Cosmic chocolate pralines: surprising discovery of physicists on the structure of neutron stars

Studying the speed of sound has revealed that heavy neutron stars have a hard mantle and a soft core, while light neutron stars have a soft mantle and a hard core – much like different chocolate pralines. Credit: Peter Kiefer and Luciano Rezzolla

Physicists are modeling more than a million equations of state to uncover other previously unexplained properties of neutron stars.

Through extensive model calculations, physicists at Goethe University Frankfurt have come to general conclusions about the internal structure of neutron stars, where matter reaches enormous densities: Depending on their mass, stars can have a nucleus very stiff or very soft. The results were published simultaneously in two articles.

Neutron stars are extremely compact objects that can form after a star dies. They have the mass of our sun or even more, but are incredibly compressed into a sphere the diameter of a large city. Since their discovery more than 60 years ago, scientists have been trying to decipher their structure. So far, however, little is known about the interiors of neutron stars.

Since their extreme properties prevent them from being recreated on Earth in the laboratory, the biggest challenge is simulating the extreme conditions inside neutron stars. There are therefore many models in which various properties – from density to temperature – are described using so-called equations of state. These equations attempt to describe the structure of neutron stars from the stellar surface to the inner core.

Now physicists at Goethe University Frankfurt have succeeded in adding other crucial pieces to the puzzle. The working group led by Professor Luciano Rezzolla of the Institute of Theoretical Physics has developed more than a million different equations of state which satisfy the constraints imposed by the data obtained from the theoretical nuclear physics of a hand, and by astronomical observations on the other hand.

While evaluating the equations of state, the working group made a startling discovery: “light” neutron stars (with masses less than about 1.7 solar masses) appear to have a soft mantle and a rigid core. , while “heavy” neutron stars (with masses greater than 1.7 solar masses) tend to have a rigid mantle and a soft core.

“This result is very interesting because it gives us a direct measure of the center compressibility of neutron stars”, explains Professor Luciano Rezzolla, “Neutron stars apparently behave a bit like chocolate pralines: light stars look like to those chocolates that have a hazelnut in their center surrounded by soft chocolate, while heavy stars can be considered more like those chocolates where a hard layer contains a soft filling.

The speed of sound, a study goal of undergraduate student Sinan Altiparmak, was crucial to this idea. This quantitative measure describes the speed at which sound waves propagate through an object and depends on the stiffness or softness of the material. Here on Earth, the speed of sound is used to explore the planet’s interior and discover oil deposits.

By modeling the equations of state, physicists were also able to uncover other previously unexplained properties of neutron stars. For example, regardless of their mass, they most likely have a radius of only 12 km. Thus, they have a diameter as large as the hometown of Goethe University, Frankfurt.

Author Dr. Christian Ecker explains: “Our extensive numerical study allows us not only to make predictions for the maximum radii and masses of neutron stars, but also to set new limits on their deformability in binary systems. , that is, how strongly they each deform. the other through their gravitational fields. This information will become particularly important in identifying the unknown equation of state with future astronomical observations and detections of

” data-gt-translate-attributes=”[{” attribute=””>gravitational waves from merging stars.”

So, while the exact structure and composition of matter inside neutron stars continue to remain a mystery, the wait until its discovery can certainly be sweetened with a chocolate or two.

References:

“On the Sound Speed in Neutron Stars” by Sinan Altiparmak, Christian Ecker and Luciano Rezzolla, 10 November 2022, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ac9b2a

“A General, Scale-independent Description of the Sound Speed in Neutron Stars” by Christian Ecker and Luciano Rezzolla, 10 November 2022, The Astrophysical Journal Letters.
DOI: 10.3847/2041-8213/ac8674