Distant stars spiraling toward a collision help unravel the mysterious forces that hold subatomic particles together

Massive core physics

The physics of massive nuclei can be studied by measuring the “note” at which the tidal resonance between fused neutron stars causes the solid crust of neutron stars to break apart. Credit: University of Bath

Space scientists have found a new way to probe the internal structure of neutron stars, giving clues about the composition of matter at the atomic level.

Space scientists at the University of Bath in the UK have found a new way to probe the internal structure of neutron stars, giving nuclear physicists a new tool to study the structures that make up matter at the atomic level.

Neutron stars are dead stars that have been compressed by gravity to the size of small cities. They contain the most extreme matter in the universe, which means that they are the densest objects in existence (in comparison, if the Earth were compressed to the density of a neutron star, it would measure only a few hundred meters in diameter, and all humans would fit in a teaspoon). This makes neutron stars unique natural laboratories for nuclear physicists, whose understanding of the force that binds subatomic particles together is limited to their work on atomic nuclei attached to Earth. Studying how this force behaves under more extreme conditions offers a way to deepen your understanding.

Enter astrophysicists, who search for distant galaxies to unravel the mysteries of physics.

In a study described in the Monthly Notices from the Royal Astronomical SocietyAstrophysicists in Bath have discovered that the action of two neutron stars moving faster and faster as they spiral into a violent collision gives a clue to the material composition of neutron stars. From this information, nuclear physicists will be in a stronger position to calculate the forces that determine the structure of all matter.


It is through the phenomenon of resonance that the Bath team has made their discovery. Resonance occurs when force is applied to an object at its natural frequency, generating a large, often catastrophic vibrational motion. A well-known example of resonance is found when an opera singer breaks glass by singing loud enough at a frequency that matches the glass’s modes of oscillation.

When a pair of coiled neutron stars reach a resonance state, their solid crust, believed to be 10 billion times stronger than steel, breaks apart. This results in the release of a bright burst of gamma rays (called a Resonant Piercing Flash) that can be seen by satellites. The spiral stars also release gravitational waves that can be detected by instruments on Earth. The Bath researchers found that by measuring both the flare and the gravitational wave signal, they can calculate the ‘energy of symmetry’ of the neutron star.

The energy of symmetry is one of the properties of nuclear matter. It controls the proportion of subatomic particles (protons and neutrons) that make up a nucleus, and how this proportion changes when subjected to the extreme densities found in neutron stars. Therefore, a reading of the symmetry energy would give a strong indication of the composition of neutron stars and, by extension, the processes by which all protons and neutrons couple together and the forces that determine the structure of the entire subject.

The researchers emphasize that measurements obtained by studying the resonance of neutron stars using a combination of gamma rays and gravitational waves would be complementary, rather than a replacement, for the laboratory experiments of nuclear physicists.

“By studying neutron stars and the cataclysmic final motions of these massive objects, we can understand something about the tiny nuclei that make up extremely dense matter,” said Bath astrophysicist Dr. David Tsang. “The huge difference in scale makes this fascinating.”

Astrophysics PhD student Duncan Neill, who led the research, added: “I like that this work looks at the same thing that nuclear physicists are studying. They look at tiny particles and we astrophysicists look at objects and events many millions of light years away. We are seeing the same thing in a completely different way. “

Dr. Will Newton, an astrophysicist at Texas A&M University-Commerce and a collaborator on the project, said: “Although the force that binds quarks together in neutrons and protons is known, it is not well understood how it actually works when large amounts of neutrons and protons. The quest to improve this understanding is aided by experimental data from nuclear physics, but all the nuclei we probed on Earth have similar numbers of neutrons and protons bound together at roughly the same density.

“In neutron stars, nature provides us with a very different environment to explore nuclear physics: matter composed mainly of neutrons and covering a wide range of densities, up to about ten times the density of atomic nuclei. In this article, we show how we can measure a certain property of this matter, the energy of symmetry, from distances of hundreds of millions of light years away. This can shed light on the fundamental functioning of the nuclei. “

Reference: “Flashing Resonants as Nuclear Symmetry Energy Multiple Messenger Probes” by Duncan Neill, William G Newton, and David Tsang, March 26, 2021, Monthly Notices from the Royal Astronomical Society.
DOI: 10.1093 / mnras / stab764

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