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Source of new stars

  1. Feb 16, 2015 #1
    When a quite big star consumes its fuel, it explodes with supernovae. So, if it consumes almost all of its fuel up to the iron, then how can the new stars born after explosion. I mean, Hydrogens already turned into Helium and Helium into Carbon and so on. Does star consume all the fuel it has or some fraction of fuel is not fused or during the supernovae some fraction of heavy nucleus break apart into ligther ones ?


    Thanks in advance ...
     
  2. jcsd
  3. Feb 16, 2015 #2

    mfb

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    New stars get some contributions from old stars, but most of their material has never been in a star.
     
  4. Feb 16, 2015 #3
    You mean interstellar medium. if so, interstellar medium has much more ligther nucleus than stars have. But why do not come together and form a star ?
     
  5. Feb 16, 2015 #4

    mfb

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    More than stars have at the time they form a supernova (for the few stars that do that), yes.
    I thought we were discussing the situation where this happens?
     
  6. Feb 16, 2015 #5

    Bandersnatch

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    In most types of stars only a fraction of total hydrogen content undergoes fusion in the core. Only very dim, low mass stars (red dwarfs) have their fuel fully mixed (i.e., fully convective interior), allowing nearly 100% hydrogen-helium conversion. Red dwarfs don't progress past that stage, nor do they explode or otherwise expel their material.

    The stars of masses comparable with the Sun will expel their outer layers of mostly unenriched material back into the interstellar medium, leaving behind a remnant of fused carbon and oxygen (white dwarf).

    Only the upper-range massive stars (as supernovae type II), as well as some white dwarfs in binary systems (supernovae type Ia), will significantly enrich the interstellar medium with heavy elements.

    Over time, more and more of the freely-floating gas clouds in the galaxy get enriched by the heavier elements. But these elements remain a low fractions of the total mass. Mostly it's just the primordial as well as recycled hydrogen and helium.


    The reason all hydrogen in the galaxy doesn't collapse to form stars is that clouds of gas floating in the void are in a hydrostatic equilibrium - they are held together by their gravity, but their pressure keeps them from collapsing further.
    A cloud of gas needs to be either significantly cooled (hard to do if other stars are nearby) or compressed by some process (like a nearby supernova explosion) to initiate further collapse.
     
  7. Feb 16, 2015 #6

    ChrisVer

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    The big deal is in understanding that the Hydrogen of the star is not totally burned. The fusion process takes place into the core, because there is where the temperatures allow it. The external layers remain as they were for the previous star.
     
  8. Feb 16, 2015 #7
    There is a onion shell model, in the outermost layer Hydrogen burns and then migrates to the next layer as helium nucleus. Obviously, the nuclei heavier than hyrogen is burning very rapidly compare to hydrogen itself. But while the fuel is burning in the inner layer, more and more hydrogen has to burn to overcome gravitational collapse until consuming all the hydrogen in the outermost layer?
     
  9. Feb 16, 2015 #8

    Ken G

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    Another important point to realize is that high-mass stars are quite rare, so a lot of the material that is in the interstellar medium has never been processed through a high-mass star. Some of it has never been in any star at all, and a lot of it may have only ever been in low-mass stars like our Sun. Note that the Sun will expel a lot of hydrogen when it is in its two giant phases, and high-mass stars spew out a lot of hydrogen before they go supernova. So if you select a particle at random from the interstellar medium, it can have a very great variation in the kinds of stars it has been in, and how it was expelled from those stars, if it has ever been in a star at all. And even if it has been in a supernova, the definition of a "Type II" supernova is that the ejecta contains a significant amount of hydrogen. Even in the cases where a type I supernova comes from a high-mass star (called a "Wolf-Rayet" star), so there is not hydrogen in the supernova ejecta, the star has already spilled out a lot of hydrogen in prior stages of its evolution, before going supernova.
     
