Why do Stars Stop at Iron?

Although our Sun will never fuse elements up to silicon and Iron, the largest stars in out universe sure do! But why no more than Iron? Couldn’t a star make Platinum or even Plutonium?

Let’s have a look at the fusion process and find an answer to our questions!

Let’s Explore some Stellar Nucleosynthesis!

Technically, a powerful star has the capability of fusing heavier elements, so the question isn’t really a case of why stars stop fusion, but why wouldn’t they stop.

One of the reasons stars don’t fuse heavy elements is because it doesn’t actually benefit the star’s life. This is due to binding energy on the atomic scale.

If you want to split a nucleus up, you need to put some energy into it to overcome nuclear forces.

In the simple case of Hydrogen into helium, we put a bit of energy into the fusion process in order to rearrange 4 hydrogen particles to make 1 helium and more energy than we put in.

Where did this energy come from? Quite simply, the mass of the helium nucleus is less than the original 4 hydrogens combined. That loss of mass has been converted into energy, which is equal to the binding energy.

Above is a quick calculation of how much energy is released by making 1 helium nucleus. The formula E = mc2 shows the relation between mass and energy, so we can use this to convert between the two. This is how we find things like binding energy per nucleon.

So Let’s Apply This to Iron!

This diagram shows the binding energy per nucleon of various elements.

All elements have a binding energy per nucleon, as shown on the left. As we progress from Hydrogen to Iron the energy per nucleon increases, which makes sense because we know fusion of things like oxygen and silicon don’t last long because of how ineffective their processes are.

Notice iron is at the top of the curve, and then it starts to slope down? You might be tempted to think that this means fusion becomes more efficient again, but remember this is per nucleon.

These heavy elements beyond iron have ginormous nuclei, so the total binding energy keeping the nucleus together is enormous! In fact, past iron fusion starts to require more energy than it releases.

So in essence a star would end up losing its precious energy, so it will instead stop fusion and die, like the drama queens they are! This is why stars stop at iron.

But wait, if stars don’t make fuse anything heavier than iron, where did all the heavy elements come from?

That’s a post for another time 🙂


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