Cosmic chocolate candy? The general structure of a neutron star has been revealed

A study of the speed of sound revealed that heavy neutron stars have a hard cover and a soft core, while light neutron stars have a soft cover and a hard core—much like various chocolate candy candies. Credit: Peter Kiefer and Luciano Rizzola

So far, little is known about the interiors of neutron stars, those extremely tiny objects that can form after a star dies. The mass of our Sun or more is compressed into a ball 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 simulating the extreme conditions inside neutron stars, as they are difficult to recreate on Earth in the lab. So there are many models in which different properties – of density and temperature – are described with the help of 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 in Frankfurt have succeeded in adding more important pieces to the puzzle. The working group, led by Professor Luciano Rizzola at the Institute of Theoretical Physics, developed over a million different state equations that meet the limitations set by data obtained from theoretical nuclear physics on the one hand, and from astronomical observations on the other. Their work was published in Astrophysical Journal Letters.

When evaluating the equations of state, the team made a surprising discovery: “light” neutron stars (with masses smaller than about 1.7 solar masses) appear to have a soft cover and a hard core, while “heavy” neutron stars (with masses greater than 1.7 solar masses) Instead it has a hard cap and a soft core.

“This result is very exciting because it gives us a direct measure of how compressible the center of a neutron star is,” says Professor Luciano Rizzola, and neutron stars appear to behave like candy chocolate chips: light stars are similar to those chocolates that have a hazelnut in their center surrounded by smooth chocolate, while they can be considered Heavy stars are more like those chocolates where the hard topping has a soft filling.”

The decisive factor in this vision was the speed of sound, which is the focus of a study by undergraduate student Sinan Altipermak. This quantitative measure describes how quickly sound waves propagate inside an object and depends on how soft or hard the material is. Here on Earth, the speed of sound is used to explore the planet’s interior and discover oil deposits.

By modeling equations of state, physicists have also been able to reveal other previously unexplained properties of neutron stars. For example, regardless of its mass, it is very likely that it has a radius of only 12 km. Thus, it is as large in diameter as the city of Frankfurt, the birthplace of Goethe University.

Study author Dr. Christian Ecker explains, “Our extensive numerical study allows us not only to make predictions of the radii and maximum masses of neutron stars, but also to put new limits on their deformation in binary systems, that is, how strongly they are distorted by their gravitational fields. These insights will become important in particular to determine the unknown equation of the case with future astronomical observations and gravitational wave detections from merging stars.”

more information:
About the speed of sound in neutron stars, Astrophysical Journal Letters (2022). DOI: 10.3847/2041-8213/ac9b2a

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