  10. Feb 17, 2015 #9
    Thank all of you. Your coments were very enlightening.
     
  11. Feb 17, 2015 #10

    D H

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    In addition to what everyone has already said, large stars are rather leaky objects. Even stars as small as our Sun expel a lot (a whole lot) of their hydrogen near the end of their life. Our tiny Sun will spew over half its mass whilst in its death throes. Larger stars expel an even greater portion of their mass before they die. Some of that expelled mass will be in the form of a planetary nebula, but a good chunk of it will be expelled out of the gravitational clutches of the white dwarf / neutron star / black hole that remains behind.
     
  12. Feb 18, 2015 #11
    A supernova occurs when a star has no hydrogen left to burn up.
     
  13. Feb 18, 2015 #12

    Ken G

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    You are probably thinking about the core of the star. A type I supernova has no hydrogen lines from its ejecta, but a type II supernova does have hydrogen lines in its ejecta.
     
  14. Feb 18, 2015 #13

    D H

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    A supernova occurs when something catastrophic happens in a star's core. This has little to do with whether the star still has hydrogen left to burn.

    Small stars (less than half a solar mass) are fully convective. These red dwarfs will eventually fuse almost all their hydrogen into helium. Larger stars have non-convective cores. The end of life for large stars involves an inert, non-fusing core surrounded by shells of fusing material. In our Sun, this end will be in the form of carbon core surrounded by a helium burning shell, which is in turn will be surrounded by a hydrogen burning shell, which in turn will be surrounded by non-fusing hydrogen. Larger stars will have a oxygen/neon/magnesium core; larger ones yet will have a iron/nickel core. This inert core essentially is a white dwarf that is hidden by shells of still-fusing material.

    The pressure that keeps that inert core from collapsing in on itself is due to electrons being squeezed very close together. That inert core is degenerate matter. Adding mass to a degenerate object makes it decrease in volume. This shrinkage gets very strong as the electrons become relativistic. It becomes very, very strong at the Chandrasekhar limit, where degeneracy pressure fails. The inert core in massive stars collapses to become a neutron star at the Chandrasekhar limit. This rapid collapse is the catastrophe that triggers a type II core collapse supernova.
     
  15. Feb 18, 2015 #14

    Ken G

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    We normally wouldn't call that the "end" for the Sun, as it still has some more evolution to go at that point. Eventually all the stuff outside the core drifts aways into space, leaving just the non-fusing core-- so that's the white dwarf the Sun will eventually become at the "end." Still, your point is that the Sun will lose all its hydrogen, and never undergo a supernova, while other stars will undergo supernovae without losing all their hydrogen, so that supports your contention that burning up all the hydrogen has no direct connection with undergoing a core-collapse supernova.
     
  16. Feb 18, 2015 #15

    D H

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    I had an overly long answer that went into that detail, but I decided to instead cut my answer short.

    I agree that what I called the "end" isn't quite the end. Eventually the white dwarf that already lives inside our dying Sun will be exposed for all to see by the Sun shedding what's left of its outer layers. This final shedding is just a continuation of a significant mass shedding process that begins when a star leaves the main sequence. (In fact, our Sun is shedding mass right now in the form of solar winds and coronal mass ejections, but if I recall correctly, this is small compared to the losses that will occur when the Sun turns into a red giant.)

    You mentioned type I supernova in post #12. Some core collapse supernovae are type I supernova rather than type II. That doesn't necessarily mean the star has used up all of its initial hydrogen. It just means that it has burned all the hydrogen that was left on the star before the supernova event. Very large stars are very leaky objects. They lose a lot of hydrogen (and sometimes helium) to the interstellar medium. That eventually can become food for a new star.
     
  17. Feb 18, 2015 #16

    Ken G

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    Yes, the mass flux in the solar wind as it is today would only account for about 10-4 solar masses lost in the Sun's main-sequence lifetime, though it may have lost a lot more than that already when it was much younger.
    Yes, that's why I mentioned that core collapse can be type II and have hydrogen, or if they are type I, they can have lost a lot of hydrogen in their past (but that was post #8 rather than post #12). So we are in complete agreement.
     
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