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Why neutron stars emit light?

  1. Oct 5, 2015 #1


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    In a neutron star gravity has overcome electron degeneracy pressure allowing the protons and electrons to combine into neutrons. But if that is the case, how do neutrons emit light if there are no electrons to swoosh the photons away?
    I have been thinking, and wonder if the surface (or "crust") of a neutron star might not have enough pressure to be able to overcome the electron degeneracy pressure, OR neutrons are released from the fast spinning surface, and in a split second turns into protons and electrons - and because of that the neutron star appear to emit light?
    Any thoughts?
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  3. Oct 5, 2015 #2
    The star emits light because it is hot. The crust is heavy metals, not neutrons, and hot heavy metals glow.

    Neutron stars are very hard to see from a distance because they are so small. Mostly we see the hot matter that orbits or is falling into them.
  4. Oct 5, 2015 #3


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    You were misinformed somewhere along the way. Photons do not need electrons "to swoosh" them away. Photons are quite capable of "swooshing" themselves away.

  5. Oct 6, 2015 #4


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    Like a white dwarf, neutron stars do not produce energy via fusion in their cores. The primary energy source for a degenerate star [neutron stars and white dwarfs] is inherited from their progenitor. They radiate away this residual energy at a very leisurely pace. Degenerate matter has poor thermal transport capability [i.e., it is an effective insulator]. Degenerate stars are expected to remain quite hot for much longer than the life span of their progenitors and should, in theory, require more time than the current age of the universe to reach thermal equilibrium [become black dwarfs]
  6. Oct 9, 2015 #5


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    The crust consist of matter that is several billion times harder and more solid than steel. I do not think any metal in the periodic table have those properties.
    I have also learned recently that neutron stars aren't solid or have uniform density, hence the crust.
    Electrons do emit photons, but neutrons alone cannot emit light, but when they leave the surface as particles they "dissolve" into protons and electrons (Or neutrinos).
    Now they can emit light. Maybe I misunderstood the program on TV "The Universe" about pulsars and quasars that I watched yesterday evening.
  7. Oct 9, 2015 #6
    No, they aren´t. Protons are emitted by accelerating charges, which require certain asymmetries to radiate. The charges need not be electrons - protons or any other charges will do. But neutrons are uncharged.
    Thus, neutronic matter devoid of electrons would be clear and colourless to photons. Even a small amount of polarizability with symmetries that do not allow absorption would only allow the neutronic matter to refract and Rayleigh scatter photons - they would look white, not black, and be unable to absorb photons or emit any, no matter how high the temperature.

    But the surface of a neutron star does not consist of neutrons alone - indeed, no matter how big the gravity field, at the surface pressure and density go to zero. The thin surface layer does contain electrons, and furthermore possesses the necessary asymmetries to absorb and emit photons at at least some wavelengths.
  8. Oct 9, 2015 #7


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    The surface of a neutron star is likely mainly composed of iron nuclei.
  9. Oct 9, 2015 #8


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    OP is - to me - an interesting question. Although I wouldn't say that gravity "allows" electrons and protons; I'd say gravity FORCES electrons and protons...Anyway, a couple of things should be mentioned here. 1st is that all this is theoretical - we've no first-hand experience with the (bulk) properties of neutronium. Specifically, it isn't clear to me that we know enough about its properties based on the models we've built to have good answers about its appearance and composition. 2nd is that it is an ERROR to assume that a neutron star (or its interior) is composed soley of neutrons. Thats just not right, there will be some electrons, positrons, protons and antiprotons present...at least that is my current (but unsophisticated - I'm NOT well versed in this area) understanding. This is not to say that those particles are stable and any one of them exists for more than a short time. Please consult the wikipedia article! 4th - Its not very meaningful to speak about the Periodic table; matter in the conditions on a neutron star are hardly meaningfully related to matter on Earth - at least in terms of its properties and characteristics. (Just consider the hydrogen in the Sun's core - its density is estimated(!) as more than ten times the density of solid iron (on Earth), and yet we call it "a ball of gas".) Lastly, we ought not to forget that we ascribe HUGE magnetic fields to these critters, claiming that they aren't electromagnetic should invoke a "D'oh!"
  10. Oct 9, 2015 #9
    Oh yes they do. The surface of a neutron star is iron nuclei. They are extremely compressed by the pressure so the density is much great than that of iron here on Earth. The iron nuclei are much closer together. The extreme magnetic fields make a difference as well, as they polymerize the iron nuclei. That's why they are so much stronger.

    Right. The density increases the deeper one goes.

    The crust is about a kilometer thick, of iron and heavier nuclei. Then it is heavier metals with extra neutrons packed into the nucleus, then free neutrons get mixed in. Eventually there is the core, which is superfluid and superconductive. It is mostly packed-together neutrons, but always has at least a few percent of electrons and protons.

    There is a recent theory that all metals in the universe heavier than iron come from neutron star crusts. It could be.

    There are no free neutrons on the surface of a neutron star. There isn't enough pressure there.
  11. Oct 12, 2015 #10
    How does the radiation from neutron stars compare to white dwarfs? Are neutron stars white? Which is brighter?
  12. Oct 12, 2015 #11
    I guess the emission signature of Iron and maybe one or two other heavier elements ought to be detectable, but quite likely redshifted.
  13. Oct 13, 2015 #12


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  14. Oct 13, 2015 #13
    There could be a reasonably (10%?) accurate gravitational pressure calculation for the core pressure of a 2 SM neutron star. It would be interesting to compare this pressure to (rho c^2)/3. I trust the Newtonian calculation more than the relativistic calculation but would like to see both. Isn't the best density profile about the same as that of a gas star: 1 -(r^2)/(R^2)? I think this gives a core density of 2.5 times the average density.
  15. Oct 13, 2015 #14
    I have read that density increases more slowly in degenerate matter than it does in a gas.
  16. Oct 13, 2015 #15
    I think it's as others here have already stated - the gravitational force near the surface of a neutron star is usually not enough to produce a pressure greater than the neutron degeneracy pressure. The outer surface of a neutron star will mostly consist of ionized gas which can emit EM radiation.

    If the neutron star is spinning (a pulsar), then there will usually be a misalignment between the axis of rotation and the magnetic field axis of the star. The moving magnetic field will generate an electric field, which will accelerate any electrons in the vicinity of the neutron star (mainly electrons in the outer ionized gas 'shell' of the neutron star). These electrons emit EM radiation as a result of being accelerated in an electric field (as a consequence, pulsars lose energy and slow down).

    I'd like to restate what snorkack said: neutrons, though electrically neutral themselves, are composed of one up quark and two down quarks, which are charged themselves. It is possible for a neutron to interact via the EM force (scattering of the quarks can take place inside the neutron when it interacts with a high energy photon), although it cannot absorb a photon.
  17. Oct 13, 2015 #16
    The surface of a neutron star is iron. On at least one star there is a 4-inch-thick atmosphere of ionized carbon which emits X and radio waves.
  18. Oct 13, 2015 #17
    I think the pressure formula for ultra relativistic degenerate matter is the same as for light, P = (rho)(c^2)/3. This describes a gas.
  19. Oct 13, 2015 #18
    Yes, but in some hot neutron stars, the surface is in a fluid state.
  20. Oct 13, 2015 #19
    I'm always ready to learn something new, but I'm not quite willing to take your word for it. How do you know that?
  21. Oct 13, 2015 #20
    But are those neutrons ultra relativistic? I've been told that the electrons are, but that is a small component of the mass.
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