Why do they call Iron a "Star Killer"?

A star is at war. The energy created in the Fusion process wants to rip the star apart but gravity wants to crush it down into a solid mass. What you then get is a star that is stable. Fusion's expansion is held in check by the pull of gravity and the crushing pull of gravity is held in check by fusion's expansion.

Stalemate.

This is where our star and thankfully will be for the next roughly 5 billion years.

Fusion is important to a star since you get far more energy out of fusing two atom of hydrogen than you put into fusing them in the first place. Which is important since it takes a heck of a lot of energy to get two atoms to get past the repelling nature of their nuclear charges. Two positively charged protons is like the positive ends of two magnets. They're going to repulse each other unless a lot of energy is used to force them together.

So you whack two hydrogen atoms together. You use a lot of energy and get a hell of a lot more back and now you have helium. So some hydrogen hits some helium and you get Lithium and a hell of a lot more energy, just not as much as two hydrogen hitting. Two helium smashing and you get Beryllium and a lot of energy, just not as much as two hydrogen or even a hydrogen and a helium atom. And so on and so forth down the chain to iron.

Here's where we address your question.

Iron is the first of the elements that does not release energy when it's fused. It actually absorbs it. It gives off *less* energy than was needed to create it. To a star that depends on the released energy to keep it ahead of gravity, this is a MASSIVE problem.

As a super-massive star (the only ones that can do this sort of atom squeezing) starts developing an onion skin like pattern. Hydrogen being the lightest is at the surface of the star, with layers of increasingly heavier elements as you approach the core and as the star ages. Younger super-massive stars are mostly hydrogen, the older ones look like onions if you could see a cross section.

Finally a star gets old enough and the fuel is starting to run out that it's running out of the lighter elements. At the core the star starts producing iron. The iron core is absorbing the energy needed to keep the core expanded against gravity's pull and so as more and more is created, gravity wins.

It always does.

The iron core collapses into a black hole at the center of the star. This happens so fast that the rest of the star is still chugging along like nothing has happened...for a very brief period. Seconds at most. The shock-wave of the core collapse punches out of the star and it rips itself to hell. It can reach temps of three billion degrees which is where you get the energy needed to create the elements heavier than iron as well as the energy to blow the matter away from the newly formed black hole at the center of the now very dead star.

This is why the stuff that allows us to live (the iron in our blood) is the same stuff that kills stars.

This video is from a show called "The Universe" and this episode covers nothing but the various types of supernovae.

https://www.youtube.com/watch?v=DNDnZb-nEOc

The rest of the series is fun to watch too and is highly recommended as they explain the workings of the universe in plain and simple English without talking down to us. Alex Filippenko is a hoot when he's talking about how the universe is going to blow up. He's so chipper and excited that my friends and I have referred to him as the "Weird Al" of astrophysics.

Iron In Stars

Iron is the starting point in the Periodic Table for swallowing energy to cook larger elements (of higher Z). By then the life energy of the star ceases.

Too much iron can also kill you. An iron core is just the final stage of nucleosynthesis before a star goes supernova. No more energy can be achieved through fusion and so the star dies, metaphorically.

You can't get energy from iron. Heavier elements can be split to obtain net energy output and lighter elements can be fused. But iron is the one element with no potential for energy production.

As Alex has superbly said. Iron doesn't give off energy when fusing. Much to the relief of arc welders everywhere!