Why do neutron stars have solid crusts?

A long time ago I read that neutron stars have a solid crusts that are several orders of magnitude harder/stronger than alloys here on the Earth. So how is this possible ?

A neutron star has a surface temperature of some 50,000 °K, so how can anything "solidify" at these temperatures ?

I understand that a solid is hard because of the chemical bonds and sometimes the crystals that form in the solid, so the only way that a star with a 50,000 °K can have a solid crust is if the matter there is solid because of other means, and that's because neither chemical bonds nor molecules can exist at these temperatures.

So how can a neutron star crust (and matter in general) become solid at these high temperatures where molecules and neutral atoms don't even exist ? And can this solid matter really reach strengths several order of magnitudes the strength of our alloys ?

• There is no bond in the neutron star. You need electrons and protons for chemical bonding. And for being a liquid should have weaker intermolecular forces which is possible only in bonds. – manshu Nov 18 '15 at 21:16
• @manshu The outer layers of a neutron star have as many electrons and protons as normal matter. – user10851 Nov 18 '15 at 21:52
• The surfaces of a neutron star can turn to liquid if the temperature exceeds 10^6 kelvin. Also, I would assume that the incredible force of gravity (and thus pressure) on the matter of the star would play a factor in the conditions of its phase change, causing the surface to turn to stay solid under much more extreme conditions than on earth – Ryan Nov 18 '15 at 22:06

The neutron star crust is separated into outer and inner regions. The outer is a crust of neutron-rich nuclei surrounded by degenerate electrons. The inner is similar, but the nuclei are even more neutron-rich and there are degenerate neutrons too.

The (qualitative) answer to your question looks at the ratio of electrostatic (Coulomb) energy to the thermal energy of the ions in the crust.

$$\frac{E_c}{E_{th}} \simeq \frac{Z^2 e^2}{4\pi r_0 \epsilon kT},$$ where $T$ is the temperature, $Z$ is the atomic number of the nuclei and $r_0$ is a characteristic separation between the nuclei.

This ratio increases with: decreasing temperature, with decreasing nuclei separation (ie increasing density) and increasing atomic number. When it reaches some critical value, the plasma "freezes" into a crust, with the ions locked into some solid lattice. The same phenomenon occurs in the cores of white dwarfs at similar temperatures and densities, and the process has been "observed" to occur via asteroseismology.

So what is going on here, is that although the crust is hot ($10^{7}$ K would not be unreasonable actually), the densities ($10^{11}-10^{15}$ kg/m$^3$) are high enough to solidify the plasma.

This is of course not the whole story. At very high densities, when the neutrons drip out of the nuclei, one has to consider surface energy terms and ultimately the neutron fluid "dissolves" the crust at about $10^{16}$ kg/m$^3$, possibly via several bizarre "nuclear pasta" phases,eventually forming a fluid of neutrons, protons and electrons.

• The word "crust" implies that the surface is somehow harder, stronger, or more rigid than the material below; is this true? – Daniel Griscom Nov 18 '15 at 22:45
• @DanielGriscom It refers to the microscopic structure. I guess macroscopically the shear modulus would be much larger. In terms of compressibility, no, the crust material is comparatively compressible compared with the neutron star fluid interior. – ProfRob Nov 18 '15 at 22:49
• Crikey, those crusts must make for some interesting chemistry! :) – Gert Nov 18 '15 at 23:03
• @Gert - And some interesting hard science fiction along those lines. See Dragon's Egg by Robert Forward. – mmesser314 Nov 19 '15 at 5:47
• @RobJeffries But where do the ions that make the lattice come from ? You just mentioned that the crust is made of neutron-rich nuclei surrounded by degenerate electrons, so what forms the lattice ? – Abanob Ebrahim Nov 20 '15 at 17:13