Why Does Iron Take More Energy to Create Fusion Than Other Elements?

[stardust]

Hello, this is my first post(so, hopefully I'm not breaking any decorum).

Something that I've always been puzzled about. I understand that a star will keep fusing elements until it hits iron. What I'm curious of, is why? Why does iron take so much more energy to create fusion than other elements? What property of iron causes this effect? Also, does iron experience fusion during supernovae when temperatures are very high?

 

[SteamKing]

Once the stellar core is composed of silicon and sulfur, further fusion results in nickel-56. All of the silicon and sulfur in the stellar core fuses in about 24 hours. The nickel-56 is unable to fuse into heavier elements because it takes additional energy to make the fusion occur, which can only cause the core to collapse, since there is no energy being given off by fusing elements anymore. The core collapse occurs in less than a second, and the shock wave given off by the collapsing core causes additional fusion reactions to occur in the stellar envelope, which produce elements heavier than iron.

http://en.wikipedia.org/wiki/Silicon_burning_process

 

[stardust]

So, basically the fusion of iron would not produce enough energy to oppose the forces of gravity, causing inevitable collapse? Why does nickel/iron take more energy than say, carbon, to fuse?

 

[stardust]

Okay, so I've looked at the article on nuclear binding on wikipedia. But, I'm still lost, what causes nuclear binding? It would seem that protons should fly away from each other, because of electromagnetism. I get that the strong nuclear force, or binding, keeps them together. But, what causes this force?

 

[Drakkith]

So, basically the fusion of iron would not produce enough energy to oppose the forces of gravity, causing inevitable collapse? Why does nickel/iron take more energy than say, carbon, to fuse?

Fusion of iron, or in this case nickel-56, produces NO energy. It is an endothermic reaction. This reaction takes so much energy to occur because there are a larger number of protons in iron/nickel than there are in lighter nuclei. Due to the way the strong force drops off with distance, iron and nickel happen to have enough nuclei so that the attraction due to the strong force and the repulsion due to the EM force are balanced extremely well. Adding more nuclei to get heavier elements would increase the repulsion from the EM force more than it would increase the attraction from the strong force.

Okay, so I've looked at the article on nuclear binding on wikipedia. But, I'm still lost, what causes nuclear binding? It would seem that protons should fly away from each other, because of electromagnetism. I get that the strong nuclear force, or binding, keeps them together. But, what causes this force?

Electromagnetism and the strong force (also known as the color force) are two of the four fundamental forces of nature. They are fundamental because they cannot be explained in terms of being caused by something else. They simply exist.

 

[D H]

Okay, so I've looked at the article on nuclear binding on wikipedia. But, I'm still lost, what causes nuclear binding? It would seem that protons should fly away from each other, because of electromagnetism. I get that the strong nuclear force, or binding, keeps them together. But, what causes this force?

Right back at ya: What causes electromagnetism?

The strong interaction is one of the four fundamental interactions. It is, as the name suggests, much stronger than electromagnetism. It is what makes it possible for atomic nuclei to form. If it weren't for the strong interaction, electrical repulsion would indeed make protons fly away from another. There would be no helium, no lithium, no iron, no uranium, etc. Just hydrogen.

What causes this force? One answer is that just as photons are the carriers of the electromagnetism, gluons are the carriers of the strong interaction. One minor problem: That explanation is begs the question, what causes photons and gluons to exist? We don't know (yet). Even if we do eventually have a deeper explanation, there's still going to be some even deeper "what causes that" kind of question that future physics won't be able to explain.

 

[stardust]

Adding more nuclei to get heavier elements would increase the repulsion from the EM force more than it would increase the attraction from the strong force.

Wouldn't it follow then that heavier elements than iron/nickel would rip themselves apart? If the EM force is stronger at the point, why does the nucleus stay intact?

Even if we do eventually have a deeper explanation, there's still going to be some even deeper "what causes that" kind of question that future physics won't be able to explain.

I'm glad too, this is what makes physics so interesting.

 

[Drakkith]

Wouldn't it follow then that heavier elements than iron/nickel would rip themselves apart? If the EM force is stronger at the point, why does the nucleus stay intact?

The EM force isn't stronger than the strong force at that point. It's that adding more nuclei causes the repulsion from the EM force to increase more than the strong force increases. The strong force still overcomes the EM force.