Other meanings of Quark star
COMPACT-OBJECT ASTROPHYSICS
A quark star is a hypothetical compact astronomical object composed of quark matter, an ultra-dense phase in which atomic nuclei are no longer intact. The idea extends the physics of neutron stars into regimes where quarks may become deconfined and, possibly, form stable bulk matter.
A quark star would be a compact star whose interior is made primarily of deconfined quark matter rather than ordinary nuclei. At densities several times greater than nuclear saturation density, theoretical calculations allow neutrons and protons to dissolve into their constituent up and down quarks; matter containing a substantial strange-quark component is often called strange quark matter.1
The proposal depends on whether strange quark matter is energetically more stable than ordinary nuclear matter at zero pressure, the premise known as the strange-matter hypothesis. If that condition holds, a star could consist almost entirely of self-bound quark matter, potentially with only a thin crust of conventional ions and electrons. If it does not, quark matter might still occur only as a core inside a conventional neutron star.
Quark-star structure is controlled by the equation of state of dense quark matter, which relates pressure, density, temperature, and composition. Unlike a purely gravitationally bound neutron star, a self-bound quark star can have finite density at its surface and may support very small masses with radii far below those of ordinary neutron stars.2
Candidate signatures include unusual mass–radius relations, rapid rotation, distinctive cooling, and changes in the frequencies of stellar oscillations. Some models permit color superconductivity, in which quarks form paired states analogous in broad outline to electron pairing in superconductors. Predicted signals are not unique: magnetic fields, atmosphere composition, crust physics, and uncertain nuclear interactions can produce similar observational effects.
No observed object has been established as a quark star. The principal observational test is comparative: measurements of masses, radii, cooling, rotation, and mergers are used to eliminate equations of state that cannot support the observed compact-star population. The existence of neutron stars near two solar masses already rules out many overly soft models of dense matter.3
Observations of gravitational waves from binary neutron-star mergers, especially GW170817, and electromagnetic measurements have narrowed the range of viable stellar radii and tidal deformabilities, but they have not uniquely distinguished hadronic matter from a quark-containing interior.4 A transition to quark matter could also occur during a merger or stellar collapse without leaving a permanently stable quark star.
Quark stars are not a single sharply defined model but a family of possibilities. Some are proposed as completely self-bound strange stars; others are hybrid stars with a hadronic mantle surrounding a quark core. The transition between these descriptions may be a smooth crossover or a first-order phase transition with a density discontinuity, depending on the still-uncertain behavior of quantum chromodynamics at high baryon density.3
A small quark star could, in principle, have a much smaller radius than a neutron star while retaining an intense gravitational field. Another niche possibility is conversion: a neutron star encountering a stable seed of strange matter might undergo rapid transformation, releasing energy and changing its radius and spin.1 These scenarios remain theoretical because terrestrial experiments cannot directly reproduce the relevant densities.
Quark-star properties remain model-dependent because quantum chromodynamics at the densities inside compact stars is not yet fully constrained by experiment.
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