This is a vein of iron ore.
If you dig it out and purify it in a furnace, you’ll get iron.
It makes up 35% of the Earth’s mass, and most of it sunk into the Earth’s core when it was being formed.
Stars have made so much of it because it’s the final waste product that they make during their lifespan.
If we can get a bit conceptual for a moment, this happens because iron atoms have the highest ‘binding energy’ of all the elements.
If you make a graph and plot all the elements by weight on the x-axis (e.g. hydrogen, carbon, oxygen… then iron and uranium), and on the y-axis you plot the inverse of their ‘binding energy’, the graph looks a valley.
If you fuse atoms together (like with the fusion that happens in the centre of stars) the atoms get heavier, so you’re pushing them the right. For light elements like hydrogen, you’re pushing them into the valley, like rolling a ball down a hill.
Splitting atoms (which is what atom bombs do) do the opposite. It makes atoms lighter, so you’re pushing them to the left. So if you split something heavy like uranium, you’re also rolling a ball down the hill.
So long as you’re pushing atoms down the hill, you’re releasing their energy from the nuclear forces, which is like the glue that holds atoms together.
But when you reach iron at the bottom, you are stuck in a rut. You need to spend energy to fuse or split it’s atoms, like pushing the ball back up the slope.
Once a star starts producing a lot of iron, it’s a sign that it has used up all of its lighter atoms as fuel. All that’s left is the waste that it can’t extract any energy from. The star is near the end of its life.
Depending on its mass, a star may then erupt as a supernova or a planetary nebulae, in which the chaos of the explosion will fuse a wild assortment of other elements together, creating heavy elements like uranium.
The explosion expels iron and other elements into space, where they will form new asteroids and planets. Iron is still relatively heavy, so it often sinks into their core, with some deposits left over in their crust.
On Earth, iron has been one of the most useful elements in human history. It’s relatively straightforward for us to extract from the ground and melt into new shapes, which has been hugely important for the development for our species.
Iron may be a dead end for stars, but it was the beginning for our advanced civilisation.
The ‘Iron Age’ defines the era of the ancient city states of Athens, Sparta, Troy, Carthage, and the Persian Empire. In mainland Europe, it was the era where Celts built hill forts to defend their small agricultral communities. In India, the Vedas were being written. In every one of their cities you could hear the clank of the blacksmith’s anvil, working iron into swords, armour, and equipment.
Iron is more difficult to work with than copper and bronze as it is melts at a higher temperature, so most early civilisations used them first before they developed the technology to use iron.
The earliest known iron goods were beads found in Egypt, made from metorite iron, but the first iron that was extracted from ore was in Mespotamia and Turkey. By modifying their furnaces to burn hotter and by adding charcoal, they sparked a chemical reaction which gave them lumps of iron which they beat into shape. Iron’s abundance, as well as the trade and cultural adoption of this technology, began the ‘Iron Age’.
Though iron is harder than metals like copper or bronze, it is also more brittle. Over time our use of iron has advanced to solve this problem. We’ve purified it then created mixtures with other things to create substances that are stronger than their constituent parts. Substituting charcoal with coke (a type of fuel that looks like coal) produces steel – one of the hardest substances yet developed.
Today iron is still heavily used. It’s inside stainless steel benches, cutlery, and appliances, in drill bits and in reinforced concrete, in ship hulls, and in the structural frames of cars, buildings, and aircraft.
It is a fascinating story from the big picture. Iron, as the waste product of stars, has been recycled as the material backbone of our civilisation.
It’s a reminder of the interconnectedness of everything in the cosmos, and how vast cosmic events billions of years in the past still shape our lives today.