Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

B "Gravitational Compression in Neutron Stars"

  1. Jan 14, 2016 #1
    What happens to the neutrons in a neutron star as it collapses Into a black hole?
  2. jcsd
  3. Jan 14, 2016 #2


    User Avatar
    Gold Member

    Same thing as happens to ALL matter that gets into a black hole, it disappears into the singularity. Now this is not believed to be physical but it's what the current model shows. Expectations are that if/when loop quantum gravity becomes a solid theory we might understand what's REALLY happening, but for now we don't.
  4. Jan 14, 2016 #3


    User Avatar
    Science Advisor

    We don't really know what happens to anything inside a black hole.
  5. Jan 17, 2016 #4
    We don't know that neutron stars collapse into black holes. Maybe a better question is what happens to neutrons if there is core collapse in a neutron star.
  6. Jan 17, 2016 #5
    Rephrasing: Total neutron collapse would mean star collapse, but does that happen in reality? If some neutrons collapse in a neutron star do all neutrons collapse? Maybe a better question would be: What happens if there is some neutron collapse in a neutron star?
  7. Jan 17, 2016 #6


    User Avatar
    Staff Emeritus
    Science Advisor

    Pretty much, yes, because if the core of the star starts to collapse, the rest of the star suddenly has nothing supporting it and collapses inward as well.
  8. Jan 17, 2016 #7
    Collider experiments show that when a nucleus collapses what is produced is from 1% quark type matter and 99% energy to 10% quark type matter and 90% energy. What if a small percentage of core neutrons (<0.01R) collapsed into this instead of nothing? Normally we think of “photons” as weightless but here there would briefly be zillions of tons of photons with a pressure of (rho)(c^2)/3. I think this explosive pressure would temporarily heat and support the neutron star, or blast out of the star if it had a channel. A magnetic solenoid is an easy way out.
  9. Jan 18, 2016 #8
    If the pressure at the center of a neutron star were to exceed the limits of neutron degeneracy pressure, then the neutrons would presumably start to collapse into a black hole. If this black hole were small enough (e.g., on the order of a few thousand tonnes), then the radiation pressure from Hawking radiation could potentially be high enough to arrest further collapse. Don't know whether it would be stable, though.
  10. Jan 20, 2016 #9
    That would only be true if the collapsed neutrons had a volume that approached zero.
  11. Jan 20, 2016 #10

    Jonathan Scott

    User Avatar
    Gold Member

    What makes you think so?

    As far as I know, even if neutrons did not collapse, GR says that a large enough neutron star would become a black hole anyway.

    If neutrons do collapse, then if they collapse to a sufficiently dense form, it would merely cause a black hole to occur at a lower mass.

    On the other hand, if they collapse to a form which is not sufficiently dense to cause an immediate black hole, then what happens beyond that would depend on the nature of that form and in particular the pressure it could support, but that form would also be certain to collapse to a black hole at a smaller mass than if it were able to remain as a neutron star because it would have greater density.
  12. Jan 20, 2016 #11
    Let me rephrase the statement:
    That would only be true if the collapsed neutrons had significantly less volume.
  13. Jan 20, 2016 #12
    Not necessarily. The density at the center of a neutron star is believed to exceed that of an atomic nucleus: 8e17 kg/m3. Of course, such high gravity is going to warp space pretty significantly, so Euclidean geometry doesn't exactly hold here...but taking the Euclidean approximation, a core which grows to 4.8 solar masses at this density will become a black hole in its own right without needing to collapse at all. If quark-degenerate matter starts to form at the core of a neutron star as neutrons begin to break down, then the density is expected to be around 1.7e18 kg/m3; such a quark-matter core would satisfy the condition for a black hole with just under 3.5 solar masses. A non-Euclidean formation would likely decrease these requirements significantly.
  14. Jan 20, 2016 #13
    So far there are about 2000 observed neutron stars all with a maximum mass limit of about 2M☉. If neutron stars collapsed directly into black holes there should be black holes starting at 2M☉ but none have been observed yet. To me it looks like there is some kind of process intrinsic to neutron stars that limits their mass to about 2M☉.
  15. Jan 20, 2016 #14
    What if that new form was ultra relativistic quark matter? Ultra relativistic matter would either heat or escape the star.
  16. Jan 20, 2016 #15

