Neutron stars are like cosmic pralines

Neutron stars, among the densest objects in the universe, remain a mystery to physicists. But a new theoretical analysis may explain the internal structures of these ultra-dense celestial bodies.

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

The neutron star’s massive mass is concentrated into a sphere the size of an average city. One 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 isn’t something that can be replicated in a lab. So, since it was first discovered 60 years ago, scientists have been trying to work out its internal structure.

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

Physicists at Goethe-University Frankfurt in Germany have succeeded in adding more important information to these equations in a paper published in the journal Astrophysical Journal Letters.

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

“Light” neutron stars — masses less than 1.7 times that of our sun — have a smooth mantle and a hard core, while “heavy” neutron stars — masses more than 1.7 times that of the sun — are the opposite, with a rigid and soft mantle. essence.

“This result is very exciting because it gives us a direct measure of how compact the center of neutron stars is,” said senior author and project leader Professor Luciano Rizzola. “It seems that neutron stars behave a bit like chocolate fudge candy: light stars are like those chocolates that have a hazelnut in their center surrounded by smooth chocolate, while heavy stars can be thought of more like those chocolates where the hard layer has a soft filling.”

Tasty comparisons aside, the research shows the power of computer simulations in modeling extreme conditions that would be difficult to investigate otherwise.

The team used an analysis of the speed of sound from their neutron star models to arrive at their insights. How quickly sound waves propagate through a material can tell scientists how hard or soft the material is. Such an analysis is used on Earth to explore the interior of our planet, including finding oil deposits.

Other previously unexplained properties of the neutron star have also been revealed by equations of state modeling.

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Interestingly, the researchers found that regardless of the star’s mass, these compact objects probably have a radius of about 12 kilometers, making them about the size of Frankfurt, the researchers’ hometown.

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

The structure and composition of neutron stars remains elusive, but such developments take us a step toward exploring the denser objects in the universe.

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