Scientists have uncovered new details about the mysterious matter that makes up neutron stars, the incredibly dense remnants of exploded stars.

“The determination of the internal structure of neutron stars presents a long-standing and important challenge for nuclear theory,” wrote the team in their paper published in Physical Review Letters. “Since neutron stars were first predicted in the 1930s and observed 30 years later, many models for the structure of their interiors have been proposed, including various phases of nuclear matter.”

These incredibly dense celestial objects pack a star’s entire mass into a sphere just 10 kilometers wide, making them far denser than atomic nuclei. This extreme density makes it incredibly challenging to study their properties — like temperature and composition — which is what makes the researchers’ breakthrough so impressive.

Understanding neutron stars 

Neutron stars are the remnants of massive stars that had ended their life cycle in a spectacular supernova explosion. Afterward, the core that is left behind is compressed into an extremely small space, and the intense gravity crushes atoms so tightly that it forces atoms to collapse.

In normal matter, electrons orbit atomic nuclei made up of protons and neutrons. However, in a neutron star, the gravity is so intense that it pulls electrons into the nuclei, where they combine with protons to form neutrons. This process creates a star made up mostly of densely packed neutrons, hence why it’s called a neutron star.

While their extreme properties made them fascinating celestial objects, one of the biggest obstacles to studying neutron stars is the difficulty of probing their interiors. 

Their small size and vast distance from Earth — the closest being RX J1856. 5−3754, which is about 400 light-years away — make direct observations nearly impossible. As a result, scientists have to rely mainly on theoretical models to understand them. 

The properties of ordinary matter — the kind that makes up most objects around us and most celestial bodies — are determined by how the electrons of neighboring atoms interact. As a result, quantum electrodynamics, which describes electromagnetic interactions, is sufficient to explain their behavior. However, to describe the matter inside a neutron star, researchers need to rely on quantum chromodynamics, a fundamental theory in physics that explains how the building blocks of matter, called quarks, interact with each other through the strong force. This is the force that holds the nucleus of an atom together and binds quarks inside particles like protons and neutrons.

Due to the extreme density and gravity inside a neutron star, understanding how quarks interact is key to studying the star. However, applying quantum chromodynamics in these conditions is challenging because conventional mathematical tools struggle to account for the particles’ behavior in such an intense environment.

This means there are limits to what we can predict or know about the star’s inner workings.

Lattice quantum chromodynamics offers a solution

The scientists behind the current study have got around this problem by turning to a theory that helps simplify the problem.

In lattice quantum chromodynamics, space is divided into a discrete grid, or “lattice,” which makes it easier to calculate and model the interactions of hadron particles, which includes neutrons, under extreme conditions. 

Using this method, the team was able to figure out how different factors, like temperature, density, and pressure, relate to each other in the matter inside neutron stars. 

With this new information, the team could predict how mass is distributed inside a neutron star, its size, and how neutron stars behave in extreme astrophysical scenarios, such as mergers with other neutron stars or black holes. 

The connection to these mergers is significant because they generate gravitational waves that can be detected on Earth. By comparing gravitational wave signals with theoretical predictions, the team can test how realistic their theory is in predicting the interiors of neutron stars.

Surprising findings about the speed of sound

One of the most intriguing results of the study concerns the speed of sound inside neutron stars. The researchers found that sound waves can travel at nearly three quarters the speed of light, which is about 30% faster than previous models suggested.

This is significant because the speed of sound in a material tells us how resistant it is to being compressed. If the speed of sound is higher, it means the material is stiffer and harder to compress. The new findings suggest that neutron star matter is stiffer than scientists previously believed.

This means that neutron stars can support more mass before collapsing into black holes, which could change our understanding of how massive neutron stars can get before they reach the point of no return.

Future observations and theoretical challenges

While the study provides valuable new insights, several challenges remain. The calculations are computationally demanding, requiring significant resources to simulate the vast range of conditions found inside neutron stars. Additionally, the team’s results apply to neutron-rich matter composed only of the two lightest types of quarks: up and down quarks. However, in the ultra-dense cores of neutron stars, it is possible that strange quarks also play a role. 

Future research will need to incorporate these additional degrees of freedom to provide a more complete picture of neutron star interiors.

Testing these findings observationally will also require more precise measurements of neutron star properties. While current gravitational wave detectors such as LIGO, Virgo, and KAGRA have provided valuable data, they are not yet sensitive enough to fully probe the equation of state. Next-generation observatories may be needed to test the researchers’ predictions.

Reference: Ryan Abbot, et al., (NPLQCD Collaboration), QCD Constraints on Isospin-Dense Matter and the Nuclear Equation of State, Physical Review Letters (2025). DOI: 10.1103/PhysRevLett.134.011903

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