Cosmic chocolate pralines? General neutron star structure revealed.

 

The study of the sound speed has revealed that heavy neutron stars have a stiff mantle and a soft core, while light neutron stars have a soft mantle and a stiff core—much like different chocolate pralines. Credit: Peter Kiefer & Luciano Rezzolla
The study of the sound speed has revealed that heavy neutron stars have a stiff mantle and a soft core, while light neutron stars have a soft mantle and a stiff core—much like different chocolate pralines. Credit: Peter Kiefer & Luciano Rezzolla

The innards of neutron stars, those very compact structures that may develop following the death of a star, is still little understood. A sphere with a diameter of a big metropolis is crushed to contain the mass of our sun or even more. Scientists have been attempting to understand their structure ever since they were discovered more than 60 years ago.

Since they can barely be duplicated on Earth in a laboratory, simulating the severe circumstances within neutron stars is the biggest obstacle. As a result, there exist several models that use so-called equations of state to explain a variety of variables, ranging from density to temperature. From the stellar surface to the inner core, these equations make an effort to characterize the structure of neutron stars.

Now, physicists from Goethe University Frankfurt have been successful in completing the jigsaw by introducing new essential components. More than a million different equations of state have been created by the working group at the Institute of Theoretical Physics under the direction of Prof. Luciano Rezzolla that satisfy the constraints imposed by data from theoretical nuclear physics on the one hand and astronomical observations on the other. Their study appears in Letters from The Astrophysical Journal.

The working group unexpectedly found that "heavy" neutron stars (with masses more than 1.7 sun masses) have a stiff mantle and a soft core, in contrast to "light" neutron stars (with masses fewer than 1.7 solar masses), which seem to have a soft mantle and a stiff core.

Neutron stars appear to behave somewhat like chocolate pralines: Light stars resemble those chocolates that have a hazelnut in their center surrounded by soft chocolate, whereas heavy stars can be thought of more like those chocolates where a hard layer contains a soft filling. Prof. Luciano Rezzolla says, "This result is very interesting because it gives us a direct measure of how compressible the center of neutron stars can be."

The speed of sound, which Sinan Altiparmak, a bachelor's student, focused on studying, was essential to this realization. This quantitative metric, which relies on how stiff or flexible the matter is, defines the speed at which sound waves move about within an item. On Earth, oil reserves are found and the planet's interior is explored using the speed of sound.

The scientists were also able to identify other, previously undiscovered characteristics of neutron stars by modeling the equations of state. For instance, they most likely have a radius of only 12 kilometers, regardless of their mass. They have the same circumference as Frankfurt, the city that is home to Goethe University.

Dr. Christian Ecker, the study's author, explains, "We can predict the radii and maximum masses of neutron stars using our in-depth numerical analysis, and we can also place new restrictions on how strongly neutron stars can deform one another in binary systems, or how much their gravitational fields can interact with one another. With more astronomical measurements and the discovery of gravitational waves from merging stars, these insights will become especially crucial for identifying the unknown equation of state."

Also Read: Death of a star reveals midsize black hole lurking in a dwarf galaxy.

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