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Thread: Could a sun have a solid core?

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    Could a sun have a solid core?

    Extremely unlikely. but maybe. But it would not be a Sun if no fusion occured. What is the maximum possible melting point at a million times a million times a million, times a million, times a million = 10^30 times sealevel air pressure?

    Possibly, If we could assemble the mass of our Sun in a sphere about that size, without getting it extremely hot, fusion would not occur and the core would be solid. If the assembling took place very slowly = billions of years, the heat of compression might be radiated away, so that fussion did not start even though the core pressure was huge. Possibly a white dwarf or neutron star would occur without there being any fusion. By definition it is not a star without fusion. Brown dwarf stars are not considered stars for this reason, and some brown dwarf stars are cool enough to have solid cores. All brown dwarfs are thought to have less than 8% of the mass of our Sun, but possibly there are very rare brown dwarf stars with more mass and no fusion. Neil

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    What do you think "solid" means at such a pressure? In our day to day life it has something to do the the state of intermolecular bonds. At those pressures there are no molecules, or even atoms, so I'm curious to know what you mean by "solid".
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    There are atoms. At those conditions, though, it is fair to say that our notions of material states are no longer appropriate. The term "gas" is usually used, but "fluid" is probably more appropriate.
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    Moved from Space Exploration to Q&A as the more appropriate host forum.
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    Solid and fluid are distinct at no matter how high pressures - as the neutron star structures show.

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    If I am not mistaken, the degenerate material in a white dwarf is expected to crystallize when it cools off enough. However, that will take a very long time.

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    Wouldn't the iron generated in more massive stars causing said stars to go supernova be considered solid?

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    Quote Originally Posted by PlutonianEmpire View Post
    Wouldn't the iron generated in more massive stars causing said stars to go supernova be considered solid?
    If that Iron is 25 or 26 times ionized, with nuclei swimming in degenerate electron soup at 100 million K, would you call it solid? I'd call it a dense plasma. As far as states of matter go that is a different state than the familiar three (solid, liquid, gas).
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    @Hornblower :

    "If I am not mistaken, the degenerate material in a white dwarf is expected to crystallize when it cools off enough. However, that will take a very long time.
    Pretty sure I've read somewhere that it will take so long that the universe isn't yet old enough for any of these so-called "black dwarfs" to form. There are some extremely ancient white dwarfs known that are only about yellow-hot and as cool as our Sun's surface temperature such as Van Maanen's Star.
    Last edited by Messier Tidy Upper; 2012-May-09 at 12:54 PM. Reason: Adding quote for clarity. Fix'n typos.

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    Quote Originally Posted by swampyankee View Post
    There are atoms.
    Only near the surface of neutron stars, which were among the objects neilzero was asking about. And neutral atoms don't exist under anything close to these conditions.

    And even if you add an atom at a time and allow the object to cool afterwards, at some point fusion will start. Some pair of tightly packed nuclei fuse via random tunneling, release enough energy to push their neighbors over the edge in the process, there's a great big kaboom and you have a star. Or a remnant of one.

    And yeah, white dwarfs are expected to form crystalline cores, but calling them solid would probably be misleading. We're talking about something like arrays of nuclei forced into a space-efficient low energy lattice by the extreme pressures, embedded in a degenerate electron gas, not something held together by molecular bonds.

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    The Sun's core has a high density at ~150gm/cc it is over 13 times more dense then gold. That said it still isn't a solid.

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    Quote Originally Posted by WayneFrancis View Post
    The Sun's core has a high density at ~150gm/cc it is over 13 times more dense then gold. That said it still isn't a solid.
    Tangential question, but wouldn't we expect heavier nuclei (e.g. iron) to sink toward the center of a star, like the big rocks end up on the bottom when you shake a box of rocks? Since elemental ratios are taken from spectra of the stellar atmosphere, we must compensate for this. It seems that we would need to know a lot about this process in order to know the true overall composition.

