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Neutron star question

  1. Jun 21, 2012 #1
    Ok, I know neutron stars are mainly composed of neutrons. But also, they have some protons and normal nuclei at their surfaces. Is this crust of protons needed to keep the neutrons below stable? As in, if it disappeared, would the neutrons below start decaying back to protons to form its 'protective' layer? Or would the neutrons be stable and not decay - the protons are just leftovers from the creation of the neutron star.
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  3. Jun 21, 2012 #2
    The neutrons and protons are in a stable dynamic equilibrium. If the crust somehow suddenly disappeared pressure would be greatly reduced, the equilibrium would change drastically, and many of the neutrons would decay. My guess is that there would be a cataclysmic explosion.

    The crust is made of iron nuclei polymerized by the intense magnetic field.
  4. Jun 21, 2012 #3
    Thanks. I didn't know there was an equilibrium between the core and surface of the neutron star. :)
  5. Jun 21, 2012 #4


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    Idea of neutron star is based on some rather shaky speculations. You have to realise that idea of neutron star was proposed around the same time as discovery of beta plus decay.

    Basically all the observations we have say that proton decays into neutron when it leads to stable configuration of nucleons. And electrons have no role in determining that stability. But idea of neutron star is based on reasoning that you can "push" electrons into protons and that way make them decay into neutrons.

    Yet another thing is that heavy version of electron - "muon" was recognised as such long after idea of neutron star was proposed. And the problem here is that when electrons become relativistic and very very energetic then speculation that they will decay into muons would be much more reasonable than speculation about them being "pushed" into protons. And muons are much more compressible than electrons as they have much bigger mass (and experience less degeneracy pressure).
  6. Jun 22, 2012 #5
    You misunderstand me. You can think of it that way, but I think it is an inferior view. The "interaction" of the crust and core is due to gravity and pressure. The extra pressure due to the mass of the crust affects the equilibrium in the core. If somehow there were an impermeable barrier between the crust and core I think it wouldn't make very much difference.

    While there is surely some sort of nuclear equilibrium between the two, I would expect it to be insignificant. There are protons and electrons in the core but they come from the decay of neutrons, not from elsewhere.
  7. Jun 22, 2012 #6


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    Have you heard of electron capture? Your post seems to suggest that you either haven't heard of it or don't believe it. Electrons are not "pushed" into the protons, at least not in any sense of the way I use the word "push".
    See here: http://en.wikipedia.org/wiki/Electron_capture
  8. Jun 23, 2012 #7


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    Yes, I know about electron capture proton decay. And I have read this wikipedia article.
    It describes quite specific case of proton decay. Say couple of quotes from wikipedia to illustrate this:
    "Note that a free proton cannot normally be changed to a free neutron by this process: The proton and neutron must be part of a larger nucleus."
    "Radioactive isotopes that decay by pure electron capture can, in theory, be inhibited from radioactive decay if they are fully ionized ("stripped" is sometimes used to describe such ions)."
    So it is believed that this kind of decay can not happen in plasma with fully ionized ions.

    This of course might change for high densities but there are just not enough experimental facts to make reliable speculations IMHO.
  9. Jun 23, 2012 #8


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    We work with the knowledge we have at the time. It's simply the best explanation we have at the moment. If in the future we learn something new, then we will apply that knowledge to neutron stars and see if something changes. Call it "shaky" if you want to.
  10. Jun 23, 2012 #9
    Not really. The neutrons at the center are that way because of the pressure and temperature.

    If you take something at the center of the neutron star and then reduced the pressure and temperature, it would turn into iron nuclei.
  11. Jun 23, 2012 #10
    This really isn't the case. You can do nuclear experiments to see what happens with the material. There might be some weird stuff happening in the core, but it's mostly neutrons. There is still a lot of "unknown stuff" when it comes to quark physics, but the physics of leptons is pretty well known.

    This won't work. In order to change an electron to a muon, you need a source of muon neutrinos. Electrons just don't spontaneously turn into muons. It violates conservation of electron number and muon number.

    Now you *can* get muons via things like reactions with protons and neutrons, but people have already added them into the calculations

    Last edited: Jun 23, 2012
  12. Jun 23, 2012 #11
    There are. The densities we are talking about are nuclear densities and you can simulate the reactions by throwing nuclei at each other. The other thing is we are talking about lepton processes, and the theory behind that (i.e. electroweak theory) is very well established. One thing that is very well established is the electrons don't spontaenously turn into muons. They just don't. We don't see this happening in our experiments.

