about what ’caused’ the Big Bang, because ’causes’ have to happen before their effects, but without time, what does ‘before’ mean?

So, the Big Bang occurred. At the very beginning – in the first 10^-34 seconds – space-time, matter-energy and the laws of physics ‘condensed’ out and began to exist. Quarks – the ultimate building blocks of matter – began to exist, but the energy levels were so high that they could not combine into matter. The universe began to expand rapidly outward from a single point. As it expanded it cooled. Initially there was as much antimatter as matter, but something (yep, I know that’s vague – partly because it’s complex to explain, partly because our explanations are still being developed) happened that led to a large imbalance. There is now much more matter than antimatter in our universe.

As the universe cooled, the quarks combined to form protons, neutrons and electrons, and as it cooled further these were collected by gravity into clouds, some of which condensed further to form stars. Some of the stars were larger and some were smaller. Most of the matter in the universe is in the form of plasma, where the electrons are separate from the nuclei, and this is the case in stars. The protons by themselves are hydrogen nuclei. With one neutron they form the hydrogen isotope deuterium and with two they form tritium.

The heat and pressure in the stars allowed nuclear fusion to begin – small, simple elements combining to form larger elements, releasing large amounts of energy. The main reaction is the fusion of hydrogen to form helium. There are a number of variants, but two deuteriums forming a helium-4 is a common example. This is the energy source of the sun shining outside right now – nuclear fusion of hydrogen into helium.

In the universe as described so far, there are no heavier elements than helium. No oxygen (and hence no water), no carbon (and hence no life), and certainly no iron. As long as the stars had plentiful supplies of hydrogen, they kept burning that.

Some of the larger stars had shorter life spans (on the order of a few to 5 billion years, rather than the 10-15 billion years a smaller star like our sun will live). They burned through their fuel fast, and as they ran out of hydrogen, began fusing the helium to form heavier elements. As this happened they became smaller, darker and more dense… and then, if they were at a certain size, exploded.

(If they were larger than that size, they would form black holes instead of exploding – their own gravity so great as to hold in even light. If they were smaller – like our sun – they would just flame out – but, given the age of the universe, are probably still around.)

It was the explosion of these stars – their supernovae – that spread the heavy elements out into the universe. Over time they spread… and then gravity pulled them together again in some places – including our solar system. A cloud of dust and gas formed, that formed both the sun and the planets. This happened about 4.5 billion years ago – so something like 9 billion years after the universe began.

In the end, those iron atoms, in the haemoglobin molecules in the red cells in your blood, that make the next breath you take keep you alive, came from the heart of an ancient giant star, dispersed in a supernova.

You’re a star.