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Gravitational collapse

  1. Aug 19, 2013 #1
    Assuming a planet suddenly got enough mass to be a few orders of magnitude more massive than the sun, the planet would collapse to form a black hole, right(irrespective of the core material?)?
  2. jcsd
  3. Aug 19, 2013 #2

    D H

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    Planets don't do that. Planets are small. Bigger objects? They become stars.
  4. Aug 19, 2013 #3


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    DH is basically right, but you are asking a curious question. A "few" is at least two. Two orders of magnitude is a factor of 100 or so.
    So you are thinking of a planet suddenly acquiring 100 solar masses?

    Wouldn't it depend on what the planet was made of, and HOW suddenly, and in what manner?

    You could google "Type IA supernova"
    That is an explosion that leaves no remnant (no black hole, no neutron star) and that happens when mass is added to a small dead star by its larger binary partner.

    A small dead "white dwarf" star is not unlike a planet. It might have fused to where it's core is carbon and too cool and unmassive to fuse carbon. The supernova happens because the thing was NOT fusing and so, when the additional mass is added it starts thermonuclear fusion abruptly all at once

    But depending on what the planet was made of it might start fusing earlier, after you had added half a solar mass, say, so then as you added more mass it would simply turn into a larger star. It's energy would tend to support the mass you were adding, or even blow it off. A star's own light-pressure can tend to blow off outer layers and prevent further accumulation.

    But then you might hurl neutron stars at it, or other very compact objects. You might find some way to increase its mass beyond what could be done just by dumping ordinary matter.

    I think what would happen would be rather sensitive to the original composition of the planet and the MANNER you went about adding ≥ 100 solar masses to it.
    Last edited: Aug 19, 2013
  5. Aug 19, 2013 #4
    I know a planet can never become that massive. What I'm proposing is a hypothetical situation. Suppose mass is suddenly added(think of it as a computer program where we can add as much mass as we want without any effect and THEN run the program to see what happens) Now I want to know if the core materials will break down irrespective of what they are. What I'm basically asking is, will ALL core materials break down(iron,gold,lead, etc.) provided enough mass is added?
  6. Aug 20, 2013 #5


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    If atomic matter is subjected to enough pressure it will turn into neutron matter.

    This is how "core collapse" supernovas happen.
  7. Aug 20, 2013 #6
    Thanks. That cleared that doubt of mine. I'm studying star formation, and have several doubts. I think I can ask those questions here itself as they are directly or indirectly related to gravitational collapse(or the absence of it).
    Ok, here go:
    1. When the temperatures in the core of the star get sufficiently high, the electrons of hydrogen are ripped out of orbit and we get a plasma of protons and electrons. If the temperatures further rise the protons can come close enough for nuclear force to take over and fusion can take place to give helium. I wanted to know all the main forces at play in such a plasma. Of course there is the electrostatic repulsion between the protons, and the nuclear attraction if they come close enough. Also there is electrostatic attraction between the protons and the electrons but due to high kinetic energy the electrons ever get into orbit. Are these all?
    2. During the formation of a Red Giant, the helium core collapses under its own gravity, but the outer layer of the star swells up. Why? What force pushes them outward against gravitational collapse?
    3. And lastly, in some stable neutron stars, as shown by Landau, the repulsion between the nucleons keeps the nucleons from coming too close together and this is the neutron degeneracy pressure. The neutrons have their own identity here. If the mass of the star is too large and the neutron star ultimately becomes a black hole, I'm assuming the neutrons all collapse to a singularity and have no individual identity anymore?

    Thank you.
    Last edited: Aug 20, 2013
  8. Aug 20, 2013 #7


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    Do you want to include the weak force, which is responsible, alongside the strong force, for allowing proton-proton fusion to occur in the first place? Without the weak force the two protons would immediately separate again after they fused and no reaction would occur. It turns one proton into a neutron so that the two particles can fuse, releasing energy in the process.

    Per wiki: http://en.wikipedia.org/wiki/Red_giant

    When the star exhausts the hydrogen fuel in its core, nuclear reactions in the core stop, so the core begins to contract due to its gravity. This brings additional hydrogen into the a zone where the temperature and pressure are adequate to cause fusion and so fusion continues in a shell about the core. The higher temperatures lead to increasing reaction rates, producing enough energy to increase the star's luminosity by a factor of 1,000–10,000. The outer layers of the star then expand greatly, thus beginning the red-giant phase of the star's life. As the star expands, the energy produced in the burning shell of the star is spread over a much larger surface area, resulting in a lower surface temperature and a shift in the star's visible light output towards the red – hence it becomes a red giant.
  9. Aug 20, 2013 #8
    FYI, the protons actually don't get so close as for the strong force to overcome the electrostatic force, the fusion process (at least for a main sequence) is mediated by protons tunneling through the electrostatic barriers into the strong interaction potential well (then fuse).

    A quick answer would be... we have no idea what happens to matter as it is crushed at the singularity... but I'd guess that the answer would be a no, they have no identity anymore since a BH only has 3 observational properties (the no hair theorem) : Mass/Charge/Spin
  10. Aug 20, 2013 #9
    "FYI, the protons actually don't get so close as for the strong force to overcome the electrostatic force, the fusion process (at least for a main sequence) is mediated by protons tunneling through the electrostatic barriers into the strong interaction potential well (then fuse)."

    Could you elaborate on this please? I didn't get what you mean.

    Ok I got the Red Giant part. But regarding the fusion process, if a neutron has greater mass than a proton how does the fusion release energy?
  11. Aug 21, 2013 #10


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    Without quantum tunneling the protons would come very close, but not close enough, to fuse. Quantum tunneling is what gets them just a bit closer without actually needing more kinetic energy.

    If you add up the masses of the two protons before fusion, you will find that it is MORE than the combined masses of the neutron and proton after fusion. Part of the energy of the reaction is needed to turn one proton into a neutron, but the rest is released.
  12. Aug 21, 2013 #11
    Don't all protons have the same mass??!
  13. Aug 21, 2013 #12
    Yes, all protons have same mass.
    The details of Main Sequence fusion can be found here http://en.wikipedia.org/wiki/Proton–proton_chain_reaction#The_pp_I_branch

    since sticking two protons together into a diproton makes them unhappy (due to electrostatic force), it would want to decay into something more stable (deuterium) which is composed of proton+neutron in its nucleus. Now, the electrostatic repulsion is gone so it is in essence has less energy than a diproton thus it gives out energy in the process.
    BTW, don't confuse this with a neutron wanting to spontaneously decay into proton+electron+neutrino, that would be the case for an isolated neutron. Our current case has two protons involved thus raises the potential energy which would manifest itself as mass.
  14. Aug 21, 2013 #13


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    It seems that this depends on whether the hoop conjecture is true.
  15. Aug 22, 2013 #14
    Anything would turn into a black hole given that its massive and dense enough. A planet given more mass than the sun could turn into a black hole, but it would likely become a star or supernovae first.
  16. Aug 23, 2013 #15
    Ok thanks guys for all the answers.
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