Neutron Star vs Quark Star in the Multimessenger Era

Deep in the heart of the universe, where gravity crushes matter to densities impossible to recreate on Earth, lie the most extreme objects known to science: compact stars. These are the remnants of massive stars that have collapsed under their own weight, packing more mass than our Sun into a sphere only a few dozen kilometers wide. For decades, astronomers have debated what these objects are made of. The standard view holds that they are neutron stars, cosmic lumps where protons and electrons have been squeezed together to form neutrons, creating a fluid so dense that a single teaspoon would weigh a billion tons. However, a competing idea suggests that under such extreme pressure, even neutrons might break apart into their fundamental building blocks—quarks—creating a star made entirely of a strange, free-flowing soup of quarks. This question of whether these stars are made of neutrons or quarks is not just a matter of classification; it touches on the very laws of physics that govern how matter behaves at its most extreme.

A new study by researchers at Shanghai Jiao Tong University brings a fresh perspective to this ancient puzzle by treating the universe as a giant laboratory. Instead of assuming all these collapsed stars are the same, the team used a powerful statistical method to analyze data from multiple sources, including gravitational waves from colliding stars and precise measurements of pulsars from NASA's X-ray telescopes. They asked a simple but profound question: if we look at the entire family of observed compact stars, do they all belong to one species, or are there two distinct types living side by side? By comparing the observed data against the predictions of three different scenarios—one where every star is a neutron star, one where every star is a quark star, and one where both types exist together—the researchers found that the universe is likely more diverse than previously thought.

The researchers did not rely on a single theory of how matter behaves inside these stars. Instead, they built a flexible, data-driven model that could adapt to whatever the observations suggested, allowing the data to speak for itself. They fed this model information from six specific celestial objects: four pulsars whose sizes and masses have been measured with high precision, and the two stars involved in the famous gravitational wave event GW170817, which provided clues about how easily these stars can be squashed. They also included theoretical limits on how matter behaves at incredibly high densities, derived from our best understanding of particle physics. The team then tested every possible combination of these six stars, asking whether each one was a neutron star or a quark star, to see which overall arrangement fit the data best.

The results pointed strongly toward a "two-family" scenario. In the most likely case, the universe contains two distinct branches of compact stars. The heavier, more massive stars belong to one family, while the lighter ones belong to another. Specifically, the study suggests that the most massive pulsar observed, known as PSR J0740+6620, is likely a strange quark star—a self-bound object made entirely of deconfined quarks. In contrast, the other five stars in the sample, including the two that collided to create the gravitational waves, are best described as traditional neutron stars. This finding is significant because it resolves a long-standing tension in astrophysics. Neutron stars are expected to have a maximum size limit; if they get too heavy, they should collapse into black holes. Yet, we observe pulsars that are nearly twice as heavy as the Sun. If all stars were neutron stars, their internal structure would need to be incredibly stiff to support such weight, but that stiffness would make them too large to fit the size measurements of smaller stars. By allowing the heavy star to be a different kind of object entirely, the study offers a natural solution: the heavy star is a quark star, which can be much more massive and compact, while the lighter stars remain neutron stars with a softer, more flexible structure.

The study also revealed how the speed of sound travels through the matter inside these stars, a property that tells us how stiff or soft the material is. In the scenario where all stars are neutron stars, the speed of sound shows a sharp, unexpected peak at certain densities, a feature that has puzzled physicists for years. However, in the favored two-family scenario, this peak disappears. Instead, the speed of sound rises steadily and then levels off, a smoother behavior that aligns better with theoretical expectations for how matter should act under such crushing pressure. This suggests that the internal structure of a neutron star is fundamentally different from that of a quark star, and that the universe does not force all compact stars into a single mold.

While the evidence is not yet absolute, the statistical weight of the data favors this mixed population. The researchers calculated that the two-family scenario is about six times more likely than the scenario where all stars are up-down quark stars, and significantly more likely than the scenario where all stars are neutron stars. This level of confidence is described as "moderate evidence," meaning the data strongly points in this direction but leaves room for future observations to refine the picture. The classification of the heaviest star as a quark star also has profound implications for the nature of matter itself. It supports a hypothesis that strange quark matter might be the most stable form of matter in the universe, potentially existing in a state that never decays. If true, this could mean that tiny fragments of this strange matter, known as strangelets, could have survived from the early universe and might even constitute a form of dark matter, though this remains a speculative possibility.

The study also tested how sensitive these conclusions are to the way astronomers interpret the data from one specific pulsar, PSR J0030+0451. Depending on the model used to analyze the hot spots on this star's surface, the classification of that particular star could shift between being a neutron star or a quark star. However, the identification of the heaviest star, PSR J0740+6620, as a quark star remained robust across all tested models. This stability suggests that the distinction between the two families is a real feature of the cosmos, not just an artifact of how we measure one specific object. As new telescopes and gravitational wave detectors come online, they will provide even sharper data, potentially confirming whether these two families of stars truly coexist in the night sky, revealing a more complex and varied population of stellar remnants than we ever imagined.