Thursday, December 26, 2013

Binding energy

A week or two ago I wrote about rust. Iron oxide. OK. It's a nuisance where we find it; it can be beautiful on the microscale.

But iron. Let me say a few words about iron, an element so ubiquitous it is inconspicuous, hiding in plain sight.

It's the element that brought us out of the Stone Age (after a brief fling with copper). We are still in the Iron Age. Or should we call it the Steel Age? It's hard to imagine a modern technological world without iron.

The grey eminence. The stalwart foot-soldier. Mister Make-it.

If you take the whole Earth, core and all, iron is the most abundant element. It is the fourth most abundant element in the Earth's crust (after oxygen, silicon and aluminum). Why so much iron in a universe that is mostly hydrogen and helium? Why does Mister 26 stand out?

There's a reason. Iron is the heaviest stable element created by fusion in very large, very hot stars. Remember my diagram not long ago showing how hydrogen is fused into helium in the cores of stars. As the process continues, helium nuclei are fused into ever heavier elements, generating yet more energy. Up to iron. After iron, the game goes the other way; instead of getting energy out, you gotta put energy in.

Here is the detailed explanation from Wikipedia. Read it just for the joy of it.
The process starts with the second largest stable nucleus created by silicon burning, which is calcium. One stable nucleus of calcium fuses with one helium nucleus, creating unstable titanium. Before the titanium decays, it can fuse with another helium nucleus, creating unstable chromium. Before the chromium decays, it can fuse with another helium nucleus, creating unstable iron. Before the iron decays, it can fuse with another helium nucleus, creating unstable nickel-56. Any further fusion of nickel-56 consumes energy instead of producing energy, so after the production of nickel-56, the star does not produce the energy necessary to keep the core from collapsing. Eventually, the nickel-56 decays to unstable cobalt-56, which in turn decays to stable iron-56. When the core of the star collapses, it creates a supernova.
Kablooie!! A big star blows itself apart and feeds iron into the interstellar medium. Out of which new stars are born. Eventually our Sun and its attendant planets.

Oh, there's more to the story; for example, why the inner planets lost most of their hydrogen and helium. But we'll leave that for another place or another time. Meanwhile, we beat our iron into swords, and plowshares, and automobiles, and skyscrapers. Every one of those atoms churned out in the cores of giant short-lived stars that lived and died in a younger universe before the Earth was born.