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Formation of the Sun

  1. May 28, 2010 #1

    i'm a 2nd year Aero engineer student from Australia. i've had this question on my mind for the past few days now and can't seem to find much material on it.

    i was just wondering how the lightest elements (hydrogen and some helium) managed to come together in the center of the solar system and form the sun, whereas the heavy elements (those beyond Iron) condensed away from the center of the solar system to form the rocky inner planets.

    it just seems sort of wierd that lighter elements would "sink" to the middle to form the sun. or does the mass of the atoms have no effect on the placement of them in the solar system?


  2. jcsd
  3. May 28, 2010 #2
    Zander I just wrote this entire thing from the big bang to the formation of the elements and realised you probably already know all of that and are just wondering about the formation of solar systems lololol and the question that you stated.

    I assume you are talking about the formation of stars and planets based on the nebular theory where mollecular clouds collapse due to gravity and the nebula continues to spin and form the sun and planets (if conditions are favorable) until the nebula flattens.

    The only thing I can think of is because the protoplanetary disk is spinning, the heavier elements would move further away from the center due to centrifugal force and the lighter elements would continue towards the center due to centripetal force.
  4. May 28, 2010 #3
    Cheers HeLiXe.

    yeah i guess that makes a lot of sense. i was on the lines of thinking atoms with greater mass would have greater gravitational pull, so they would congregate in the centre. however i didnt take into account the spinning of the proto disc acting as a centrifuge.

    The high percentage of Iron in Mercury sort of threw me off because i could not see how such high amounts of iron could be so close to the sun, and the sun not have similar makeup. but now i see that the sun must have formed first and the energy from the nuclear fusion would have vapourised and driven away most of the lighter elements, to the outer solar system, and left the heavier elements to condense closer to the sun.

    thanks again HeLiXe, my querie has been satisfied :P.
  5. May 28, 2010 #4


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    The proportion and origin of heavy elements in the sun and planets is a complex subject. It is the subject of an obscure field of study called cosmochemistry. The solar system was seeded by the remnants of at least 10 supernova dating back as far as 6 or more billion years ago. A somewhat disproportionate amount of heavy elements ended up freezing out of the primordial mix in the inner planets. The sun, however, still came away with a larger mass of heavy elements than all the planets combined. For a teaser see:
  6. May 30, 2010 #5
    ^^I could not have said this better myself:tongue2:

  7. May 30, 2010 #6
    Thanks for this!

    You are saying there is more of an abundance of heavy metals in the sun than all the planets combined. I have never heard of this before and I cannot find it indicated in the article you provided. I found this:
    I understand through nuclear fusion many stars produce iron and it is believed to exist in the sun in small percentages now. Can you tell me more about the heavy elements in the sun and any theories surrounding this, if any exist. I am also really interested in hearing more about Cosmochemistry, but I do not want to take the thread too far from its original topic.

    Thx Chronos!/
  8. May 30, 2010 #7

    Vanadium 50

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    The idea that centripetal force moves heavy elements in one direction and centrifugal force moves light ones in the other shows a profound misunderstanding of both forces. I would urge you to do the calculation and see for yourselves what happens. In particular, pay attention to the role of mass in both expressions and ask yourselves what this implies for separating heavy and light elements.

    The sun has a metalicity of 1.6%, meaning 1.6% of its mass is in elements heavier than helium. That's over ten times the mass of all the planets put together. So to a very good approximation, the heavy elements did end up in the sun.

    Also, most of the planetary mass is in the gas giants, which have a composition rather close to the sun's.
  9. May 30, 2010 #8
    OK, but the proportions are more interesting. Why is the Earth rocky instead of gasious as is the Sun? Apparently, this is due to an early solar blowoff that would tend to carry away a disproportionate number of lighter elements than heavier elements from a pre-planetary disk.
  10. May 30, 2010 #9

    Vanadium 50

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    Earth's gravity is too weak to capture hydrogen. Escape velocity is 11 km/s and at 300K, the hydrogen velocity distribution peaks around 16 km/s. By the same argument, the moon has no substantial atmosphere.
  11. May 30, 2010 #10
    ^^Although this is the reversal of my estimation, there is definitely room for misunderstanding on my end as I am learning. I will perform the calculations and see what I find. I ask questions for further understanding, not because I believe my understanding to be complete.

    You are saying to a very good approximation, the heavy elements did end up in the sun...but aren't the heavy elements in the sun a result of nuclear fusion?

    On what basis are you saying the composition of the gas giants are close to the sun? The fact that they are gaseous? Or their percentages of hydrogen and helium? And just to be clear are you including Uranus and Neptune in the gas giant category or are you classifying them as ice giants?
  12. May 30, 2010 #11
    I count two mechanism favoring rocky planets in the inner solar system.

    Would a disk form or persist at a temerature of 300K and a distance of one AU? Temperature alone doesn't tell us everything as the momentum of solar photon flux is directed outward having a peak temperature proportional(?) to Radiusorbit/Radiussun.
  13. May 30, 2010 #12
    There is also the notion that if you stick a gas giant near a star, it will blow the atmosphere away over time and you'll be left with a rocky remnant. This is one of those questions that seems as though it should have a clear and quick answer, but when they emerge something else muddies the water. Calcium springs to mind.
  14. May 30, 2010 #13


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    Not by the Sun itself. At present, the Sun only fuses Hydrogen into Helium. In a later stage of its life cycle, it could fuse Helium into Carbon, but that is far as it goes. Its just not massive enough to go any further.
    All the heavier elements came from the cloud the Sun formed from, and those elements were forged in much more massive stars.
  15. May 31, 2010 #14
    Ah ok. I thought it was massive enough to make iron and supernova. I did a little more research and found that it is expected to end as a white dwarf. I think I got too consumed with the red giant phase and the various theories of what would happen to the earth and the other planets!

    I have another request....hope I am not bothering you! :biggrin: If you can point me to any more resources that further explain nuclear fusion as it relates to stars, I would appreciate it (whether it be books, websites, etc.). I know I have some years of study before I reach that, but it interests me all the same. I thought that as the sun fused one element to the next (like H-->He) an extremely small percentage of the latter element would fuse into the next element in sequence (He-->C) providing enough of the latter element is present. I thought this accounted for the heavier elements in the sun. Thanks for explaining!
  16. Jun 1, 2010 #15


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    The sun can only fuse elements as heavy as carbon [and perhaps nickel after it enters its red giant phase]. Anything heavier was primordial [present when the sun formed], or captured thereafter.
  17. Jun 1, 2010 #16


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    Just for the record, a small correction:
    It's more like 2 km/s.
  18. Jun 1, 2010 #17

    D H

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    I'll correct both of you.

    Vanadium's number is correct for a temperature of around 10,000K (hydrogen dissociates between 1000 and 3200K). Your 2 km/s is using Vanadium's stated temperature of 300K.

    Temperatures in the thermosphere can reach 2500 K. That makes for a vrms of 5.9 km/s for molecular hydrogen, 8.3 km/s for dissociated hydrogen (of which there will be some at 2500 K).
  19. Jun 1, 2010 #18
    Thx Chronos. Now I just wonder why lol I'll be checking out stellar nuclear fusion.
  20. Jun 1, 2010 #19
    This has been very informative, thanks to those who cleared matters for the rest of us!
  21. Jun 2, 2010 #20
    I read that Betelgeuse can fuse iron. This is the 9th largest star we know of.
    cheers, BT
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