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I was unable to answer this question from my daughter. Is it just a coincidence or is there a connection between the following two observations: (1) the core of the earth is made of an iron (-nickel alloy) as well as the existence of iron meteorites with the fact that (2) the heaviest element that can be produced by a star (prior to supernova) is iron?

Related: Elements of a Planet reveals nearby supernova remnant?

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From what I know, iron is the element that has the lowest energy combination of protons and neutrons as nucleus. Heavier elements can produce energy by fission. Lighter elements can produce energy by fusion.

It is related to the production of stars (prior to supernova), as stars sustain themselves by producing energy, which they get from fusion. Once they have changed what matter they have available to iron, they can no longer produce energy to fight gravitational collapse, which may result in a variety of events depending mainly on their mass.(wikipedia)

So I guess iron is a very heavily produced element in a star lifecycle, which explains that it is found in abundance. It is also the heaviest of those produced abundantly, hence Archimedes sends it at the bottom of things.

If you look at the fusion cycle in stars energy production, you may notice that the more common elements play an important role. (wikipedia)

Elements heavier than iron are rarer than the ligter ones.

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I agree that during nucleosynthesis in stars, as more and more iron is produced it is this production that “kills off” the star (prior to supernova). However, I don’t see the connection yet. So let me rephrase my question: how does the earth get this iron-nickel core? –  Carlos Jul 21 '13 at 22:22
    
If I read you correctly, the iron produced at the end of a star life is abundant. When a star goes supernova, the iron-nickel combination is produced more abundantly than the other heavier elements. If so, this means that as a planet is being formed, the iron-nickel atoms (heaviest, most abundant and most tightly bound) will according to you, collect at the “bottom” or core of a planet (assuming they have an iron core) via Archimedes Principle. –  Carlos Jul 21 '13 at 22:23
    
I am in no way a specialist of stellar evolution, so I cannot say much more than I did. When a star has turned a kernel part of itself into iron nuclei it starts collapsing. Heavier elements are created by the supernovae. I am not sure about Ni, but it is close to iron in periodic table. The composition of a planet depends on composition of the cloud where it forms, and clouds are not homogenous. I am not sure why some heavy elements stayed on the surface of Earth. On the other hand, internal heat is still fueled by radioactive decay, presumably from heavy elements. –  babou Jul 21 '13 at 22:52
    
Ánother reason for the iron core is that heavier elements agglomerate more easily in the early stages of planet formation, and the lighter ones are more easily captured after the core has grown. You can find many documents on the web. Iron is produced mostly before the star goes supernova. I do not really know for Nickel. Heavier elements need to get energy from the explosion to be formed, as fusion to produce elements beyond iron consumes energy. –  babou Jul 21 '13 at 22:56
    
Look at en.wikipedia.org/wiki/Structure_of_the_Earth for more details on planet structure. It explains, for example, how some heavy elements stayed on surface by binding to lighter ones. –  babou Jul 21 '13 at 23:05
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