Neutron stars are incredibly compact objects that can form after the death of a star. However, little is known about their interiors. Since their discovery, scientists have been trying to decipher their structure.
Since they can barely be duplicated on Earth in a laboratory, the biggest obstacle is simulating the severe circumstances inside neutron stars. As a result, there are numerous models in which different attributes, such as temperature and density, are defined using so-called equations of state. These equations attempt to characterize 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. They have developed more than a million equations of state that satisfy the constraints set by data obtained from theoretical nuclear physics on the one hand and by astronomical observations on the other.
When they evaluated the equations of state, physicists found something surprising: “Light” neutron stars (with masses smaller than about 1.7 solar masses) seem to have a soft mantle and a stiff core, whereas “heavy” neutron stars (with masses larger than 1.7 solar masses) instead have a stiff mantle and a soft core.
Prof. Luciano Rezzolla said, “This result is very interesting because it gives us a direct measure of how compressible the center of neutron stars can be. Neutron stars behave a bit like chocolate pralines: light stars resemble those chocolates with a 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.”
The speed of sound is crucial to this insight. This quantitative metric, which relies on how stiff or flexible the matter is, describes the speed at which sound waves move about inside an item. On Earth, oil deposits are found, and the planet’s interior is explored using the speed of sound.
The scientists could also identify additional, 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.
Author Dr. Christian Ecker explains: “Our extensive numerical study not only allows us 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 strongly they distort each other through their gravitational fields. These insights will become particularly important to pinpoint the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars.”
- Sinan Altiparmak, Christian Ecker, Luciano Rezzolla: On the Sound Speed in Neutron Stars. The Astrophysical Journal Letters (2022) DOI: 10.3847/2041-8213/ac9b2a
- Christian Ecker & Luciano Rezzolla: A general, scale-independent description of the sound speed in neutron stars. The Astrophysical Journal Letters (2022) DOI: 10.3847/2041-8213/ac8674