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    Quote Originally Posted by TooMany View Post
    Tangential question, but wouldn't we expect heavier nuclei (e.g. iron) to sink toward the center of a star, like the big rocks end up on the bottom when you shake a box of rocks? Since elemental ratios are taken from spectra of the stellar atmosphere, we must compensate for this. It seems that we would need to know a lot about this process in order to know the true overall composition.
    The working idea is that heavy elements are proportionally better at getting pushed away from the center by photon pressure, and so, on average, there is no differentiation going on because of nucleon mass.
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    Quote Originally Posted by TooMany View Post
    Tangential question, but wouldn't we expect heavier nuclei (e.g. iron) to sink toward the center of a star, like the big rocks end up on the bottom when you shake a box of rocks? Since elemental ratios are taken from spectra of the stellar atmosphere, we must compensate for this. It seems that we would need to know a lot about this process in order to know the true overall composition.
    Yes and the centre of our star which is still mostly just helium is 13 times more dense then gold about 20 times more dense then terrestrial iron.

    The point is that the density is really high but it is due mostly to gravitational pressure. High density ≠ solid. If you could some how keep a big ball of iron cool and drop it into our sun it would not sink to the centre because it would get to a point, fairly quickly, where it's density would be less then the plasma of a much lighter element. Let it heat up and it will burst apart and probably stay pretty much evenly dispersed within the sun. Eventually it might "settle" towards the core but you are talking about millions of years for that to happen.

    I think we have a very good idea on stars. Our models are very good at making predictions. If our star had a big iron core the gravity would be much different raising the temperature, making it burn much faster then it is.

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    Quote Originally Posted by WayneFrancis View Post
    Yes and the centre of our star which is still mostly just helium is 13 times more dense then gold about 20 times more dense then terrestrial iron.

    The point is that the density is really high but it is due mostly to gravitational pressure. High density ≠ solid. If you could some how keep a big ball of iron cool and drop it into our sun it would not sink to the centre because it would get to a point, fairly quickly, where it's density would be less then the plasma of a much lighter element. Let it heat up and it will burst apart and probably stay pretty much evenly dispersed within the sun. Eventually it might "settle" towards the core but you are talking about millions of years for that to happen.

    I think we have a very good idea on stars. Our models are very good at making predictions. If our star had a big iron core the gravity would be much different raising the temperature, making it burn much faster then it is.
    I think you misinterpreted my question a bit. I'm certainly not suggesting that the sun has an iron core, I was just wondering about what we know about stratification of elements in stars. Millions of years is nothing. Aren't we talking about billions? I'll do some research because I'm curious about this mixing issue. I recently read a paper claiming that the missing Lithium had sunk. If true, that raises big questions about the whole issue of stratification and our measurements of composition.

    Why would plasma of lighter elements be more dense than heavy elements? Seems like that opposite would hold since as the neutron/proton ratio goes up, repulsive forces per unit mass should go down. Heavy elements (above H) would tend to sink unless convection prevents substantial stratification. No?

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    Quote Originally Posted by TooMany View Post
    Tangential question, but wouldn't we expect heavier nuclei (e.g. iron) to sink toward the center of a star, like the big rocks end up on the bottom when you shake a box of rocks?
    Wouldn´t we expect heavier molecules, e. g. oxygen, carbon dioxide and argon, to sink towards the surface of Earth?

    Yet they do not. The mixing ratio between nitrogen and carbon dioxide is the same on sea level and on mountain summits.

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    Quote Originally Posted by chornedsnorkack View Post
    Wouldn´t we expect heavier molecules, e. g. oxygen, carbon dioxide and argon, to sink towards the surface of Earth?

    Yet they do not. The mixing ratio between nitrogen and carbon dioxide is the same on sea level and on mountain summits.
    Mount Everest is about 8.85 kilometers high. The Earth's radius is 6,378.14 kilometers. 8.85 / 6378.14 = 0.00139% of the Earth's radius. Maybe that's why they're the same? Or is there more to the story?

