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I heard the term the other day, but it seems strange to me. My understanding is that neutron stars are made up of neutrons; and neutrons (having no charge) shouldn't be magnetised.

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The neutron has no net charge, but it does have a net magnetic moment. As an aside, this simple fact provides strong evidence that the neutron is a composite particle (made of smaller things like quarks and gluons), because if it were a neutral elementary particle we would not expect it to have any magnetic moment.

But we know that the neutron is composite, and it definitely has a magnetic moment (it can be measured). Therefore if you have a macroscopic object made of neutrons whose spins are polarized (their angular momentum vectors tend to point one direction rather than another), it will be magnetized. In fact, it will be very strongly magnetized compared to ordinary matter simply because nuclear matter is so much more dense.

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Could you please explain "net magnetic moment"? –  Smashery Jan 31 '11 at 10:01
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Everything you say in this answer is true, but it is not the mechanism which gives rise to large magnetic fields in neutron stars. Any review on neutron stars that discusses the magnetic field will emphasize the role of having a large conductivity which freezes in the field lines and the large conductivity comes from the electrons and protons. The effects due to the neutron magnetic moment are rather small I believe. –  pho Jan 31 '11 at 13:58
    
I remember that the picture of neutron stars is a layered structure, neutrons as core, a layer of neutrons/protons/electrons and on top "normal" matter in some cases. The collapse of that upper layer (whem growing by accretion) is thought to be source of Gamma bursts. Right? –  Georg Jan 31 '11 at 18:09
    
The core might be more complicated with some quark matter, or a kaon condensate or other such things. If it is more or less ordinary nuclear matter then there have to be protons and electrons as well as neutrons for chemical equilibrium due to beta decay. I don't know about gamma bursts, but "glitches" where the pulsar frequency changes abruptly may be associated with "starquakes" in which the crust breaks and rearranges itself. Not sure if that is the latest model, but it was popular some years ago. –  pho Jan 31 '11 at 18:46
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In spite of the name, neutron stars also contain protons and electrons. This is required for equilibrium with respect to weak-interaction processes which can convert neutrons into protons and electrons. Since neutron stars also contain protons and electrons, they can contain electric currents which generate magnetic fields. It is thought that the protons actually form a superconductor and the magnetic field in most of the interior of a neutron star is carried my quantized magnetic vortices, much as in a type II superconductor.

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