Hybrid Stars with Different Amount of Quark Matter

Deep in the cosmos, far beyond the reach of any telescope or spacecraft, lie the most extreme laboratories in the universe: neutron stars. These are the collapsed cores of massive stars that have died, crushed down until a single teaspoon of their material would weigh as much as a mountain. Inside these stellar remnants, matter is squeezed so tightly that it defies the rules we observe in everyday life. At the very center, where the pressure is unimaginable, the atoms themselves may break apart. The protons and neutrons that usually hold matter together could dissolve into a soup of their smaller building blocks, known as quarks. This hypothetical state of matter, where quarks roam freely rather than being locked inside particles, is called quark matter. If it exists, it would form a hybrid star—a cosmic object with a core of exotic quark matter surrounded by a shell of normal nuclear matter. Understanding whether these stars exist, and what they look like, is one of the great challenges in modern astrophysics, as it would reveal how nature behaves under the most intense conditions possible.

For years, scientists have tried to piece together the internal structure of these stars by combining theoretical models with real-world observations. They have data from telescopes that watch X-rays bouncing off the surfaces of neutron stars, and they have data from gravitational wave detectors that listen to the ripples in space-time caused by two neutron stars colliding. In a recent study, a team of researchers from the University of Coimbra in Portugal revisited this puzzle. They had previously created a set of models suggesting that the heaviest neutron stars must contain large cores of this exotic quark matter. However, their new work asked a different question: what if the quark cores are much smaller, or even barely present at all? By relaxing the strict requirement that the cores must be huge, they generated a new set of possibilities that aligns much better with the latest astronomical measurements.

The researchers used a powerful statistical method to sift through thousands of possible scenarios. They started with two main ingredients for their models: a description of the normal matter found in the outer layers of the star, and a description of the potential quark matter in the center. They then tested these models against a rigorous checklist of real-world data. This checklist included the known properties of nuclear matter, the precise masses and sizes of several specific neutron stars observed by the NICER X-ray telescope, and the details of the collision between two neutron stars detected by the LIGO and Virgo observatories. In their previous work, they had forced the models to produce stars with massive quark cores, which pushed the transition from normal matter to quark matter to happen at relatively low densities. In this new study, they removed that specific restriction. They still required that the heaviest stars have some quark matter inside, but they allowed the size of that core to be anything from tiny to large, letting the data decide what was most likely.

The results of this new approach were striking. The models that allowed for smaller quark cores, which the team dubbed the "SmallQ" set, produced neutron stars that were slightly smaller in radius and less deformable than those in their previous models. This made the new models a much better fit for the actual observations from the NICER telescope and the gravitational wave data. In the previous models with large cores, the transition to quark matter happened at a density about two to two and a half times the density of normal atomic nuclei. In the new, more successful models, this transition was pushed much deeper into the star, occurring at densities between three and four and a half times the normal nuclear density. This delay in the transition means that the normal matter shell of the star is thicker, and the exotic core is smaller or, in some cases, almost non-existent even in the heaviest stars.

This shift in the timing of the phase transition also changed the internal composition of the stars. In the models with large cores, the quark matter was dominated by up and down quarks. In the new models with smaller cores, the strange quarks—a heavier type of quark—became much more abundant, making up a larger fraction of the core's composition. Interestingly, the new models suggest that some of the heaviest neutron stars, those weighing twice as much as our Sun, might contain very little to no quark matter at all. This challenges the idea that the heaviest stars must necessarily be dominated by exotic matter. Instead, it suggests that the transition to quark matter is a rare event that only happens under the most extreme pressure, or perhaps not at all in some of the heaviest stars we have observed.

The study also looked at how the size of a neutron star changes as its mass increases. For stars made only of normal matter, the radius tends to shrink as the star gets heavier. For stars with exotic matter, the relationship can be more complex. The researchers found that in their new set of models, a significant number of stars still showed a positive slope in their mass-radius relationship at lower masses, meaning they got slightly larger as they gained weight. However, at a mass of 1.8 times that of the Sun, the vast majority of these models returned to the expected behavior of shrinking. This is a crucial distinction because, in their previous models with large cores, a positive slope was common even at these high masses. The fact that the new models mostly show the standard shrinking behavior at high masses, while still fitting the observational data, suggests that the presence of a large quark core is not required to explain the stars we see.

Ultimately, this work refines our understanding of the densest matter in the universe. By allowing the size of the quark core to vary freely rather than forcing it to be large, the researchers found a set of models that are more consistent with the universe as we observe it. These models suggest that if quark matter exists in the hearts of neutron stars, it is likely confined to a small region at the very center, appearing only when the density reaches extreme levels. The findings do not rule out the existence of quark matter, but they do suggest that the transition to this exotic state is more difficult to achieve than previously thought. As astronomers continue to gather more precise data on the sizes and masses of neutron stars, these refined models will serve as a critical guide, helping to determine whether the cores of these cosmic giants are made of the strange, free-roaming quarks that have long been theorized, or if they remain composed entirely of the familiar matter that makes up our own world.