I can remember reading somewhere (think it was Scientific American, but not sure, so sorry, no cite) that neutron stars can be covered with a shell of normal matter, especially if it is acreting. This material shines due to the extreme (near fusion) temps that itis at due to gravitational compression, and radionic heating from the intense radio and magnetic fields. Additionally, the neutronium itself is not exactly stable, and shifts (star quake) emitting vast amounts of energy.
Sorry for the lack of cites, and if my info is old, or incorrect.
I remember reading about this after I read Larry Niven’s story “Neutron Star,” which described “BVS-1”, Known Space’s first-discovered “old and cold” neutron star (that is, not a pulsar). At 1.3 times the mass of the sun, it was described as “11 miles of neutronium, covered by about half a mile of degenerate matter, followed by perhaps a dozen feet of ordinary matter.”
At the time, I was wondering if that was accurate, so I went looking. From what I found (nearly 20 years ago) it was thought to be, more or less.
Again, IANAPhysicist, but there’s several physical mechanisms by which something can emit light. It sounds like there’s a few processes going on in a neutron star (like accretion or ionized electrons), but it doesn’t sound like it’s in such a forced state of matter, like a black hole (at least at the surface), in that electrons are still free to do their quantum dance, and photons can careen through the universe like any other sufficiently heated ordinary matter. Basically… it glows like a light bulb, a hot iron poker, or… the sun (i.e. incandescence; as part of the radiation begins to enter the visible part of the spectrum).
cmyk is right, I think. In brief, everything at a certain temperature emits thermal radiation – as to how this works, consider that every moving charge emits electromagnetic radiation, a greater than zero temperature means that there’s movement somewhere, so whatever’s charged there is gonna emit – if there’s a sufficiently high temperature, this emission will be visible light; at roughly 300K (room temperature), the emission will be in the infrared, which is why you can use those neat goggles to see people in the dark (they will typically be slightly hotter than their surroundings, so glow brighter (and in a slightly different ‘colour’)). That’s why hot metal, or other hot stuff, glows visibly.
So the reason neutron stars glow is (apart from accretion) simply that they’re hot.
The general idea of there being layers is accurate, but the specific numbers aren’t. The layer made up of ordinary atoms is somewhere in the vicinity of a fraction of a centimeter. Then there’s maybe a few more centimeters of neutron-rich atoms (isotopes which wouldn’t be stable under Earthly conditions), then free neutrons start dripping out in a sort of transition region, then you have the neutronium (which still has an inmix of about 10% protons, and a corresponding number of electrons-- It’s never pure neutrons). Finally, in the core, you may (though nobody knows for sure) have more exotic particles like pions, strange baryons, hyperons, or quark-gluon plasma. And we’re not entirely sure of the total radius, but it’s probably closer to 10 km than 10 miles.
This is a nice question. In order for the motion of matter (which is what heat is) to produce electromagnetic radiation, we usually say the matter has to be charged. That is, we normally say only charges interact with radiation – which is why you need free electrons in an antenna to respond to radio waves, why the acceleration of electrons when they change states of motion leads to the absorption or emission of photons, and so forth.
So what do we do about neutrons, which are uncharged? Well, in fact the statement that electromagnetic radiation interacts only with matter that is charged is incomplete, or perhaps one might say is a shorthand for a more complex statement, which is that EM radiation interacts only with matter that has some electromagnetic nonuniformity, or can be induced to have some. It could be a net electric charge, and in fact this is the strongest and most relevant (on Earth) interaction. But EM radiation will also interact with matter that has (or in which can be induced) an electric dipole, quadrupole, et cetera moment, and with matter that has a magnetic dipole, quadropole, et cetera moment. And as it turns out, the neutron has a permanent magnetic dipole moment and an electric polarizability (because it has a finite size).
So while the interaction of accelerating neutrons with EM radiation is much much weaker than is the interaction of accelerating electrons, it isn’t zero. Consequently if a neutron star had a high temperature, which necessarily means many neutrons are in excited states, with unoccupied lower energy states beneath them, then when the neutrons fall to the lower states they can indeed emit photons. I would guess because the coupling is so much weaker that their lifetime in the excited state is way longer than it would be for electrons, but I don’t think this affects the final result, which is that even a “bare” neutron star – nothing but neutrons, if such a thing could exist – would still emit blackbody radiation if it could be described as having a finite temperature*.
Note that this is actually a severe restriction, though. Because of the exclusion principle, the neutrons will stack up into higher and higher energy states, just like electrons do in metals. Because there are so many neutrons, the highest occupied energy states even at T=0 are very high energy. (This is also the case in metals: the highest energy electrons even at T=0 have a lot of energy.) Now, in order to produce a Boltzmann-like distribution of neutrons in excited energy states, you’re going to need the thermal energy available (~kT) to be something in the ballpark of what’s called the Fermi energy, the energy the highest occupied neutron state would have at T=0. This is going to be enormous, for a neutron star. So you need a very high temperature. If the neutron star were much below that temperature, then there really wouldn’t be many neutrons in excited states. That doesn’t mean the star couldn’t emit or absorb radiation, however – I would guess it just needs to be by some kind of collective excitation process, with large coordinated movements of zillions of neutrons that shift the polarization density or magnetic dipole density over large volumes (where “large” just means “of order of the wavelength of light”). But I have no idea how to even think about describing that. I don’t think it can be done with the mean field “single particle” ideas I’m using above. You need the full many-body quantum Monty. I think.
And in fact, neutron stars do have a Fermi energy much higher than kT, and hence their physical state can be excellently approximated by zero temperature.
But they still emit blackbody radiation, because even with a weak coupling of any given particle with photons, they’re still thick enough to be quite opaque.