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I've heard that white dwarf stars are extremely dense and hard. So, if I had a piece of white dwarf matter, would it be possible to cut it (or otherwise) into a custom shape? How could one do that?

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    $\begingroup$ stones on white dwarf stars? $\endgroup$ – user24901 Jun 11 '13 at 11:06
  • $\begingroup$ How did you recon that white dwarfs have stones? $\endgroup$ – Ali Jun 11 '13 at 12:01
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    $\begingroup$ Though it sounds weird, this is a defensible question. Once a white dwarf cools down, apparently the surface can crystallize (before that, it might be a liquid). But there are many many issues, e.g. the gravity maintains it under pressure, so if you could somehow break off a chunk, the lattice of atoms might expand and rearrange radically... So far I can't find a good reference. $\endgroup$ – Mitchell Porter Jun 11 '13 at 12:13
  • $\begingroup$ Digging further, I have even found people suggesting that en.wikipedia.org/wiki/Carbonado diamonds may be formed in the carbon-rich environment around a white dwarf. Something like that might be behind this question... $\endgroup$ – Mitchell Porter Jun 11 '13 at 13:16
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    $\begingroup$ Related: physics.stackexchange.com/q/10052 Note that the same reasoning applies - the matter loses all its strange properties when removed from the high-pressure environment. $\endgroup$ – user10851 Jun 11 '13 at 15:13
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A quick, inadequately researched answer, which I post because the question is already at -5, and so probably doomed...

A white dwarf is a bit like a blob of white-hot burning liquid metal, spinning in space; the size of a planet but the mass of a star, so if you landed there you would be crushed to a smear of atoms less than a millimeter high, in an instant. Even if it had cooled down enough that the surface was solidified (and that might take billions of years to happen), you wouldn't be able to dig into it and extract a piece, because of the gravity. So there ought to be no stones, no mountains, no structures, even on an old cool black dwarf - everything will be crushed into flatness by the gravity.

But when it's still hot, the star will have an atmosphere as well, and it seems remotely possible that small crystals might form there and somehow get away. Alternatively, if some catastrophic explosion somehow blew up the star or part of the star, perhaps some blobs of "white-dwarf crystal" would escape in one piece. Away from the crushing gravity, the electrons and nuclei would expand apart and rearrange themselves into a more conventional sort of mineral (made of carbon and oxygen, the dominant elements in a white dwarf). If such a fragment somehow made its way to Earth, it would just be an ordinary crystal and could be cut like any other mineral.

Reader beware: I picked up most of this by speed-reading papers for the past hour, and also adding some physics common-sense. I may have missed something important. Also, human scientists undoubtedly do not yet understand everything about the environment in and around a white dwarf. So, believe what I wrote at your peril.

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  • $\begingroup$ The interiors of white dwarfs crystallise, not their surfaces. $\endgroup$ – Rob Jeffries Mar 12 '16 at 14:30
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We know that matter in White Dwarfs and Neutron stars become hugely dense, ten or fifteen orders of magnitude more dense than ordinary matter. What we don't know is, if such matter stays stable away from the deep gravity well of the star corpses where they form.

We certainly haven't found a single grain of such matter in nature after a couple of centuries of geological observations. But, such a dense material will not stay afloat in any kind of matter crust of any planet, quickly sinking toward the center of planets that capture such grains gravitationally.

If there is some kind of neutronium or strangelet matter with a stable low-pressure phase, the only three ways we are going to ever find out are:

  • by recreating those densities and pressures in laboratory (our highest laboratory pressure diamond anvils top out at 40 GPa or something, 7 orders of magnitude less of what you are after, so good luck with that)

  • We become extremely good at observing gravitational perturbations and optically check the sources.

  • My favorite: We adventure a mining expedition to some old moon on the solar system that we are certain that is cool enough for us to go and drill toward the center of the core, and then we get to see what kind of heavy stuff is frozen in there.

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  • $\begingroup$ Yes, we do know that degenerate matter would be unstable if you had a lump of it right here. It would be explosively unstable. Calculate the internal energy per unit volume. Neutronium is a made-up science fiction term. $\endgroup$ – Rob Jeffries Oct 15 '15 at 12:36
  • $\begingroup$ "we do know" is actually the wrong way to begin that assertion. I would rather say that there are no reasons to believe that strangelet matter exist, but no reasons either to believe it doesn't: en.wikipedia.org/wiki/… $\endgroup$ – lurscher Oct 15 '15 at 16:03
  • $\begingroup$ White dwarfs (which is what the question is about) aren't made of "strangelet matter" or "neutronium", whatever they are. There are plenty of reasons to believe this, both theoretical and observational. $\endgroup$ – Rob Jeffries Oct 15 '15 at 17:19
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The "surface" of a white dwarf is a mixture of hydrogen, helium and perhaps a trace of heavier elements. It is never (read this as many, many times the current age of the universe) going to cool down enough to solidify.

Solids exist inside the approximately isothermal interiors$^{1}$ of white dwarfs, at densities $\geq 10^{9}$ kg/m$^{3}$, once temperatures drop below a few million K. In typical white dwarfs this probably occurs within a billion years. The white dwarfs freeze from the centre outwards, because the melting point increases with density.

The typical pressure contributed by the degenerate electron gas in a white dwarf interior at these densities is $10^{23}$ Pa. So, if you want to preserve your bit of crystallised white dwarf that you have somehow mined from the interior, then you have to work out how to stop it exploding. And it is not just a matter of letting it cool down - the degeneracy pressure is independent of temperature. So this high density material simply is not stable unless you can work out a way of constraining it. The problem is similiar, though not quite as extreme, to that of constraining neutron star material.

So in summary, crystalline material at white dwarf densities will have such a high internal energy density (due to degenerate electrons), that it would be incredibly difficult to constrain or manipulate.

$^{1}$ Degenerate electrons have extremely long mean free paths and so the thermal conductivity in a white dwarf interior is very high.

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