A quark star is a hypothetical type of compact stellar remnant made almost entirely of free quarks rather than the neutrons found in a neutron star. No quark star has been confirmed to exist, but the idea rests on a provocative and still-unresolved conjecture in physics: that matter made of up, down, and strange quarks might be the most stable form of matter in the universe. If that conjecture holds, then some of the objects we currently classify as neutron stars could actually be quark stars, and future gravitational-wave detectors and X-ray telescopes may be able to tell the difference.
The Strange Matter Hypothesis
The theoretical foundation for quark stars comes from what physicists call the Bodmer-Witten hypothesis, first proposed in the early 1970s and refined in the 1980s. Ordinary atomic nuclei are built from protons and neutrons, each of which contains three quarks bound together. Under extreme pressure, those composite particles could dissolve, freeing the quarks into a soup-like state. The hypothesis goes further: it suggests that if you add a roughly equal population of strange quarks alongside the usual up and down quarks, the resulting “strange quark matter” might actually have a lower energy per particle than normal nuclear matter. In other words, it could be the true ground state of matter, and everything we see around us, from iron to oxygen to the protons in your body, would technically be in a metastable state that just hasn’t had the nudge to convert.
Whether this is actually the case depends on details of the strong force that are fiendishly difficult to calculate. Theoretical work using different models of how quarks interact finds that confinement, the force that normally locks quarks inside protons and neutrons, plays a crucial role in whether strange quark matter can exist as a stable phase at all.1arXiv. Conditions for the existence of stable strange quark matter Models that handle confinement differently can reach opposite conclusions: some predict stable strange matter, others do not. This ambiguity is part of why the question remains open after decades of work.
How a Quark Star Could Form
The most dramatic route to a quark star runs through a supernova. When the core of a massive star collapses, it crosses into a regime of density and temperature where nuclear matter might undergo a phase transition, converting ordinary neutrons and protons into a quark-gluon plasma. If this transition is sharp enough, the sudden change in pressure can actually power the explosion itself. Researchers have shown that a first-order phase transition from nuclear matter to quark-gluon plasma during core collapse can produce energetic supernovae capable of explaining a range of observed lightcurves, from unusual Type II supernovae to some super-luminous events.2Nature Astronomy. Quark deconfinement as supernova explosion engine for massive blue-supergiant stars This is especially relevant for massive blue supergiants, which standard supernova models struggle to blow up convincingly. The quark-deconfinement mechanism provides an alternative engine that works precisely for these difficult cases.3The Astrophysical Journal. Core-collapse Supernova Explosions Driven by the Hadron-quark Phase Transition as a Rare r-process Site
A second, slower route involves an existing neutron star. If the core of a neutron star is squeezed past a critical density threshold, perhaps by accreting mass from a companion, quarks could begin to deconfine. A seed of strange quark matter would form at the center and potentially spread outward, converting the entire star. This conversion process could release enormous energy, and some theorists have proposed it as the mechanism behind certain fast radio bursts and short gamma-ray bursts. Recent work argues that the collapse of a quark star’s thin nuclear crust alone could release enough energy to account for these transient events.4arXiv. Recent progresses in strange quark stars
What a Quark Star Would Be Made Of
If ordinary matter is a building made of bricks (neutrons and protons), a quark star is that building dissolved into its constituent clay. The quarks are no longer confined inside individual particles. Instead, they roam freely throughout the star, forming a continuous fluid of up, down, and strange quarks in roughly equal numbers, bathed in a sea of gluons that mediate the strong force between them. Electrons are also present in small quantities to maintain charge balance. Theoretical models describe this material using modifications of a framework called the bag model, which treats quarks as free particles inside a “bag” of vacuum energy, plus corrections for the strong force and quark masses.5The European Physical Journal C. Properties of strange quark matter and strange quark stars
The composition matters because it determines the equation of state: the relationship between pressure and density that governs how large and heavy the star can be. Different assumptions about quark interactions lead to different equations of state, and the structural predictions vary accordingly. Studies have explored how factors like the effective bag constant (a measure of the vacuum energy confining the quarks) and the strange quark’s mass control the resulting mass-radius relationship of quark stars.6arXiv. Strange Quark Stars and Spontaneous Scalarization
Color Superconductivity and the CFL Phase
Under the extreme pressures inside a quark star, quarks are not simply floating around freely. They can pair up in a phenomenon analogous to how electrons pair in an ordinary superconductor, but involving the “color charge” that quarks carry instead of electric charge. The most symmetric version of this pairing, called the color-flavor-locked (CFL) phase, involves all three quark flavors pairing with each other across all three color charges simultaneously. The result is a remarkably ordered state of matter.