    There are also some very well established princples like conservation of mass, and conservation of energy, and similar conservation laws. One conservation law is conservation of electron number and conservation of muon numbers. Also you can ask "what happens if those laws aren't conserved". If electrons were spontaenously changing into muons, this would radically increase the Chandraseakar mass to about 400 solar masses rather than 1.4. This isn't what we see.

    One reason I like neutron star physics is that we are talking for the most part about known nuclear physics, and you can't make up random stuff. Also, if you change the physics it has some pretty dramatic changes, so that "we don't know' also doesn't work.
    Last edited: Jun 23, 2012
  13. Jun 25, 2012 #12


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    Is it the "best explanations" or the "only speculation"? But please do not misunderstand me, I am not saying that we shouldn't have baseline for our research. Instead I am saying that if we haven't established that part as scientifically confirmed solid theory then we shouldn't build further speculations on top of it.
  14. Jun 25, 2012 #13


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    Yes, for weak interactions we have tested model that works quite fine. But is there updated model of white dwarf collapse into the neutron star that takes into the account all we know about weak interactions?

    Well, but muons do turn into electrons spontaneously. And in the process appropriate neutrinos are emitted. Actually W bosons decay into any of the three leptons (+ appropriate neutrino) with almost equal branching ratio. At least that is what wikipedia says about it - http://en.wikipedia.org/wiki/W_boson#W_bosons

    As I understand this paper speaks about hypothetical neutron star cooling by emission of neutrino/antineutrino pairs. So it's very specific calculation where they have added muons.
  15. Jun 25, 2012 #14


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    I believe that the sentence you where commenting was about protons not turning into the neutrons when they are bombarded by fast electrons.
    So do you think that there are experimental facts that would indicate the opposite i.e. that protons turn into the neutrons when they are bombarded by fast electrons?
  16. Jun 25, 2012 #15


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    Considering that we may never be able to make it a "confirmed solid theory" since we may never be able to visit a neutron star itself, what are our options? We have a very good understanding of atomic and subatomic particles and how they interact with each other and using this knowledge to make predictions about neutron stars doesn't seem like too big a step to me. What's the difference between this and guessing that Jupiter has a metallic hydrogen layer underneath it's outer atmosphere? We haven't been inside Jupiter that far, so we "don't know for sure", but since we know how hydrogen behaves we can make predictions based on our knowledge.

    And this is by no means just something done in astronomy and astrophysics. This is done, to some extent, in all areas of science. We use our knowledge to make predictions about the universe. When we find out something new, or that something we thought we understood is incorrect our predictions change accordingly.
  17. Jun 25, 2012 #16
    Most models used Steve Bruenn's electroweak reaction rates and the Lattimer-Swesty equation of state.

    The Appendix C of this paper goes into the gory details.....


    It turns out that we don't know how the collapse process works, but it's something other than particle physics processes and EOS. You can change those, and within the limits of known physics, it doesn't make much of a difference.

    Right. But electrons don't turn into muons without a source of muon neutrinos. The only physical process that breaks electron/muon number conservation is the MSW process in which free streaming neutrinos change flavor. That's important for solar neutrinos, but it's not for supernova because the neutrinos get reabsorbed pretty quickly after emission.

    Bruenn's paper also doesn't include massive neutrinos. It's not hard to change the equations to include those, but it doesn't make a difference.

    All these calculations are very specific. For leptons we have a firm theory that gives cross sections and we have experiments that confirm the theory. For baryons, there is a lot more that is unknown because quark-quark interactions are hard to measure.

    In cosmology, you can get away with inventing new particle processes, but with neutron stars, we are talking about energies that can be simulated in particle accelerators, so inventing a new reaction process is like inventing a new element. You have to argue *really* hard to get people to believe you.
  18. Jun 25, 2012 #17
    These reaction rates are very well studied. If you throw an electron with energy X1 at angle Y1 at proton with energy X2 and angle Y2, then event Z will happen with probability Z1. Appendix C of Bruenn's paper has the gory details on how to calculate this, and the reaction rates are in fact what we see when we in fact throw electrons at protons.

