Neutron stars are the densest observable objects in the universe. A new Finnish Centre of Excellence investigates them with a combination of astrophysics, particle physics, nuclear physics and relativity.
What happens to endlessly compacted matter? The compression can be so intense that all of the 'empty' space between atomic nuclei and electrons disappear. In such extreme conditions, even the components of atomic nuclei, that is, protons and neutrons, break down, and the state of matter changes.
Investigating this phenomenon on Earth is difficult. Such collisions can only be realised in particle accelerators on an extremely small scale, and for no longer than fractions of a second.
In contrast, far-away neutron stars in space present a perfect laboratory. A typical neutron star is only about 20 kilometres in diameter. Even the lightest ones have the mass of almost one and a half times the Sun, the heaviest slightly more.
This means that their matter is compacted to extreme density. A single teaspoon of a neutron star would weigh six billion tonnes on Earth. Packing the entire humanity together equally densely would fit us in a single sugar cube.
Conditions more extreme than these are found only inside black holes.
"Neutron stars are much more interesting, though," says Associate Professor of Astrophysics .
"Since nothing escapes black holes, we can't know anything about their insides. Neutron stars, on the other hand, emit radiation across a wide range of wavelengths, from radio waves to X-rays. This is why we are able to obtain information on their conditions."
These extreme conditions are the focus of research at the recently established Centre of Excellence in Neutron-Star Physics. The multidisciplinary group combines theoretical and observational astrophysics, particle physics, nuclear physics and the study of relativity.
"The pressure of neutron stars breaks down even the structure of atomic nuclei so that individual neutrons and protons can no longer hold together," says Professor of Theoretical Particle Physics from the University of Helsinki, who heads the Centre of Excellence.
This results in quark matter formed by elementary particles, which is not found anywhere else in the universe.
The Centre of Excellence will test this hypothesis by combining theoretical calculations and supercomputer simulations with the latest observations using statistical methods. The Centre will also investigate events on the surface of neutron stars, starquakes and neutron star collisions.
"These are the most violent events taking place in space," Nättilä says. "Probably almost all of the known heavy elements, such as gold and uranium, have been born in such collisions."
