Strange Quark Stars: Self-Bound Compact Stars

• Strange quark stars are hypothetical compact stars made entirely of stable, three-flavor quark matter that may have a bare surface or a thin nuclear crust.
• Their self-bound nature—driven by strong interactions rather than gravity—produces distinct mass–radius relations (M ∝ R³) and ultra-high surface electric fields.
• Diverse equations of state, from the MIT bag model to Dyson-Schwinger approaches, reveal unique oscillation, tidal, and merger features that help differentiate them from neutron stars.

Searching arXiv for recent and foundational papers on strange quark stars to ground the article. Strange quark stars are hypothetical compact stars composed of absolutely stable three-flavor strange quark matter, i.e. deconfined up, down, and strange quarks, possibly with electrons, and in some models with either no crust, a thin nuclear crust, or a strangelet crust. Their existence is tied to the strange quark matter hypothesis, according to which strange quark matter may be the true ground state of hadronic matter (Weber et al., 2012). In contrast to neutron stars, they are self-bound by the strong interaction, a property that leads to distinctive structural, dynamical, and observational consequences, including the scaling $M \propto R^3$ for low-mass self-bound configurations, the possible existence of strange dwarfs and strange quark planets, and, for bare stars, ultra-high surface electric fields on the order of $10^{18}$ to $10^{19}\,\mathrm{V/cm}$ (Weber et al., 2012, Geng et al., 2015). Contemporary work studies these objects across multiple theoretical settings, including the MIT bag model, Dyson-Schwinger and contact-interaction approaches to dense QCD, modified-gravity frameworks, dark-matter admixture, stellar evolution, and binary-merger simulations (Banerjee et al., 2020, Chen et al., 2016, Zhang et al., 22 Apr 2026, Zhang et al., 2024).

1. Definition, self-binding, and stellar taxonomy

Strange quark stars are proposed to be made of absolutely stable strange quark matter. In the overview of quark-star structure, such objects can be either bare or enveloped in thin nuclear crusts, and their structure differs qualitatively from that of neutron stars because self-binding by the strong force, rather than gravity alone, determines the equilibrium over a wide mass range (Weber et al., 2012). A central consequence is the low-mass scaling $M \propto R^3$, which also underlies the prediction of a continuous hydrostatically stable sequence extending from $1$–$2\,M_\odot$ strange stars down to strange dwarfs and strange planets (Weber et al., 2012, Geng et al., 2015).

Bare quark stars are described as having deconfined quark matter up to the surface, with the surface itself extremely thin, of order $1$ fm, and an electron layer several hundred fermis thick above it (Weber et al., 2012). Dressed, or crusted, quark stars instead possess a thin nuclear crust made of heavy ions in an electron gas, held out of contact with the quark core by an intense electric dipole layer. The maximum base density of such a crust is set by the neutron drip point, approximately $4.3\times10^{11}\,\mathrm{g/cm^3}$, because free neutrons would otherwise be absorbed into the quark matter core (Weber et al., 2012).

A distinctive structural feature is that quark stars with crusts form two-parameter sequences. Their structure depends on the central star density and the density at the base of the crust, whereas neutron stars are primarily described by one-parameter sequences governed by central density (Weber et al., 2012). The same self-bound character also implies that there is no minimum mass for strange stars, in contrast to neutron stars, and that compact SQM objects can extend into the planetary regime (Weber et al., 2012, Geng et al., 2015).

For bare stars, the electron cloud produces ultra-high electric fields $E \sim 10^{18}-10^{19}\,\mathrm{V/cm}$, with the associated energy density $\mathcal{E}=E^2/(8\pi)$ comparable to the quark matter energy density; the review of quark-star structure notes that this can affect the stellar mass and radius by up to $10^{18}$0 and $10^{18}$1, respectively (Weber et al., 2012). Bare stars are also not subject to the Eddington luminosity limit, which has been invoked in discussions of strong pair-creation-driven surface emission (Weber et al., 2012). These properties collectively motivate treating strange quark stars not merely as neutron-star analogues, but as a broader class of self-bound compact objects with a richer taxonomy.

2. Equations of state and microscopic descriptions of strange quark matter

A large fraction of the literature models strange quark stars using the MIT bag model or closely related variants. In the 4D Einstein-Gauss-Bonnet study, two equations of state are used: a massless-quark approximation with

$10^{18}$2

and a cold-star approximation,

$10^{18}$3

quoted for $10^{18}$4 (Banerjee et al., 2020). In other MIT-based treatments, quark matter is written as

$10^{18}$5

or equivalently

$10^{18}$6

with the bag constant fixed according to the model under study (Lopes et al., 2018, Sinha et al., 11 Sep 2025, Wang et al., 2021). The short review on recent progress presents the MIT form schematically as

$10^{18}$7

with $10^{18}$8 for massless quarks (Zhang et al., 2024).

