What neutron stars have in common with chocolates

Ein Stern, which is only as big as Frankfurt – and yet as heavy as the sun. The mere imagining of this pushes the layman to the limits of his imagination. Luciano Rezzolla and his colleagues do not limit themselves to mental games: they try to describe such a celestial body as precisely as possible mathematically. The team led by Rezzolla, professor of theoretical astrophysics at Goethe University, has now apparently taken a major step forward.

It has been known for over 60 years that neutron stars exist. They are formed when large stars burn out and collapse at the end of their lives. Their mass is extremely densely packed in an astronomically small space. A cubic centimeter of their matter can weigh up to a trillion grams and contains as many atoms as the entire Alps.

Over a million equations of state

Since no laboratory in the world can simulate such a condition, Rezzolla’s working group tried to computerize the structure of a neutron star. To do this, the physicists have drawn up more than a million equations of state, which describe the structure of such a structure from the surface to the core. Each of the equations is consistent with all astrophysical measurements of neutron stars and known results from nuclear physics, the researchers write.

The result of the calculations inspired Rezzolla – a friend of graphic equations – to a metaphorical visit to the candy store: according to his words, two types of neutron stars can be distinguished, the structure of which is reminiscent of different types of candy. “Light” stars, less than 1.7 times the mass of the Sun, have a soft outer shell and a firm core, much like nut candies. On the other hand, if the mass of the star is above the specified value, it will look more like a cream-filled chocolate ball – hard on the outside, soft on the inside.

Star with a radius of only twelve kilometers

The formulas also showed that the radius of a neutron star is most likely independent of its mass and is only about eight miles. In addition, limit values ​​for the deformability of such an object by tidal forces can be derived from the calculations in a binary system, ie a constellation in which two stars revolve around each other.

When two neutron stars collide, the violent shock leads to distortions of space-time, so-called gravitational waves. These can now also be observed on Earth. As Rezzolla’s colleague Christian Ecker explains, the boundary values ​​determined, along with future wave measurements, will help to better understand what happens in neutron star collisions.

Incidentally, other physicists close to Rezzolla also love the cosmic ‘candies’. The Frankfurt Institute for Advanced Studies is attempting to understand the structure of neutron stars using machine learning. And the Darmstadt GSI Helmholtz Center for Heavy Ion Research is involved in an EU project that aims to shed light on the role of stellar collisions in the formation of elements such as silver, gold and platinum. This means that the competence in neutron star research is distributed within a radius in the Rhine-Main area alone, which is larger than that of the research object itself.

The publications can be found here and here

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