How do neutron stars emit light?

  1. It is my understanding that when an electron drops to a lower orbital that 2 photons of light are produced. The moving electric charge produces EM radiation just like moving charges in a radio transmitters antenna produce EM radiation. Energy in the form of radiation, heat, or whatever else, can bump electrons up into higher orbitals, preparing them to drop again and emit more photons.

    If all the matter in a neutron star is neutrons then there are no charged particles. None of the neutron matter should emit, absorb, or interact with EM radiation, so how can such a star shine?
  2. jcsd
  3. Chronos

    Chronos 10,348
    Science Advisor
    Gold Member

    Obviously, they are no longer powered by fusion processes, but, there is an abundance of other energy sources. Neutron stars emit vast quantities of neutrinos [via URCA processes] and thermal photons. The 'mantle' of a neutron star is believed to largely be superfluid and rotates at various speeds relative to the crust. Given neutron stars have very high rotational speeds to begin with, this results in a lot of frictional heating. Accretion also causes photon production.
  4. Drakkith

    Staff: Mentor

    Also, the surface of the star may not be made up of Neutrons, but of normal degenerate matter, with the star turning into Neutrons further down as the pressure increases. (At least I think so)

    And remember that while a neutron is uncharged it is composed of charged quarks. Though I'm not sure if that would have an effect on the light emitted.
  5. DRAK

    Neutron stars do carry a vast amount of kinetic energy that could be released as electromagnetic radiation. [Reference magnatar v pulsar]. Still. I have to wonder if a neutron star can ever cool down! Lets assume a given neutron star no longer RECEIVES electromagnetic radiation, AND spins down to zero angular momentum.

    Does it have a temperature above absolute zero? I have a more difficult time thinking about neutron stars then I do about black holes. Black holes have relativistic properties in which time comes to a near stop. Accordingly, they can go on, literally, indeffinitely. Neutron stars, on the other hand, are just very dense matter.
  6. Drakkith

    Staff: Mentor

    How does a neutron star differ from black holes in a relativistic aspect? The effects are identical other than magnitude. You would most definately experience severe time dilation on the surface of a neutron star.

    Also, I don't see why a neutron star would be unable to cool down. If there is thermal energy in the motion of it's neutrons, then I would think it would eventually cool down by emitting radiation.
  7. One photon. But that's only one way of making light. There are others. You can get light, anytime any particle changes energy states, and those particles can be electrons, neutrons, what ever. Usually when neutrons are involved, the energies are higher so you tend to get gamma rays rather than light, but the principle is the same.

    It's doesn't. Neutron stars don't have any internal shine. Now what can happen is that if you dump some matter onto the neutron star a lot of the gravitational energy gets converted to EM radiation, and so neutron stars (and black holes for that matter) can be bright X-ray sources.
  8. Drakkith

    Staff: Mentor

    So the neutrons do not produce EM radiation themselves? I know they are neutral, but they are composite particles made up of charged quarks, so I thought that might make it different.
  9. They can. If you move electrons between different energy states, they can produce EM. Same thing happens with neutrons.
  10. Drakkith

    Staff: Mentor

    Will this happen with neutrons that make up a neutron star?
  11. I'm pretty sure that it can and does. Neutron star material is quite hot and as it cools, I'm pretty sure that it gives off E&M as the neutrons in the star rearrange themselves.

    The problem is that this energy is insignificant compared to the energy that is generated by the material that is getting dumped onto the star.
  12. Drakkith

    Staff: Mentor

    Awesome. Thanks twofish!
  13. Infalling matter producing radiation before it gets compressed into neutron matter makes complete sense to me. I still don't understand though how a neutron changing energy states produces EM. Is it EM from the charged quarks inside? I know neutrons have no E field but do they have a B field? If neither of these ideas is correct then my immagination fails me on how a neutron can emit EM.
  14. Drakkith

    Staff: Mentor

    I would assume that it is the quarks inside that produce it since they are charged, but I don't know.
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