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Shouldn't large bodies be hollow?

  1. Feb 10, 2004 #1
    At the centre of the sun, gravity is in equilibrium, as with in the centre of the earth. As you go towards the centre of the earth you will get lightr and lighter as the force of gravity pulling you upwards above you gets larger and larger slowly decreasing the net force of gravity inflicted apon you.

    Why do scientists and geologists always assume that the core is the most compressed part of a large body. It is known that the outer layer of the sun is the most hot, this is obviously due to the fact that gravity is at it's highest there and not at the centre. Why is this fact never mentionned.

    For some reason geologists believe that the centre of ther earth is a solid ball of super compressed and heated metals because S waves caused by earth quakes cannot penetrate the earth's 'solid' core. Why did they ignore the fact that S waves cannot pass through high pressure gas? Why is it also ignored that these S waves are the strongest detectable waves and easily pass through kilometers of crust after bouncing off the surface of the earth at shallow angles.

    Either I have missed something out or every geologist and scientist who has ever looked at this issue severely dumb?

    I do not think that the centre of the earth is solid, I think that it is superheated gas. If this idea just happens to be correct, it would be polite to give me due credit. Though I would not be suprised to hear that other scientists have put forward this idea, though I have not found any.
  2. jcsd
  3. Feb 10, 2004 #2


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    What you are missing is the fact that while the matter at the center of the Earth doesn't experience any gravitational pull itself, it still has to support all the weight of all the layers above it that are trying to push to the center due to their weight. It is this inward pressure that causes the core to be extremely dense.
  4. Feb 10, 2004 #3


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    "It is known that the outer layer of the sun is the most hot, this is obviously due to..."

    in a kind of basic simpleminded sense we dont really know the temperature of the core of the sun because we cant see it

    (or the temperature at the center of the earth either)

    but give the guys a break
    there are well established models for the
    temp, pressure, density inside gravitating bodies
    and we DO see a little bit by inferences
    like with seismology earthquake waves going thru the earth and stuff
    and neutrinos coming out of center of sun and fusion-energy and all
    so a lot can be inferred even tho we dont actually see

    what interests me is your saying the outer layer is the hottest!!!!

    well there are a lot of outerlayers including a very sparse ionized gas called IIRC corona
    but what do you mean by the outer layer
    dont you mean the "surface of the sun" namely what we see
    glowing there?
    from the brightness and color of glow we can tell the temp
    of that very reliably and precisely!

    I guess that is what you mean by the "outer layer"

    it is cool compared with the core AFAIK
    5000 versus 15 million degrees kelvin.
  5. Feb 10, 2004 #4
    "all the weight of all the layers above it that are trying to push to the center due to their weight."

    There would still be a hollow centre as the gravity at a certain height above the origin of the sphere will be so slight that weight is negligible compared to the pressure of the hot gas. Also bear in mind that the effect of gravity by the opposite side of the hollow planet will be less than the one which is pulling it away from the centre.

    Marcus, your right.. Though the sun has no core. How did they estimate the sun's core temperature?
  6. Feb 10, 2004 #5


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    A star cannot reach hydrostatic equilibrium unless both temperature and pressure increase steadily with decreasing radius, and have their maximum values at the center.

    - Warren
  7. Feb 10, 2004 #6
    Of course there's a core, just like the earth has a core. Given the spherical nature of the sun, and the earth, the material "presses" (this is important now) the material below it and so on due to gravity. Even though gravity weakens near the core, there is still an increadible amount of material above it pressing down in a spherical manner. Because seismic readings show readings in par with calculated high-temperature elasticity of iron, they conclude that iron is predominate at the core.

    http://www.ipgp.jussieu.fr/anglais/rub-recherche/eq09geomateriaux-pression/rech2ni-detail01.html [Broken]

    The sun is different. It is much much much larger. And because it predominately hydrogen and helium, it acts much different. The pressure from the rest of the sun's material causes fusion to happen at its core. The layers are so dense on the way to the surface that photons take millions of years to reach the surface because they are absorbed and reemitted so much. The reason they think there is a core is because of the fact that it is sitting there glowing brightly to begin with. That fact that it is stable. It's the best explaination that works.
    Last edited by a moderator: May 1, 2017
  8. Feb 10, 2004 #7


