Space pralines? Revealed the general structure of a neutron star

(ORDO NEWS) — Until now, little is known about the interiors of neutron stars, extremely compact objects that form after the death of a star.

A mass commensurate with our sun or more is compressed into a sphere with a diameter of a large city. Since their discovery over 60 years ago, scientists have tried to study their structure.

The biggest challenge is simulating extreme conditions inside neutron stars, as they are unlikely to be recreated in a lab on Earth.

Therefore, there are many models in which various properties – from density to temperature – are described using the so-called equations of state.

These equations attempt to describe the structure of neutron stars from the stellar surface to the inner core.

However, physicists at the Goethe University in Frankfurt have managed to add a few important pieces to the puzzle.

A working group led by Professor Luciano Resolla from the Institute for Theoretical Physics has developed over a million different equations of state that satisfy all the constraints set by data obtained from both theoretical nuclear physics and astronomical observations.

While evaluating the equations of state, the working group made an unexpected discovery: “light” neutron stars (with masses less than about 1.7 solar masses) appear to have a soft mantle and a hard core, while “heavy” neutron stars (with masses greater than 1.7 .7 solar masses) instead have a rigid mantle and a soft core.

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

“Neutron stars seem to behave a bit like chocolates: hazelnuts in the center surrounded by soft chocolate in one case, candies with a hard layer of chocolate and soft filling in another.”

Crucial to this find was the speed of sound, which was studied by undergraduate student Sinan Altiparmak.

This quantitative measure describes how fast sound waves propagate inside an object and depends on how hard or soft the matter is. Here on Earth, the speed of sound is used to explore the interior of the planet and discover oil deposits.

By modeling the equations of state, physicists have also been able to uncover other previously unexplained properties of neutron stars.

For example, regardless of their mass, they typically have a radius of only 12 km. Thus, their diameter is commensurate with Frankfurt, the hometown of the Goethe University.

Study author Dr. Christian Ecker explains: “Our extensive numerical study allows us not only to make predictions about the radii and maximum masses of neutron stars, but also to set new limits on their deformability in binary systems, or how much they distort each other through their gravitational fields.

These ideas will become especially important for the precise determination of the as yet unknown equation of state with future astronomical observations and the detection of gravitational waves from merging stars.”

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