The Iron-clad Element

 The other day my friend told me they heard a fun fact, but want more clarity. Specifically they asked,

Why is iron the "most stable" element?

This is one of those cases where there's a very straightforward answer, but it's not simple; the simple answer, though, is not straightforward. But this blog is all about making answers simple, so here we go!

Where does iron come from?

A little while after the Universe started, it was full of hydrogen and helium at a ratio of about 3:1. Technically some berillium (element number three) was probably in the mix but astronomers don't count that much. So where did all of the other elements come from? Well, for all of the elements that are lighter than iron, they were created in stars via nuclear fusion. This sentence already tells you that iron is a key element in this process, but let's take a second to go over nuclear fusion.

Nuclear fusion review

Stars like our Sun have a ton of hydrogen at their center, and enough heat and pressure to start doing some extreme stuff with it. Like the Kreb's Cycle, or the engine of a car, there are quite a few small, complicated steps between the starting fuel and the result: energy and exhaust. In the Sun, the fuel is four hydrogen atoms, while the result is one helium atom and a lot of energy. If you compare the mass of four hydrogen atoms, though, you'll notice that it does not exactly equal the mass of one helium atom. Where does the rest of the mass go? Well, thanks to Einstein's famous equation, you already know that it gets converted into energy:

E = mc^2

That little bit of mass difference is turned into a lot of energy. And this reaction happens [insert big number here] of times per second for billions of years! This, dear reader, is how the Sun and all of the stars like it are able to shine so brightly and burn so hot for so long; there is a huge amount of hydrogen to turn into helium, and a huge amount of energy to be released. 

And voila! Helium is created by the star itself! Now, helium was already around long before any stars were, but now we certainly have more of it. So what about the other elements?

Heavier stars make heavier elements

This title is a little misleading but it's too poetic to pass it up. Stars more massive than our Sun have an advanced life cycle where, after burning most of their hydrogen into helium, will continue to burn the helium in their cores into the next elements: oxygen and carbon. Because there are a few ways for helium to combine, both oxygen and carbon can be made, and even some berilium. If the star is massive enough, it will have enough material, heat, and pressure to continue to the next stage of nucleaosynthesis: burning oxygen and carbon into neon and magnesium. After that, silicon and sulfur can be created, then follows by nickel and iron. 

And then it stops. All of the other elements are made in more extreme conditions, like supernovae or stars colliding with each other. But this is the end of the line for stars making stuff.

The Plot of the Day

Here's our plot for today: the curve of binding energy

I've mentioned that the center of a star has enough heat and pressure to begin nuclear fusion. This is because hydrogen doesn't actually want to be turned into helium; it takes a lot of energy to begin the nuclear fusion process. But once it starts, the energy released from the reaction is more than the energy needed to start it. And so a star can continue to fuse these elements until it runs out, because energy from the first reaction will help kick start the second reaction, and so on.

So we see that helium is fairly high up on this plot, all the way on the left-hand side. It has a very good exchange rate for energy (the process is exothermic). Carbon and oxygen also do well, climbing up the binding energy curve as we move right towards larger elements. Before we get to iron at the top of the curve, let's take a look at uranium all the way on the right.

On Earth, uranium-236 is famous for its nuclear fission, where the one atom is split to make two or more smaller atoms (in this case, krypton-92 and barium-141). The process of splitting a uranium atom is also exothermic, where it might take a bit of energy to start but the result releases a ton of energy. However, the reverse process is endothermic, with a net loss of energy. That is, the energy needed to combine krypton-92 and barium-141 is greater than the energy released from the reaction. This is true for all of the elements to the right of iron on the binding energy curve: more energy is released if the elements is split than combined.

The Iron-clad element

So iron-56 here is special because it sits right at the top of the curve: fusing silicon and sulfur to make iron is not going to release enough energy to be an exothermic reaction, and splitting iron is also not exothermic. [To be entirely honest, I'm not 100% sure if it's endothermic; I just know it sits at the top]. So this is one way to call it "stable:" It doesn't want to be combined, it doesn't want to be split, it just wants to sit.

What happens to a star once it starts to fuse iron, though, is anything but stable. The star spends energy in the core to create the iron, but this reaction isn't giving enough energy in return. The star collapses rapidly as it loses energy, and BAM! It goes supernova! (This is a type II supernova, in case that means anything to you). Lots of super interesting processes go on in fractions of a second, but that's neither simple nor straightforward, so I'll end it here. Hope that helped answer your question! 🌠