Strength of materials - neutron stars

[chill_factor]

I have a scientific background in materials science. However this is one type of material that we don't exactly learn about!

Let's say you have a neutron star approaching a comparable mass black hole. As the neutron star approaches the event horizon for such a comparable black hole, tidal forces increase extremely quickly.

Can the neutron star's structure be disrupted by tidal forces outside the horizon? If it can, what will the fragments look like and what will be their properties, and how will they differ from that of a bulk neutron star? indeed, what are the material properties (electrical and thermal conductivity as well as their optical, magnetic and mechanical properties) of a bulk neutron star? Can neutron stars be even considered solids?

 

[PhysicoRaj]

Neutron stars densities are due to immense gravity. But as the gravity of black hole acts upon, I think it loses its density to become a fragmented mass of neutrons held together..( a neutron star is an atom with no proton!). I don't know of the properties much.

 

[twofish-quant]

Can the neutron star's structure be disrupted by tidal forces outside the horizon?

Yes.

If it can, what will the fragments look like and what will be their properties, and how will they differ from that of a bulk neutron star?

My guess is that it's less fragments than "silly putty." Also for neutron star material, there is an equation of state that describes how the material behaves for a given density, temperature, and electron fraction. The black hole is going to strech the neutron star and then you can calculate the reaction of the material.

Indeed, what are the material properties (electrical and thermal conductivity as well as their optical, magnetic and mechanical properties) of a bulk neutron star?

see http://arxiv.org/abs/0807.2343

Also it turns out that if you want to ask what happens if a neutron star hits a black hole, the bulk viscosity, electrical and thermal conductivity are not likely to be important since you can show that they would be much smaller than the turbulent viscosity and radiative and conductive heat transfer.

Magnetic properties are likely to be important, as are optical properties.

The one thing that's hell to simulate is magnetic fields.

Can neutron stars be even considered solids?

Open question. The problem is that solids involve collective effects so we don't know enough about nuclear matter to say how they would behave once you increase the density passed a certain point.

 

[zhermes]

'Tidal tails' are a characteristic of compact binary mergers, see:
http://users.monash.edu.au/~dprice/research/nsmag/movies/pricerosswog_halfstars_3ivx.mov
Thats for two neutron stars merging; NS-BH looks very similar, except asymmetric (with only 1 tail).

 

[Chronos]

You are not going to get stable 'neutronium' from such a process, if that is what you were wondering. Once freed from the immense gravity of the neutron star, any fragments liberated will decompose [i.e., go boom]. The crust of a neutron star is indeed unlike any material found on Earth [or pretty much anywhere elses]. It is believed to be billions of times stronger than steel - e.g., http://arxiv.org/PS_cache/arxiv/pdf/0904/0904.1986v1.pdf

 

[chill_factor]

thanks for the replies. those are very interesting papers and movies. the 2nd paper is interesting in that they model the neutron star crust as an actual solid. the failure mode is also interesting in that neutron stars are "all or nothing", unlike almost all terrestrial materials which yield over a range.

the movie showed matter being expelled and vaporized. would this be a gas of neutrons?

 

[twofish-quant]

the movie showed matter being expelled and vaporized. would this be a gas of neutrons?

iron vapor. Once the density drops then it becomes more stable for the neutrons to form nuclei, so you'd end up with iron vapor.

 

[Chronos]

Decompressed neutron star material is believed to have varying composition. See http://arxiv.org/abs/1105.2453 for discussion.