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Neutron star core material could be a superfluid mixture of neutrons with a few protons and electrons, or it could incorporate high-energy particles like pions and kaons in addition to neutrons, or it could be composed of strange matter incorporating quarks heavier than up and down quarks, or it could be quark matter not bound into hadrons. (A compact star composed entirely of strange matter would be called a strange star.) However, so far, observations have neither indicated nor ruled out such exotic states of matter.
This means that even an extremely hot neutron star is only about a tenth as bright as the sun. To top it off, most of this light output is in the ultraviolet and X-rays, so they're hard to find with optical telescopes.The reason we only see them when they're young is that they cool over time. In fact, we think that after only 10 million years (a short time on cosmic scales) they'll cool to 105 K. You can see from the above equation that this will bring their luminosity down by about a factor of 10,000.
I've learned that as a neutron star forms the electrons fuse with the protons and become neutrons. However if you have a single neutron it very rapidly decays into an electron/proton pair to become an atom of hydrogen.
Even as you sent this post I was also making the same point, no electrons, no interaction with light.
To be more precise: even if neutrons are not charged particles, they have a magnetic moment, and this does make them interact, even if extremely slightly, with EM radiation. But, doing the correct calculations (it's not simple, I just relate what some more prepared of me wrote in another NG) the free path of a visible photon in a pure neutron star would be of thousands of km.
Ah ha; you have just discovered dark matter !! It may be invisible neutron star matter.But don't neutron stars have a skin of ordinary matter a few meters thick? I seem to remember someone making that assertion.
I don't think there is a size limit for photons. We would have to consider the wave length of a photon to be an indicator of its size. I don't remember an upper limit for radiation frequency. It is probably just that we don't have a way to manipulate the high frequency stuff.Edit: I'm sure you meant photons in the visible range.
Is it possible that the temperature of a Neutron star could ever drop to room temperature ?.I have seen the figure quoted that it would drop from 10^6°K to 10^5°K in a million years but as the radiated power accords with T^4 then to drop to 10^4°K would take 10 billion years so to get to 300°K we just don't have an old enough universe.Is there some flaw in my arithmetic ?.