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    Quote Originally Posted by PlutonianEmpire View Post
    Mount Everest is about 8.85 kilometers high. The Earth's radius is 6,378.14 kilometers. 8.85 / 6378.14 = 0.00139% of the Earth's radius. ...
    I get it as 0.139%, or just 0.00139 of the radius. On the other side, I'd like to point out that Radon collects in people's basements.
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    Given time, and an undisturbed environment, CO2 and N2 may differentiate, but thermal processes within even a closed room would make this take a very long time. It probably wouldn't take much turbulent convection to make differentiation by mass impossible; I'll poke around in my h/x books to see if there's a number (Rayleigh, Grashof, etc) where natural convection is likely to become turbulent.

    Radon collects in basements because it comes through cracks and porosity in the basement walls and floor, and basements have poor air exchange with the outside.
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    If I am not mistaken, a mixture of gases will not separate gravitationally by molecular weight unless it is rarefied enough to make the mean free path of the molecules large compared to the inherent motions (turbulence, convection, etc.) that we find in a typical atmosphere. In Earth's atmosphere that is high in the ionosphere. Below that altitude these motions keep the gases mixed, even in the stratosphere where there is very little convection. See this Wiki article for some details.

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

    As usual, don't take Wiki as the last word, but most of their work on scientific topics looks good to me. Lots of dedicated participants are doing their homework and protecting it from tampering.

    In the Sun there is plenty of convection in the outer parts. In the radiation zone my educated guess is that there would be plenty of action to keep the gases mixed, though I would need to hear from the astrophysicists to be sure. I would guess that most of the iron that started in the envelope is still there.

    To illustrate the power of convection, consider the technetium that is found in the photospheres of some evolved red giants. Its relatively short half life indicates that it must have been formed deep inside the star recently, and then dredged up by convection.

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    Quote Originally Posted by Hornblower View Post
    To illustrate the power of convection, consider the technetium that is found in the photospheres of some evolved red giants. Its relatively short half life indicates that it must have been formed deep inside the star recently, and then dredged up by convection.
    Yes but the question is quantitative not qualitative. I guess if the stars are perfectly uniform in composition due to convection then I can throw that paper I mentioned explaining away the missing Lithium in the trash.

    Denser gases can and do sink depending on conditions. We need a quantitative analysis to decide how important this is in stars. When the earth was cooling, there was certainly convection at work (and there still is) but the entire core is supposed to be iron and nickle while the mantel is lighter stuff and the lithosphere still lighter.

    Diffusion probably won't do much for stars near the core because they are so incredibly dense. I forget just how long it takes "a photon to reach the surface", but I believe it's in the millions of years.

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    Quote Originally Posted by TooMany View Post
    When the earth was cooling, there was certainly convection at work (and there still is) but the entire core is supposed to be iron and nickle while the mantel is lighter stuff and the lithosphere still lighter.
    Water does not sink in atmosphere because water molecules are heavy - they are actually lighter than air, molecular mass water 18, nitrogen 28, oxygen 32. Water sinks in air because and when it condenses into droplets or crystals which are heavier than gaseous air and large enough to sink in air against brownian motion and turbulent convection - aided by surface tension causing the droplets to get even bigger. Salt will not sink into sea but mud will. Uranium is much heavier than iron, but uranium readily dissolves in rocks and iron does not - which is why iron and nickel sank into core and uranium did not.

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    Quote Originally Posted by chornedsnorkack View Post
    Water does not sink in atmosphere because water molecules are heavy - they are actually lighter than air, molecular mass water 18, nitrogen 28, oxygen 32. Water sinks in air because and when it condenses into droplets or crystals which are heavier than gaseous air and large enough to sink in air against brownian motion and turbulent convection - aided by surface tension causing the droplets to get even bigger. Salt will not sink into sea but mud will. Uranium is much heavier than iron, but uranium readily dissolves in rocks and iron does not - which is why iron and nickel sank into core and uranium did not.
    What's your point? That uranium is saved from sinking because it is bound to another phase? Does this have something to do with the issue of homogeneity of the sun?

