We know how to define the Temperature of a neutron star, as answered in this question. After watching the Sixty Symbols video about Forging Weapons from Neutron Stars, where the temperature and resulting black body radiation is a crucial element, I wondered how objects made (almost) entirely of neutral particles can radiate electromagnetic waves, since radiation is caused by acceleration of charged particles (at least in classical physics).
I can imagine a few possible answers, but I would like the expert's statement:
- Neutrons can be excited and radiate when relaxing
- Neutron stars aren't entirely neutral. The tiny fraction of protons and electrons radiate
- Neutrons are made up of charged quarks, and the quarks radiate
I believe this question is different from "Are neutron stars transparent" because I'm asking about the generation of black body radiation, not the lack of absorption of any radiation.
Answer
Most materials that emit photons are made from neutral constituents (atoms), but they radiate because atoms have charged constituents (electrons) that can be thermally excited. Indeed, because of the beauty of thermodynamics the total emissivity is independent of the precise mechanism.
Neutron stars are optically thick, and photons are made in a very small layer (a few inches, on top of star 10km radius) called the neutron star atmosphere. The composition of NS atmospheres is not always known, it may depend on the history of the NS (accretion events, etc). Typical models are H, C, or Fe atmospheres. In any case, atmospheres are made from atoms, and radiate in the usual way. If it was not for atmospheres, NS would radiate because of protons and electrons in the crust. The p,e fraction is not tiny, just small (a few per cent).
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