Research into this phase has found that CFL quark matter is more stable than unpaired strange quark matter. Under strong magnetic fields, CFL matter also behaves differently: oscillation effects tied to how quarks fill up energy levels in a magnetic field get smoothed out as the pairing strength increases, making CFL matter more homogeneous under conditions that would cause irregular behavior in ordinary strange quark matter.7Acta Physica Sinica. Quark matter and quark star in color-flavor-locked phase The enhanced stability of the CFL phase means that if quark stars exist, their deep interiors might be in this paired state rather than in an unpaired quark soup. The distinction is not academic: it affects how the star cools, how it responds to rotation, and what gravitational-wave signal it would emit during a merger.
How Massive and How Large
One of the most important observational constraints on any compact star model is whether it can produce objects as heavy as the heaviest neutron stars we have actually measured. Several pulsars have been clocked at around two solar masses, and any viable model of a quark star has to be able to reach that threshold. Solving the equations governing stellar structure for quark matter with various parameters shows that quark stars can in principle reach masses up to about 2.5 times the mass of the sun, comfortably above the two-solar-mass floor.8The European Physical Journal C. Massive compact stars beyond two solar masses: quark stars admixed with dark matter in regularized 4D Einstein–Gauss–Bonnet gravity The predicted mass-radius relations are consistent with observational constraints from the gravitational-wave event GW170817 and from measured pulsars like PSR J0348+0432 and PSR J0952-0607.
Quark stars would generally be more compact than neutron stars of the same mass, meaning they pack their weight into a smaller radius. This compactness has downstream effects: it influences how strongly the star warps spacetime, how quickly it can spin before flying apart, and how much it deforms under the tidal pull of a companion in a binary system. Structural calculations show that the precise radius depends sensitively on the equation of state parameters, with different quark interaction models predicting radii that range from roughly 8 to 12 kilometers for a star near the maximum mass.9The European Physical Journal C. Anisotropic quark stars with an interacting quark equation of state in extra dimension
Telling a Quark Star from a Neutron Star
If quark stars look roughly the same mass and size as neutron stars, how would anyone know the difference? Several observable signatures have been proposed, though none has yet been confirmed.
One approach involves how the star spins. Neutron stars and quark stars obey certain relationships between their moment of inertia, their tidal deformability (how much they squish under gravity from a nearby object), and their quadrupole moment (how their shape deviates from a perfect sphere). For neutron stars, these three quantities follow nearly universal relationships that hold regardless of the specific equation of state. For quark stars made of strange matter, this universality breaks down at rapid rotation rates. Different strange-matter equations of state produce completely different relationships, meaning a precise measurement of these properties for a fast-spinning compact object could reveal whether it is a quark star.10The Astrophysical Journal Letters. BREAKDOWN OF I–LOVE–Q UNIVERSALITY IN RAPIDLY ROTATING RELATIVISTIC STARS
Another proposed test involves the star’s surface. A “bare” strange star, one without any nuclear crust, would have dramatically different surface properties from a neutron star. The quark matter would extend all the way to the surface, creating an extremely sharp boundary with a huge electric field at the edge, rather than the gradually thinning atmosphere that neutron stars possess. This could affect the thermal X-ray emission from the star. One early theoretical paper speculated that if future observations of objects like the isolated neutron star RX J1856 show no peculiar polarization in their thermal X-ray emission, or if no gravitational waves are detected during post-glitch recovery phases, it would be consistent with a solid quark star rather than a conventional neutron star.11The Astrophysical Journal. Solid Quark Stars
Gravitational-wave astronomy offers another window. When two compact objects spiral into each other and merge, the tidal deformations each object undergoes leave an imprint on the gravitational-wave signal. Because quark stars are typically more compact and stiffer in certain density ranges, their tidal signatures differ from those of neutron stars. The LIGO and Virgo detectors already constrain tidal deformability from events like GW170817, and future detectors with greater sensitivity could narrow the range enough to distinguish quark-star mergers from neutron-star mergers.
Magnetic Fields and Magnetar-Like Behavior
Magnetars are compact stars with absurdly strong magnetic fields, capable of producing violent bursts of X-rays and gamma rays. If some magnetars are actually quark stars, their magnetic field dynamics would differ from those of a neutron star in interesting ways. In dense quark matter, a phenomenon called the chiral magnetic effect can drive the evolution of magnetic fields. This effect arises from an asymmetry between left-handed and right-handed quarks in the presence of the weak nuclear force, and it produces currents along magnetic field lines that can amplify or restructure the field over time.
