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An international team of researchers from McGill University, California Institute of Technology and Indiana University has calculated the strength of nuclear pasta — extremely dense material deep inside the crust of neutron stars. The results, published in the journal Physical Review Letters, show that nuclear pasta may be the strongest known material in the Universe, with a shear modulus of up to 1030 ergs/cm3 and breaking strain greater than 0.1.
Neutron stars are the smallest, densest stars in the Universe. True to their name, neutron stars are composed almost entirely of neutrons. Image credit: Casey Reed, Penn State University.
Neutron stars are born as extremely hot objects when the cores of highly evolved, massive stars become gravitationally unstable and collapse.
They have diameters of only 5-10 miles (10-15 km), but contain 1.5 times the mass of our Sun.
Their immense gravity makes their outer layers freeze solid, making them similar to Earth with a thin crust enveloping a liquid core.
This high density causes the material that makes up a neutron star, known as nuclear pasta, to have a unique structure.
Below the crust, competing forces between the protons and neutrons cause them to assemble into shapes such as long cylinders or flat planes, which are known in the literature as ‘lasagna’ and ‘spaghetti,’ hence the name ‘nuclear pasta.’
Together, the enormous densities and strange shapes make nuclear pasta incredibly stiff.
“The strength of the neutron star crust, especially the bottom of the crust, is relevant to a large number of astrophysics problems, but isn’t well understood,” said lead author Dr. Matthew Caplan, a postdoctoral researcher at McGill University.
Nuclear pasta, the hardest substance in the known Universe. Image credit: McGill University.
Dr. Caplan and co-authors successfully ran the largest computer simulations ever conducted of neutron star crusts, becoming the first to describe how these break.
“Our results are valuable for astronomers who study neutron stars,” Dr. Caplan said.
“Their outer layer is the part we actually observe, so we need to understand that in order to interpret astronomical observations of these stars.”
The findings could help astrophysicists better understand gravitational waves like those detected last year when two neutron stars collided.
The results even suggest that lone neutron stars might generate small gravitational waves.
“A lot of interesting physics is going on here under extreme conditions and so understanding the physical properties of a neutron star is a way for scientists to test their theories and models,” Dr. Caplan noted.
“With this result, many problems need to be revisited. How large a mountain can you build on a neutron star before the crust breaks and it collapses? What will it look like? And most importantly, how can astronomers observe it?”
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M.E. Caplan et al. 2018. Elasticity of nuclear pasta. Phys. Rev. Lett, in press; arXiv: 1807.02557