    To begin with, there needs to be an explanation on the formative side of things. Current models suggest strongly that there is a certain mass/metallicity threshold required for the formation of a black hole by a collapsing star which ensures black holes will have at least five solar masses. Below this threshold, the collapse is much more energetic and will accelerate most of the star's material away, leaving no more than two solar masses to collapse into a stellar remnant. However, this fails to explain why a neutron star could not subsequently grow above this threshold. There are a few possibilities for neutron stars which exceed approximately two solar masses (by accretion or by a different kind of collapse):
    1. They immediately collapse, with the collapse generating enough strong-interaction-bound and gravitational-potential-bound energy to exceed the relativistic gravitational binding energy of the object, blowing it apart completely.
    2. The pressure at the core begins to "burn" neutrons by collapsing them into quarks, and that energy somehow escapes into polar jets.
    3. They exceed two solar masses without incident, but this is so rare that we have not yet discovered one. Or, if we have discovered one, it isn't in the right place to have its mass measured so we don't know yet.
    If 1 or 2 above are correct, it should be noted that this eliminates the need for an explanation on the formation side of things; a neutron star COULD form with a mass greater than 2 solar masses, but it would blow itself up (or, in the other case, shrink) rapidly. If the answer is 3, then the formative explanation is still needed.
  17. Jan 20, 2016 #16

    Jonathan Scott

    User Avatar
    Gold Member

    In the case you quote, the energy comes from the collider.

    As far as I know, unless baryon number can be violated (which would be a non-mainstream assumption outside the scope of these forums), the effective rest energy (including internal kinetic energy) of the components of a neutron cannot be less than that of a proton, and quarks cannot be isolated, so very little additional kinetic energy can be obtained by breaking down a neutron into its components.
  18. Jan 20, 2016 #17
    So the binding energy between the quarks in quark-degenerate matter or a quark-gluon plasma is exactly identical to the binding energy between the quarks in a neutron? That doesn't quite make sense; breaking a bunch of neutrons down into quark-degenerate matter ought to release at least some of the strong-interaction-binding energy that kept the quarks in a baryonic configuration. Baryon number wouldn't be violated because you still have the same number of quarks, right?
  19. Jan 20, 2016 #18
    So are you saying when a 1000 MeV neutron disintegrates all we get out of it is some quarks with about 10 MeV rest mass?
  20. Jan 20, 2016 #19

    Jonathan Scott

    User Avatar
    Gold Member

    This appears to be a personal theory of yours which you have already posted in some other threads, and I pointed out that you should start a new thread and provide acceptable references if you wished to continue to discuss it.

    Your idea that something inside the neutron star could have enough energy to rise to the surface and escape does not make sense from an energy point of view.

    As most of the energy per particle is simply derived from gravity, the only way for anything other than electromagnetic radiation and neutrinos to escape from the surface is if there is some effect such as a significant fusion explosion of accumulated matter which generates a huge amount of energy over a very short time. That could then result in a flash of neutron star surface material being ejected into space, as a cloud or shell containing traces of elements such as iron.
  21. Jan 20, 2016 #20

    Jonathan Scott

    User Avatar
    Gold Member

    Baryon conservation and the fact that quarks can't be isolated together mean that per original neutron the internal kinetic energy of the bound systems of quarks plus the rest mass of any components with rest mass cannot add up to less than the mass of a proton. If there is sufficient energy around, then obviously one can create additional matching particle/antiparticle pairs or equivalent, but the quarks and gluons cannot "cool" back to anything less than a proton.
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook