So far, not much is known about the inside of neutron stars, which are very dense objects that can form when a star dies. The mass of our sun or even more is packed into a sphere the size of a large city.

Since its discovery over six decades ago, scientists have endeavored to unravel their structure.

Since they can barely be reproduced on Earth in a laboratory, simulating the severe circumstances inside neutron stars is the biggest obstacle.

Because of this, there are many models in which so-called equations of state are used to describe properties like density and temperature.

With the use of these equations, an attempt has been made to characterize the structure of neutron stars from the stellar surface to the inner core.

Now, physicists from Goethe University Frankfurt have been successful in completing the puzzle by adding more significant pieces.

More than a million different equations of state were 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.

The working group unexpectedly found that “heavy” neutron stars (with masses greater 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 appear to have a soft mantle and a stiff core.

“This result is very interesting,” says Professor Luciano Rezzolla, “because it gives us a direct measure of how compressible the centre of neutron stars can be.

“Neutron stars apparently behave a bit like chocolate pralines: light stars resemble those chocolates that have a hazelnut in their centre surrounded by soft chocolate, whereas heavy stars can be considered more like those chocolates where a hard layer contains a soft filling.”

Sinan Altiparmak, a bachelor’s student, devoted a lot of time and energy to researching the speed of sound and realized that it was crucial to his discovery.

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, the speed of sound is used to find oil deposits and learn more about the inside of the planet.

The scientists were also able to 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, the city that is home to Goethe University.

“Our extensive numerical study not only allows us to make predictions for the radii and maximum masses of neutron stars,” explains Author Dr. Christian Ecker, “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,” according to the author, “will become particularly important to pinpoint the unknown equation of state with future astronomical observations and detections of gravitational waves from merging stars.”

So, while the precise structure and composition of materials inside neutron stars remain unknown, the wait for its discovery can certainly be made more bearable with a chocolate or two.

Source: 10.3847/2041-8213/ac9b2a

Image Credit: Peter Kiefer & Luciano Rezzolla