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If there are no orbitting electrons in a neutron star's makeup to interact with EM, what happens to light that strikes it?

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The details of what the surface of a neutron star would look like is poorly understood, involving lots of poorly understood physics. However, shining a light on it is going to be tricky since the radiation temperature of the surface of a neutron star is about a million degrees Kelvin and, even in visible light, would be hundreds of times brighter than the surface of the Sun.

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    But in theory they could cool down over time, right? – Florin Andrei Jul 05 '11 at 20:27
  • Well, if it gives off its own light, we know it can't be white because that would violate the laws of thermodynamics. So it would therefore be black or light gray or any color between. –  Jul 06 '11 at 00:02
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    I don't follow that argument, Dietrich. Can you elaborate? – Andrew Jul 06 '11 at 23:11
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    In practice they cool down, not just theory. You can observe it happening. A neutron star has a tiny heat capacity. Neutron stars might by a million K for only a few thousand years. The vast majority will be way cooler than this. – ProfRob Jan 25 '15 at 02:38
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A neutron star would shine brilliantly with its own light for a very long time indeed*, especially if it were being heated by matter falling onto it. The emitted light would be dominated by blue, with decreasing intensity towards the red end of the spectrum. The net result overall is bluish white, but not a perfect blue mixed with a perfect white.

But you specifically want to know what a neutron star's incident-light properties are? That is much more difficult to answer. Because a neutron star is so dense, only its outermost layer is going to matter to its optical properties. A neutron star has a thin atmosphere of extremely hot gas, but due to the extreme surface gravity, it will only be about a meter thick. Neutron star atmospheres are currently an active area of research. I'm not sure if that is enough to be opaque ala Venus, or if the true surface would be visible.

In any event, in the top layer of the crust, the density drops below the neutron drip density and the nuclear saturation density, so the composition will have the normal distribution of precisely equal numbers of protons and electrons, which will probably be approximately equal to the number of neutrons. The reflective properties will therefore be like hot, ionized iron, helium, or hydrogen, depending on which is actually present.

*White dwarfs are cooler than neutron stars and have a larger surface from which to emit radiation, yet even their cooling time scale is longer than the age of the Universe. Unless I'm missing something important about a neutron star, a neutron star should be much much hotter than the surface of a normal star for as long as you could possibly care to watch or wait.

Andrew
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  • "The emitted light would be a light blue, with a little green."

    Rather, a bluish white. But I'm nitpicking. :)

     – Florin Andrei Jul 07 '11 at 00:58
  • Did you look at the link? The high temp limit of the Planckian locus is clearly well past blue-white. – Andrew Jul 07 '11 at 10:18
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    Great answers. Thanks to all. Now, here's the rub. I know this is more of a mind experiment but I remember hearing that if i had a teaspoon of a neutrons from a neutron star it would weigh more than the earth or whatever. The question now is what would a pile of neutrons look like with a light shining on them with no electrons with whicn to interact. I really wonder about this. Thanks again for the great answers. –  Jul 07 '11 at 13:18
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    A teaspoonful of real neutron star matter still contains about 1% protons and electrons, which even if neutrons were completely transparent, would still be incredibly dense by terrestrial standards and thus have optical properties of their own. Hypothetical, pure neutrons might be electrically neutral, but they do have a magnetic moment, which by Maxwell's Laws means they can fully take part in EM dynamics, if not electrostatics. Beyond that is past my expertise and a question better suited for the physics stackexchange. – Andrew Jul 08 '11 at 15:14
  • @Andrew - did you look at the link? Quote: "It goes from deep red at low temperatures through orange, yellowish white, white, and finally bluish white at very high temperatures." The real hue of that diagram cannot be correctly reproduced on a computer monitor, only a portion of it is close to reality, and that assumes a calibrated display, which few people have. You can't just eyeball the hue and expect to derive any useful information from it. It's approximate only. Hot objects never appear green, no matter what's the temperature - just ask any astronomer. – Florin Andrei Jul 16 '11 at 00:08
  • I suppose now we're both nitpicking. I wanted to distinguish between say, a pure-blue mixed with white color that could be represented in RGB as 50, 50, 256, and the strong cubic extinction at low frequencies in the blackbody curve. The latter would produce an RGB with a dominant B, a middling G, and smallest R. I'll edit my answer to reflect that. PS I am an astronomer. – Andrew Jul 18 '11 at 19:50
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    About that last paragraph: Neutron stars cool predominantly via neutrino losses, rather than photons leaving the surface, at least when young. Predicting their temperatures as a function of time is still the subject of open research. –  Mar 22 '13 at 01:43
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    Cooling timescales are formed from a ratio of heat capacity to luminosity. Neutron stars have tiny heat capacities and cool extremely rapidly compared to white dwarfs. Photon cooling at low temperatures is expected to dominate for the vast majority of their lifetimes. – ProfRob Jan 25 '15 at 02:43
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It should be reflective, since the crust of the NS is not made of pure neutrons (even the interior has some non-negligible fraction of free electrons), and the "normal" matter should be highly ionized, thus conductive.

Re: the extreme thermal brightness of a million-degree black body, you could still see the visible light if you shined a coherent source like a laser. Also, very old NS's cool off below that temp, but they are much harder to find.

Jeremy
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I think, honestly, it would look like a glass ball or a concave mirror with flashes of light where matter gets sucked in. A neutron star would have significant gravitational lensing. I don't think the surface itself would reflect any light, even though it's guaranteed to be the smoothest surface imaginable.

Russell S
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    This is rather speculative in tone... can you back it up with any references or elaborate a bit more on your claims? – Kyle Oman Jul 31 '14 at 04:08
  • Just speculation, but I took into account gravitational lensing and the naive assumption that neutron stars were pure neutrons. Looking into this to give you a good answer showed me that the surface of a neutron star is super-dense iron aligned strongly with the magnetic field. Electrons only flow along the magnetic lines so it is a good bet that light reflecting off the surface would be polarized. There also would be a significant red shift due to the gravity. – Russell S Jul 31 '14 at 05:28
  • Heck with this. This is what a neutron star looks like: http://en.wikipedia.org/wiki/Neutron_star#mediaviewer/File:IsolatedNeutronStar.jpg – Russell S Jul 31 '14 at 05:39