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Why is Earth made of Iron and Stone?

  1. Aug 10, 2011 #1
    In Cosmos, Sagan explains to us laymen that stars fuse hydrogen into helium, and then if a star is big enough it creates higher order elements in later stages, before going supernova.

    What isn't explained is why such a high proportion of heavier elements (heavy relative to hydrogen) collect to form planets. I can *assume* that the heavier elements come together first, in the earth, in the sun, in all of the planets, and that only the larger objects capture and hold the lighter elements. IF this were so, then I'd expect that the Sun contains far more iron and stone than the Earth in terms of mass, but less as a percentage of total mass. That is, I'd expect the sun to be built upon a foundation of heaver elements which had been ejected from supernovae. I'd *expect* this, but it wasn't stated or explained, so I just don't know.

    Now, if this is the case, does it suggest that the sun also contains a center consisting of Uranium, maybe even Plutonium or other elements? If traces of Uranium are a part of the Earth's constitution, I'd think there should be much more within the sun. Or, do these elements undergo fission in the early evolution of a small star?
    Last edited: Aug 10, 2011
  2. jcsd
  3. Aug 10, 2011 #2


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    The sun does contain a far greater amount of iron and the elements that comprise stone. The planets nearest the sun are 'rocky' because the gaseous elements were too warm to freeze out and have very low densities in the gaseous state. So, the 'wind' from the birthing sun carried biggest part of them off to the outer solar system - where the gas giants now reside. There is stone out there too, just a much higher percentage of these lighter materials.
  4. Aug 11, 2011 #3


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    Well, I doubt that they stay in the center. The Sun is pretty hot, so they'd be diffused throughout it. But at any rate, yes, we can be pretty certain that there's a lot more heavy elements in the Sun than there are in the Earth, it's just that the Sun is over 300,000 times more massive.

    So the picture here is that the Sun formed first, and then when it formed nearly all of the lighter elements were blown away from the inner solar system, leaving only rocky planets behind.
  5. Aug 11, 2011 #4
    Thanks folks, this makes sense.
  6. Aug 12, 2011 #5
    That also explains why outer-Solar-System objects are so icy -- water and ammonia and methane could condense out there.
  7. Aug 13, 2011 #6
    The Sun contains about 1.5-2.0% of elements heavier than helium, what astrophysicists confusingly call "metals". Most of the "metals" are carbon, nitrogen and oxygen. You can look up the abundances of the elements relative to hydrogen on the Web. You're quite right about there being more of the metals in the Sun than in the planets - there's about 16-22 Jupiter masses of "metals" total mixed in with all the hydrogen and helium.
  8. Aug 13, 2011 #7
    ^ I think this is most right. Still in proportion, its not too significant. A freshly born sun is still mostly H and He. I never really thought about that though. That probly explains the huge magnetic field as well. Who knows whats truely at the very very center. Maybe the H fuses around a radius of liquid metal soup composed of everything from Li -> Fe, with trace amounts of heavier elements from a previous supernova. It could even be the same for the gas giants at their centers, but not massive enough to fuse.
  9. Aug 13, 2011 #8


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    I doubt the metalicity has much to do with the magnetic field. The material in the Sun is ionized, which means you've got large electric currents running around, and those electric currents are going to produce magnetic fields.
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