Utrecht University: Nature paper sheds more light on the stuff neutron stars are made of

Researchers from the National Institute for Subatomic Physics Nikhef and Utrecht University, among others, have gained clearer insight into the properties of the extremely compact material of which neutron stars are made, by combining data from nuclear collisions and gravitational waves. They published their new insights in the scientific journal Nature.

Neutron stars are the small and extremely dense remnants of collapsed massive stars. The matter in these strange objects is so closely packed together by gravity that in a relatively small sphere only neutrons are left. A teaspoon of the stuff would weigh tons on Earth.

Rendering of neutron star
Accelerator data
Astronomers and physicists are trying to deduce the properties of the neutron material on paper and from observations. To do this, the research team including Peter Pang (Utrecht University), has now for the first time brought together two types of data: observations of gravitational waves from neutron star collisions, and measurements from particle accelerators. Tsun Ho (Peter) Pang is shared first author of the Nature publication.

The team used two existing observations of colliding neutron stars via gravitational waves, from 2017 and 2019. In the light of the first collision they saw that traces of heavy metals such as gold were produced in the process. From the waves, the researchers could deduce the mass and dimensions of the colliding neutron stars.

Space-time ripples
Gravitational waves are ripples in the fabric of space and time, that Einstein’s 1916 theory of relativity predicts to be possible if compact objects like neutron stars or black holes collide. Such tiny ripples were first measured in 2015 by detectors in the US and Europe.

A neutron star 1.4 times the mass of the sun compresses all its matter into a sphere about 24 kilometers in diameter.
To those observations the researchers added data from nuclear physics: existing measurements of colliding gold nuclei in laboratories. Such collisions are not able to produce the giant density of matter that exists in a neutron star, but they compress the nuclear material denser than in ordinary matter, and they do appear to be able to sharpen models for neutron stars.

The combined measurements suggest that a neutron star 1.4 times the mass of the sun compresses all its matter into a sphere that is about 24 kilometers in diameter. This is close to earlier estimates, but more precise due to the combination of observations from different fields. The analysis also used data from NICER, a measuring instrument aboard the ISS space station.

In the future we can bridge the gap between nuclear theory and astrophysical observations.

Prof. Dr. Chris van den Broeck
Bridging the gap
These improved density estimates from heavy-ion collisions show we can bridge the gap between nuclear theory and astrophysical observations in the future, by complementing each other, says Prof. Chris Van Den Broeck, co-author of the Nature paper for Utrecht University and Nikhef.

The researchers expect that in the future, new data from both fields can be easily combined to further improve the understanding of extremely dense matter. At the GSI laboratory in Darmstadt, the FAIR experiment is currently being built, in which densities can be achieved similar to those that exist in neutron stars.

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