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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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3 Answers

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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 5 '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. – Dietrich Epp Jul 6 '11 at 0:02
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I don't follow that argument, Dietrich. Can you elaborate? – Andrew Jul 6 '11 at 23:11
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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.

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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 7 '11 at 0:58
Did you look at the link? The high temp limit of the Planckian locus is clearly well past blue-white. – Andrew Jul 7 '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. – pion Jul 7 '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 8 '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 0:08
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up vote 4 down vote

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.

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