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    Quote Originally Posted by TooMany View Post
    What's your point? That uranium is saved from sinking because it is bound to another phase? Does this have something to do with the issue of homogeneity of the sun?
    Yes. Molecules find sinking much harder than materials which have segregated as condensed solid or liquid.

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    Quote Originally Posted by TooMany View Post
    ... Does this have something to do with the issue of homogeneity of the sun?
    No, obviously, this is nice information about a side issue. As I stated in post 13 above, the homogeneity of the Sun is maintained by photon pressure. I'm not sure why you didn't want to discuss it then.
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    Quote Originally Posted by antoniseb View Post
    No, obviously, this is nice information about a side issue. As I stated in post 13 above, the homogeneity of the Sun is maintained by photon pressure. I'm not sure why you didn't want to discuss it then.
    Sorry, somehow I missed that post. Hmm... Not by mixing or convention, but somehow photon pressure compensates for the tendency of nuclei with neutrons to sink into a sea of protons?

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    Quote Originally Posted by Hornblower View Post
    If I am not mistaken, the degenerate material in a white dwarf is expected to crystallize when it cools off enough. However, that will take a very long time.
    And indeed there have been popular news items suggesting that there are old white dwarfs that are diamonds. http://news.bbc.co.uk/2/hi/3492919.stm

    I share antoniseb's scepticism about thinking that matter at white dwarf density can be thought of as being in the same state of matter as solids and crystals we are familiar with. I don't see that it can have the same kind of bonding structure, which of course is critical to descriptions like "diamond".

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    Quote Originally Posted by TooMany View Post
    I think you misinterpreted my question a bit. I'm certainly not suggesting that the sun has an iron core, I was just wondering about what we know about stratification of elements in stars. Millions of years is nothing. Aren't we talking about billions? I'll do some research because I'm curious about this mixing issue. I recently read a paper claiming that the missing Lithium had sunk. If true, that raises big questions about the whole issue of stratification and our measurements of composition.

    Why would plasma of lighter elements be more dense than heavy elements? Seems like that opposite would hold since as the neutron/proton ratio goes up, repulsive forces per unit mass should go down. Heavy elements (above H) would tend to sink unless convection prevents substantial stratification. No?
    I was wrongly applying iron's density to individual nuclei. Those heavier elements are still going to get stirred up. The energy levels are huge which leaves gravity little chance to settle trace amounts. That is my understanding at least

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    Quote Originally Posted by WayneFrancis View Post
    I was wrongly applying iron's density to individual nuclei. Those heavier elements are still going to get stirred up. The energy levels are huge which leaves gravity little chance to settle trace amounts. That is my understanding at least
    Intuitively if you jiggle the pot vigorously enough, everything stays mixed. I'm curious about the details due to an article (mainstream) explaining the Lithium abundance problem as due to some combination of settling and turbulence. But, if the Lithium can hide this way, surely other nuclei can too, throwing the observed composition overall into doubt.

    I'd be very happy to know that physics tells us without much doubt that atmospheric compositions are precisely representative. Guess I need to do some research.

    On the other hand, when certain supernova are discussed, you typically hear of the collapse of an "iron core" that is no longer able to support fusion. Perhaps in this case separation has occurred and lighter elements have been shed?

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    Quote Originally Posted by TooMany View Post
    ... if the Lithium can hide this way, surely other nuclei can too, ...
    I saw that article, and immediately discounted it enough that I didn't bother searching for the original paper. You are correct that IF Lithium can hide this way, than all stellar (and solar) abundances are in doubt... but it can't hide this way, and neither can the other elements heavier than Hydrogen.
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