Theoretical modeling of this process, including the effects of turbulence in the quark fluid, shows that under certain initial conditions, small-scale magnetic fields in quark matter can undergo rapid fluctuations that resemble the electromagnetic flashes observed from some magnetars. The researchers behind this work suggest that these field fluctuations, originating deep in the star’s interior, could propagate outward and trigger the bursts we observe.12International Journal of Modern Physics D. Magnetic fields in turbulent quark matter and magnetar bursts If confirmed, this would be a case where the internal composition of the star (quark matter rather than neutrons) actually shapes the observable behavior of one of the most extreme types of objects in the universe.
The Relationship to Gamma-Ray Bursts and Fast Radio Bursts
Beyond magnetar-like flashes, quark stars have been invoked to explain other high-energy transient events. Short gamma-ray bursts, which last less than about two seconds and are traditionally attributed to neutron star mergers, could also be triggered by the sudden conversion of a neutron star into a quark star or by the collapse of a quark star’s thin nuclear crust. The crust of a strange star, if it has one, would be a fragile layer of ordinary nuclear material sitting atop the quark matter surface, held in place by an enormous electric field at the boundary. If something destabilizes that crust, perhaps a starquake or accretion of material, its collapse releases energy on a scale consistent with the energetics of observed short gamma-ray bursts.13arXiv. Recent progresses in strange quark stars
Fast radio bursts, the mysterious millisecond-duration flashes of radio waves detected from cosmological distances, are another transient phenomenon where quark stars have been suggested as a possible engine. The exact mechanism differs across models, but the common thread is that the extreme energy densities and surface properties unique to strange quark matter could produce coherent radio emission in ways that ordinary neutron star models struggle to replicate. This remains speculative, and multiple competing explanations for fast radio bursts exist. Still, the quark-star connection is considered plausible enough that it continues to attract modeling effort.
Candidate Objects and the Current State of the Search
No compact star has been definitively identified as a quark star. The fundamental difficulty is that quark stars overlap with neutron stars in many of their gross observable properties: mass, spin period, magnetic field strength, and X-ray luminosity can all fall within ranges that either type of object could produce. The distinguishing features, like precise tidal deformability during a merger, surface polarization signatures, or the breakdown of universal rotation relations, require measurements at the edge of current instrumental capability.
One object that has attracted particular attention is HESS J1731-347, a compact remnant whose mass and radius estimates, derived from X-ray observations, place it in a region of the mass-radius diagram that is difficult to explain with conventional neutron star equations of state but fits comfortably with some strange quark star models. Modeling work has shown that mass-radius relations predicted for quark stars are consistent with the constraints placed by this object, alongside better-established measurements from pulsars and gravitational-wave events.14The European Physical Journal C. Massive compact stars beyond two solar masses: quark stars admixed with dark matter in regularized 4D Einstein–Gauss–Bonnet gravity Consistency is not confirmation, of course, but it keeps the hypothesis alive.
The next generation of gravitational-wave detectors, along with X-ray missions capable of precise radius measurements and polarimetry, will tighten the constraints considerably. If tidal deformability measurements from multiple merger events cluster in a range that neutron star models cannot reach, or if the universal relationships between spin, shape, and tidal response break down for specific objects in exactly the way quark star models predict, the case will strengthen. Until then, quark stars remain one of the most physically motivated hypothetical objects in astrophysics: not confirmed, but not ruled out, occupying a peculiar position where the answer depends on unresolved questions about how matter behaves at densities we cannot recreate on Earth.
Strangelets and the Question of Stability
If strange quark matter truly is the ground state of matter, an unsettling implication follows: any chunk of ordinary matter that comes into contact with a large enough seed of strange matter could, in principle, be converted. Small lumps of strange quark matter, called strangelets, have been hypothesized as cosmic-ray particles that quark stars might shed from their surfaces or produce during collisions. Pulsars, with their intense electromagnetic fields, could accelerate these strangelets to high energies and inject them into the galactic cosmic-ray population.
Searches for strangelets have been conducted in cosmic-ray detectors and in heavy-ion collision experiments at facilities like RHIC and CERN. None has been found. This null result does not disprove the strange matter hypothesis, because the production rate might simply be too low, or strangelets might be unstable at the small sizes accessible in experiments. But it does mean that one of the most dramatic predictions of the hypothesis remains unverified. The stability of strange quark matter may require a critical minimum number of quarks, perhaps on the order of hundreds, below which the surface energy penalty makes small lumps unstable. A quark star, with its astronomical number of quarks, would be well above any such threshold. The question is whether anything in between, too small to be a star but large enough to be stable, can exist and survive long enough to be detected.