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I would know what would happen if we put a small part (let me assume 1 cm$^3$) of White Dwarf or Neutron Star on Earth. Will it explode or maybe it collapses to the Earth's core?

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up vote 6 down vote accepted

A piece of a white dwarf would explode, because atoms would expand to their normal size. This would hold for outer layers of a neutron star, too. For inner parts of the neutron star, that's not quite clear, but that kind of matter would probably decay rapidly, also resulting in an explosion.

If the material of a neutron star could be kept stable, it would fall down to Earth's core, up to the other side of the Earth, and oscillate a while that way, before being slowed down by friction, and stopped close to Earth's center. (1 cm³ neutron star matter has a mass of about 400 million tons.)

One cm³ white dwarf matter would have a mass of about 1 ton. Assumed it could be kept stable, solid rock could withstand this load.

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Thank you for your clear and helpful explanation. I can accept this answer. – Tom Feb 14 '14 at 18:38

If we take neutron star material at say a density of $\sim 10^{17}$ kg/m$^{3}$ the neutrons have an internal kinetic energy density of $3 \times 10^{32}$ J/m$^{3}$. So even in a teaspoonful (say 1 cc), there is $3\times10^{26}$ J of kinetic energy (similar to what the Sun emits in a second, or 10 billion or so H-bombs) and this will be released instantaneously.

The energy is in the form of around $10^{38}$ neutrons travelling at around 0.1-0.2$c$. So roughly speaking it is like half the neutrons (about 50 million tonnes) travelling at 0.1$c$ ploughing into the Earth. If I have done my Maths right, that is roughly equivalent to a 30 km radius near-earth asteroid hitting the Earth at 30 km/s.

So this material would instantly vapourise and take a large chunk of the Earth with it, probably destroying most of life on Earth.

The situation for a white dwarf is much less extreme. The density would be more like $10^{9}$ kg/m$^3$ and the energy density more like $10^{22}$ J/m$^3$ - so 10 orders of magnitude less kinetic energy density. Nevertheless that is still $10^{16}$ J, which is like a 2.5 megatonne H-bomb.

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