A group of scientists at the Goethe University Frankfurt recently published two articles about their work on determining the internal structure of neutron stars. The papers can be found in the Astrophysical Journal Letters (On the Sound Speed in Neutron Stars; A General, Scale-independent Description of the Sound Speed in Neutron Stars).
Neutron stars form as the result of the death of massive stars. They are extremely dense and small – or compact – objects. To put this into perspective, these objects weigh more than our Sun, but have that mass compressed into a sphere with the diameter of a large city. Ever since their discovery over 60 years ago, scientists have struggled to understand the interior and structure of these extreme objects.
The kinds of conditions present inside neutron stars cannot be replicated in a laboratory on Earth, but they can be modeled in complicated computer simulations. These theoretical models use various equations of state to describe conditions, such as temperature and density, throughout the star. The team of scientists at Goethe University Frankfurt ran extensive computer simulations to try to recreate data obtained from theoretical nuclear physics and astronomical observations. The results of these simulations have allowed the team to describe the structure of neutron stars from their surfaces down to their cores.
The results of the simulation were quite surprising. They found that light neutron stars – or those with masses less than 1.7 times the mass of the Sun – have a soft mantle and a stiff core. On the other hand, heavy neutron stars – or those with masses greater than 1.7 times the mass of the Sun – have a stiff mantle and a soft core. These results give the team a direct measure of how compressible the center of neutron stars can be.
Luciano Rezzolla, the scientist leading this study, found that chocolate pralines are a good metaphor for describing these types of stars. He said that neutron stars “behave a bit like chocolate pralines: light stars resemble those chocolates that have a bit of hazelnut in their center surrounded by soft chocolate, whereas heavy stars can be considered more like those chocolates where a hard layer contains a soft filling.”
Source: Goethe University Frankfurt