Cosmic chocolate pralines? The general structure of neutron stars revealed

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

The biggest challenge is to simulate the extreme conditions inside neutron stars, as they can hardly be recreated on Earth in the laboratory. There are therefore many models in which different 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 from the Institute of Theoretical Physics, has developed more than a million different equations of state that satisfy the constraints imposed by data obtained from theoretical nuclear physics. on the one hand, and by astronomical observations on the other. Their work is published in Letters from the Astrophysical Journal.

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,” says 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 are just as large in diameter as Goethe University’s hometown of Frankfurt.

Study author Dr. Christian Ecker explains: “Our extensive numerical study allows us not only 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 much they deform. each other through their gravitational fields. This information will become particularly important for identifying the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars.