[thedoc (OP)]

@JayTip_SA asked the Naked Scientists:
   
What is the density of a Neutron Star?

What do you think?

[yor_on]

Depends on its size and density, but it can't be greater that the size/density given for becoming a black hole. So first you have ordinary matter, then you have a further state due to compression called degenerate matter, from which we get a neutron star. But if the compression from that is being feed by some other mass you might get it to pass that state and instead become a black hole. I saw someone claim that about five percent of all neutron stars are binary. And adding that mass should be sufficient for it to pass the Schwarzschild radius from where you get a Black Hole.

NASA use this calculation approximately defining where you get a neutron star.

[distimpson]

"Neutron stars have overall densities predicted by the APR EOS of 3.7×10¹⁷ to 5.9×10¹⁷ kg/m³ ... which compares with the approximate density of an atomic nucleus of 3×10¹⁷ kg/m³. The neutron star's density varies from below 1×10⁹ kg/m³ in the crust, increasing with depth to above 6×10¹⁷ or 8×10¹⁷ kg/m³ deeper inside (denser than an atomic nucleus). This density is approximately equivalent to the mass of a Boeing 747 compressed to the size of a small grain of sand." this information from http://en.wikipedia.org/wiki/Neutron_star

Several years ago I asked an astrophysicist (can not remember name) "if somehow a spoonful of this dense matter could be removed from the star, would it expand into  a large volume of ordinary matter?". Her answer was yes it would, any thoughts here?

[yor_on]

Never thought of that one, but she should be right
That would be something to see.

[yor_on]

If it had been defined the way a black hole is I wouldn't expect it to expand, as described outside a event horizon, assuming a 'infinite center' is all there is behind that. But a Neutron star, even if in a equilibrium? Then again, after a compression it will consist of neutrons, not electron-proton pairs any more?? so what would it 'expand' into? Maybe we both are wrong there?
=

No, she's right, it will decay into electron-proton pairs again I think. And that should happen in about fifteen minutes of your local time, after you lifted it up, through (radiation) beta decay. But I'm still not sure? What happens to the nucleus as it becomes a degenerate 'neutron gas'? Does that exist any more?

[yor_on]

You could imagine a Einstein Rosen bridge opening up inside a neutron star. If she is right then that should mean a explosion and a he of a lot of radiation coming from that 'portals' other end. But it builds on a idea of nucleus's, and, does that still exist inside a degenerate neutron gas?

That one was very tricky, to me at last:).
=

Then again, is there something leading to particles of a very compressed state, and also if treated as 'matter waves' being compressed into a very small 'volume', to expand into electron-proton pairs again? Is there some quantum mechanical rule that guarantees it? I don't think there is myself?

Pressure is something defined and restricted to a constricting volume, no matter if we discuss it as waves or rest mass. And so it belongs to dimensions, and distances? I don't know?

[distimpson]

Very far out of my area of understanding, I was OK with the idea of some sort of decompression of the dense matter, unless there is some other force to keep it together, some conditions where the strong force would hold the material together like a macroscopic radioactive super nucleus. That is what you are saying with beta decay to get some protons in the mix? Sorry if this is really out there, I don't even know what questions to ask.

[Pmb]

"Neutron stars have overall densities predicted by the APR EOS of 3.7×10¹⁷ to 5.9×10¹⁷ kg/m³ ... which compares with the approximate density of an atomic nucleus of 3×10¹⁷ kg/m³.

I didn't check you figures but I'm assuming their correct. You can think of a neutron star as a large nucleus of an atom where all the nucleons are neutrons. Think of it as if all the electrons have been absorbed by the protons to form neutrons.

[yor_on]

The weirdest thing I know about a neutron star, is that the more mass you add, the smaller it becomes. (As defined by you looking at it, from afar.)