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    what I remember from astronomy class was some estimate on the order of
    10,000 years to 100,000 years
    (that estimate may be out of date)
    you say "millions" of years
    which feels about the same----not a big qualitative difference
    ---either way its a long time.

    but I am curious about what the current estimate is.
    of the time it takes some energy, made in the core, to
    percolate out to the surface
    (from whence it can take off and fly to us)

    could you find a link with an estimate of that time, neutroncount?
  9. Feb 10, 2004 #8
    Sure can.

    http://www.deepspace.ucsb.edu/ia/nineplanets/sol.html [Broken]

    The Sun's energy output (3.86e33 ergs/second or 386 billion billion megawatts) is produced by nuclear fusion reactions. Each second about 700,000,000 tons of hydrogen are converted to about 695,000,000 tons of helium and 5,000,000 tons (=3.86e33 ergs) of energy in the form of gamma rays. As it travels out toward the surface, the energy is continuously absorbed and re-emitted at lower and lower temperatures so that by the time it reaches the surface, it is primarily visible light. For the last 20% of the way to the surface the energy is carried more by convection than by radiation. It takes 50 million years for a photon to reach the surface.

    I'll find more sources to back it up.

    Edit: http://www.genesismission.org/science/module4_solarmax/structured_sun.html [Broken]

    http://www-astronomy.mps.ohio-state.edu/~pogge/Ast162/Unit2/energy.html [Broken]

    And for some reason this link says 200,000 years.

    So I don't know if new calculations show a discrepancy over old values or what.
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  10. Feb 10, 2004 #9


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    Thanks, I have been wondering. If you find other estimates, I'd be glad to see them.

    these links say
    50 million
    1 million
    ~1 million (Richard Pogge)
    200,000 (Ben Chandran)

    the last three are all within a factor of 5 of each other
    so the one that says 50 million seems to be the exception
    the last link, which says 200,000 is by a guy teaching
    Astro in 2001, who does Theoretical Astrophysics and got his
    phd from princeton in 1997. Here is his page, with snapshot
    and list of articles he has published


    The one before the last, that says ~1 million, is also recent around
    2001, by Richard Pogge, an observational astronomer also into instrument design, here's his picture and link to a list of some
    160 scientific articles he has authored

    http://www-astronomy.mps.ohio-state.edu/~pogge/ [Broken]

    Pogge's notes are for an introductory course, so if he says
    "on the order of 1 million years" or ~1 million, he could
    mean the same as roughly 200,000.
    It is not a very exact number in any case and 200,000 is the same order of magnitude as 1 million (they dont differ by a factor of ten)

    I will keep an open mind for now as to whether it is closer to 200,000 years or 1 million years. Maybe Nereid will show up with some very recent authoritative figure. For me the main thing is how strange it is to think that the energy I see looking outside at the light is roughly half a million years old
    the energy that was in my breakfast cereal, which I am now burning as I sit here, was made in the core of the sun that long time ago.

    Individual photons, of course, dont percolate out. the lifetime of an individual photon is very brief.
    Maybe in the core there is a 1000 eV photon and he dies and
    is absorbed and re-emitted many times and after many generations
    his energy is divided up into a couple of 500 eV photons
    and they percolate out a bit and die
    so the energy gets absorbed and divided up and re-emitted
    as four 250 eV photons
    and they percolate out a bit and get absorbed and
    the energy gets divided up and re-emitted, and so on.
    finally there are these thousand photons with average 1 eV.
    and they take off from the surface of the sun

    but the ancestor 1000 eV photon only had a brief life
    some 200,000 years ago or say something on the order of a million,
    that's a terrible oversimplification, more an animated cartoon
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  11. Feb 11, 2004 #10
    Thanks, how silly of me not to visualise a 'vice' effect acting on the core of the sun and other large bodies. Also thanks for teh links.
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