Why is there so much Iron in the Universe?
I get asked this question a lot: Why is there so much iron in the Universe?
If you look at the big picture, our Universe, our Solar system, and even our own planet, are most abundant in hydrogen and helium, the two lightest elements. Then, it’s all-downhill from there: elements get less and less abundant the heavier they are.
This is the general rule, but like every rule, there are exceptions. There are ups and downs, peaks and valleys, in the distribution of elements. Some of them are common, like ups for even elements and downs for odd elements. This is also called odd-even staggering, and it appears simply because even elements are more stable than their neighbors; protons like to pair-up.
Sometimes the features in the element abundances appear because the mechanism that makes certain elements is just very rare (like for lithium and beryllium, elements 3 and 4). And other times, nature simply favors certain elements and creates peaks in the abundance distribution. The most prominent peak appears around iron, hence the original question of this post.
So why is that? Why do we have more iron (element 26) than, let’s say, titanium, which is lighter (element 22)? Why more iron than calcium (element 20)? There is even more iron than argon, which has half the mass.
Considering that this is a nuclear physics discussion, you might have guessed that the answer comes from the nuclear properties of iron. It happens that iron, and its closest neighbors, have the largest “binding energy” out of the 7000 different isotopes. Binding energy is simply the amount of energy we need to give to the atomic nucleus to break it apart into the protons and neutrons it’s made of. High binding energy means it’s harder to break this nucleus; it simply prefers to stay the way it is. On the other hand, a nucleus with low binding energy will be easily transformed into a different nuclear species, a different isotope or even a different element. In nature, this translates into having more of a particular type of element (high binding energy) and less of another (low binding energy).
Iron and its neighbors (elements between 22-30) occupy the top 38 places in the binding-energy ranking. And this basic nuclear property affects the stellar processes that create elements in the Universe. As a result, the heaviest stars will create an iron core in their centers, but never go beyond that. A lot of the stellar processes will end around iron, and therefore matter accumulates in the form of iron or its neighbors.
So next time you are tending your fire with your fire-iron, remember that all that iron came from a massive star because iron is one of the most bound elements in the Universe.
