Neutron stars are like cosmic pralines

Neutron stars, among the densest objects in the universe, are still a mystery to physicists. But new theoretical analysis might explain the internal structures of these super-dense celestial bodies.

A neutron star is the collapsed core of a supergiant star (10-25 times bigger than our Sun) which has run out of fuel. The central region of the star, 1-3 times the mass of the Sun, collapses in on itself, pushing electrons and protons into each other under so much pressure they become neutrons.

The immense mass of a neutron star is concentrated into a ball about the size of an average city. A single teaspoon of neutron star matter would have a mass of about a trillion kilograms.

Being light years away from Earth, neutron stars are difficult to study. And their extreme compactness is not something which can be replicated in a laboratory. So, since they were first discovered 60 years ago, scientists have been trying to work out their internal structure.

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To describe neutron star properties, physicists have to use “equations of state” to model their various properties – from temperature to density.

Physicists at Goethe University Frankfurt in Germany have successfully added further crucial information to these equations in research published in the Astrophysical Journal Letters.

The researchers developed more than a million equations of state for neutron stars. The equations are set by data from theoretical nuclear physics and astronomical observations. And the results revealed some surprising conclusions.

 “Light” neutron stars – masses smaller than 1.7 times that of our Sun – have soft mantle and a stiff core, whereas “heavy” neutron stars – mass greater than 1.7 times the solar mass – are the opposite, with a stiff mantle and soft core.

“This result is very interesting because it gives us a direct measure of how compressible the centre of neutron stars can be,” says senior author and project lead Professor Luciano Rezzolla. “Neutron stars apparently behave a bit like chocolate pralines: light stars resemble those chocolates that have a hazelnut in their centre surrounded by soft chocolate, whereas heavy stars can be considered more like those chocolates where a hard layer contains a soft filling.”

Delicious analogies aside, the research shows the power of computer simulations to model extreme conditions which are otherwise difficult to probe.

The team used an analysis of the speed of sound through their modelled neutron stars to attain their insights. How fast sound waves propagate within a material can tell scientists how stiff or soft the matter is. Such analysis is used on Earth to explore our planet’s interior, including finding oil deposits.

Other previously unexplained neutron star properties were also uncovered by modelling the equations of state.

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Interestingly, the researchers found that, regardless of the star’s mass, these compact objects are probably all around 12 kilometres in radius, making them roughly the size of the researchers’ university hometown, Frankfurt.

“Our extensive numerical study not only allows us to make predictions for the radii and maximum masses of neutron stars, but also to set new limits on their deformability in binary systems, that is, how strongly they distort each other through their gravitational fields,” explains co-author Dr. Christian Ecker. “These insights will become particularly important to pinpoint the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars.”

Neutron star structure and composition remains elusive, but advances such as these take us a step toward probing the densest objects in the universe.