    It's high energy chemistry.
  19. Jun 25, 2012 #18
    Our theories make some very strong statements on some things. For example, lepton interactions have a very firm theory, and that theory just says that "nothing weird will happen with electrons, muons, and tau leptons" in neutron stars. You ask electroweak theory what happens to stuff at neutron star densities, you get firm numbers.

    The theory might be wrong, but there is no reason to think that the theory is wrong. One thing is that we can get electrons up to 90 GeV on earth, whereas the energies for neutron stars is 100 MeV.

    For stuff to do with quarks, we can't get numbers that are quite as firm. When you increase things to neutron star densities, you get all sorts of effects that we don't know how to calculate, so we don't get firm numbers.

    Some things we have strong knowledge of, some things we don't. Lepton processes are something that we have strong knowledge of. Baryon processes we don't, and then magnetic fields and convection are total mysteries.

    And sometimes it doesn't matter. If you build a dog house using Newtonian physics, then it turns out that GR effects don't matter. Same with neutron stars. In a lot of situations, you can ask "how wrong do we have to be before it makes a difference?" and it turns out that there is a huge room for error.
  20. Jun 26, 2012 #19


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    This paper does not have Appendix C - it ends with references.

    And we know about particle physics in "open" environment. We have theoretical ideas about particle physics in a volume of dense plasma. These ideas might work just fine if we have reasonably correct model for degeneracy pressure. And I doubt that.

    Electrons do not turn into muons directly with or without neutrinos. Just like muons do not turn into electrons directly with or without neutrinos. They can do that only through intermediate state of W- boson. Are we on the same line so far?
  21. Jun 26, 2012 #20


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    Give some reference that works. Or alternatively we can try to go trough the details ourselves.
  22. Jun 27, 2012 #21
    It's on page 822. Appendix C - Derivation of the Weak Interaction Rates

    Any particular reason? Degeneracy pressure is just Fermi-Dirac statistics. You count the number of states, count the number of particles, put it into a canonical ensemble, and boom, you have the equation of state. It's pretty well covered in any graduate level textbook on solid state physics or thermodynamics (i.e. Kerson Huang's or Kittel/Kromer).

    If there is a particular solid state/HEP phenomenon that you think isn't being modeled that will make a big difference, then it would be useful to state what that is.

    I just care about the cross sections, and conservation of electron/muon number makes that cross-section zero for the energies that are relevant here.

    I'd be *very* interested if you can dig up a reference for someone that has done the calculation that Bruenn did which shows a significant generation of muons in neutron stars.
  23. Jun 27, 2012 #22
    Last edited: Jun 27, 2012
  24. Jun 27, 2012 #23
    Also here is a paper that describes what happens if you put in "new physics" (i.e. flavor violation lepton processes)


    IMHO, it doesn't make much of a difference.....
  25. Jun 27, 2012 #24


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    Thanks, I will look what I can make out of it. And thanks for other papers too.

    Yes there is particular reason.
    When degeneracy pressure is calculated for astronomical bodies it is assumed that distribution of charged particles is homogeneous. But electrons might (should) be partially squeezed out of the body due to larger degeneracy pressure.
    Another thing is that degeneracy pressure should be higher in the middle of the body.

    Look, I am trying to get us on the same page. If I read about muon decay in wikipedia the things you say do not make much sense.

    Can you read that page (the part about muon decay)? It does not speak about any cross sections. It speaks about "decay width" and "Fermi's golden rule" and these do not seem to be related to any cross sections.

    So what I am missing?
  26. Jun 27, 2012 #25
    Also, if you are interested in the nuclear equation of state.....


    And several other EOS that are mentioned in this paper

    New Equations of State in Simulations of Core-Collapse Supernovae

    And this one

    Equation of State for Proto-Neutron Star

    And if you want to take a walk on the wild side (hyperions!!!! strange quark matter!!!!)

    Hadron-Quark Crossover and Massive Hybrid Stars with Strangeness

    I'm a lot less familiar with that physics, since I was able to get numbers out by using Lattimer-Swesty as a "black box". For the radiation hydro part, I had to tinker with the particle cross sections and implement some of these equations, so I know a bit more about what goes into that part of the code.
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