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Some questions on star evolution

  1. Feb 14, 2006 #1
    ok, some stuff doesnt make sense to me, so i'll just sum up all i know... maybe it will clear things up, and i'll mark the parts im unsure of\want clearification on.
    for your convenience i'll write numbers for each remark so you can just write the number and your response to it.

    at first we have a gas cloud containing mainly hydrogen and little helium (and much less other heavier elements) all in molecular form.
    then part of this cloud starts to collapse much like water dropplets that form out of vapour.
    the gas collapses and heats-up, the molecules of hydrigen are broken when the temp rises to 1800k, then its ionized on 10^4 (in both temps the collapse is faster because the heat energy is used up for breaking the bonds or ionizing), but the protostar isnt in equilibrium yet, and its still collapsing.
    after the protostar reaches 10^5k, all the matter in it is ionized.

    1: the reaidation pressure is greater as the precentage of ions gets higher beacuse the ions wont let the radiation out, thus the protostar reaches equilibrium, and collapses in a much slower rate. (am i right?, why is it called equilibrium if the protostar still gets smaller?)

    by this time the protostar is 10^-2 - 10^-3 of the original gas cloud it started from, and you can draw it on HR diagram on the hayashi track, which will take it to the main sequence.

    2: here it continues to collapse (why did they say hes in equilibrium???), reducing its luminosity and heating the core untill most of it is radiative (what do they mean by that?, i mean its always radiating because its hot isnt it? but the radiation is absorbed by the ions so no radiation escapse... what different now? - im confused...)

    after a while nuclear reactions (not a full blwn p-p chain reaction, but lithium berrilium and boron may light-up in the ppII and ppIII branches for example) takes place in the core, and it contributes to the heat and luminosity.

    untill now ive described the hayashi track - first the protostar goes down in the HR diagram and then it goes slightly up and left to enter the main sequance.
    the main sequence starts officialy when the star begins to fuse hydrogen with the main branch of the p-p chain, it starts at around 4*10^6.
    (well, on large mass stars CNO cycle is also active at the stage of main sequence).

    3: is it still collapsing on hayashi track (as i suspect) and only gets to stable equilibrium in the main sequence?, and why is there another drop in luminosity just before it enters the main sequence?

    now, a high mass stars have convective core, because the radiation is too extensive and can escape the core 4: (am i right here?), so the hydrogen in the core decreases uniformly.
    outside the core the energy is transfered by radiation and the 5:hydrogen abundance increases outwards (why?).
    slowly the core mass is getting smaller because of fusion, and the radius of the star begins to grow while the temperature of its surface decreases.
    after most of the hydrogen is used, the core collapses and heats-up making the star hotter and with higher luminosity 6: (why doesnt it just make the star larger due to more radiation pressure?)
    the hydrogen in the shell around the core will soon ignite again due to the heat from the collapsing core.

    as for the low mass stars:
    the core is radiative 7: (what does that mean?), but at the outer shells theres high opacity because of the low temperature 8: (i thought ionized matter is better for high opacity... why does lower temperature have higher opacity?)
    and so the outer shells are convective, as opposed to the high mass stars.
    since there is no mixing of material in the core, the hydrogen in the innermost core will burn faster, and the hydrogen abundance increases outwards.
    as the hydrogen in the core is transformed to helium the star will climp up the HR diagram (almost along the main sequence line) and become more luminous and hotter 9: (why?, i mean less hydrogen is being fused at this stage... and the helium doesnt ignite yet... is it because the hydrogen is burning closer to the surface - around the helium core?)

    well, this post is getting too big... so please comment on this one, and look for the next one on the later stages of star evolution (giant and dwarf stages).
    i'll write it in about 10hr, im going to sleep =P
  2. jcsd
  3. Feb 16, 2006 #2
    Depends on what kind of equilibrium you mean. Its not in hydrostatic equilibrium if its still shrinking though.

    Young stars will typically have convective interiors (at least so our computer models of solar evolution say) until a few hundred million years. When they say its radiative what they really mean is 'not convecting'. Although, radiative energy transport is generally insignificant in convective regions compared to energy transport via convection.

    To the first question, in solar simulations we consider the point where the sun is at Zero Age Main Sequence to be when it reaches its minimum radius. This typically occurs at a few million years, when the core is still convective. I've seen others count ZAMS as when the core is no longer convective though, at a few hundred million years.

    Hydrogen abundance increases outwards because of diffusion. The heavier nuclei will over time drift down towards the core of the star, hydrogen will float on top.

    The core in the sun (for example) is not convective, fluid flow is not used to transport energy. Ionized matter gives higher mean free paths. Absorption dominates over scattering in determining the opacity.

    Last edited: Feb 16, 2006
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