More microscopic approaches attempt to encode nonperturbative QCD effects directly. In the Dyson-Schwinger quark model, the strange quark matter pressure is written as

$10^{18}$9

with model constraints imposed by the stability of ordinary nuclear matter and by the requirement that strange quark matter be more bound than iron if the strange-matter hypothesis is to hold (Chen et al., 2016). Respecting these constraints, the maximum mass is about $10^{19}\,\mathrm{V/cm}$0 solar masses and typical radii are $10^{19}\,\mathrm{V/cm}$1–$10^{19}\,\mathrm{V/cm}$2 km (Chen et al., 2016). In a later Poincaré-covariant study using a symmetry-preserving vector$10^{19}\,\mathrm{V/cm}$3vector contact interaction, the equation of state is constructed from a momentum-independent quark propagator at finite chemical potential, with

$10^{19}\,\mathrm{V/cm}$4

subject to beta equilibrium and charge neutrality (Zhang et al., 22 Apr 2026). That analysis finds that reducing the coupling constant stiffens the EOS, whereas increasing the ultraviolet cutoff softens it, and identifies parameter sets $10^{19}\,\mathrm{V/cm}$5 and $10^{19}\,\mathrm{V/cm}$6, together with $10^{19}\,\mathrm{V/cm}$7, as compatible with multi-messenger constraints (Zhang et al., 22 Apr 2026).

Other effective descriptions include the quasi-particle model and the extended MIT bag model. In the dark-matter-admixed study, the quasi-particle model uses effective masses and a running coupling, with pressure

$10^{19}\,\mathrm{V/cm}$8

while the extended MIT bag model incorporates medium effects through density-dependent effective masses and writes

$10^{19}\,\mathrm{V/cm}$9

(Yang et al., 2024). The review on recent progress further lists the NJL model and quasi-particle model as two of the three popular phenomenological descriptions besides MIT, emphasizing that the resulting mass-radius relations depend sensitively on model assumptions (Zhang et al., 2024).

A recent scaling analysis abstracts away from specific microphysics and links macroscopic observables to the energy per baryon at zero pressure $M \propto R^3$0 and repulsive interactions. In that work, the maximum mass obeys

$M \propto R^3$1

with $M \propto R^3$2 in the QMDD model and $M \propto R^3$3 in the vector MIT model (Lugones et al., 14 Mar 2025). This suggests that self-binding and repulsion can be parameterized in a way that exposes robust macroscopic trends across substantially different microscopic models.

3. Structure equations, stability, and compactness

Given an equation of state, stellar structure is obtained by solving the Tolman-Oppenheimer-Volkoff equations or modified variants. In the review of recent progress, the standard GR equations are written as

$M \propto R^3$4

(Zhang et al., 2024). These equations also appear in the Dyson-Schwinger study as the basis for deriving mass-radius curves (Chen et al., 2016).

The self-bound character of strange quark matter changes both boundary conditions and stability diagnostics. In the 4D Einstein-Gauss-Bonnet treatment, the pressure gradient and mass equation are modified by the Gauss-Bonnet coupling $M \propto R^3$5,

$M \propto R^3$6

$M \propto R^3$7

with the GR limit recovered as $M \propto R^3$8 (Banerjee et al., 2020). Positive $M \propto R^3$9 increases the maximum mass and radius, while negative $1$0 reduces them (Banerjee et al., 2020). For the massless quark EOS with $1$1, the quoted maximum mass changes from $1$2 and $1$3 in GR to $1$4 and $1$5 for $1$6 (Banerjee et al., 2020).

The same study defines compactness as

$1$7

and reports values in the range $1$8, with compactness slightly higher for negative $1$9 and lower for positive $2\,M_\odot$0 at fixed mass (Banerjee et al., 2020). Stability is tested by the condition $2\,M_\odot$1 and by the adiabatic index

$2\,M_\odot$2

with all studied configurations satisfying $2\,M_\odot$3 (Banerjee et al., 2020).

In mimetic gravity, the equilibrium equation acquires an additional force $2\,M_\odot$4,

$2\,M_\odot$5

and equilibrium is reported not to be achieved unless the mimetic extra force is included (Sinha et al., 11 Sep 2025). The same paper checks NEC, WEC, DEC, and SEC, quotes $2\,M_\odot$6 as the adiabatic stability criterion, and finds $2\,M_\odot$7 together with surface redshift below $2\,M_\odot$8 in all configurations studied (Sinha et al., 11 Sep 2025).

Universal and scaling relations specific to strange quark stars have also been proposed. The 2025 scaling study gives an EOS-independent relation for the normalized moment of inertia,

$2\,M_\odot$9

with $1$0, $1$1, $1$2, $1$3, $1$4, as well as

$1$5

with $1$6, $1$7, $1$8, $1$9, and

$4.3\times10^{11}\,\mathrm{g/cm^3}$0

with $4.3\times10^{11}\,\mathrm{g/cm^3}$1, $4.3\times10^{11}\,\mathrm{g/cm^3}$2 (Lugones et al., 14 Mar 2025). That work emphasizes that these relations differ significantly from those previously established for hadronic stars.

4. Oscillations, tidal deformability, and response to perturbations

Tidal deformability is a central observable in strange-star phenomenology. In the recent review, it is defined through

$4.3\times10^{11}\,\mathrm{g/cm^3}$3

with $4.3\times10^{11}\,\mathrm{g/cm^3}$4 obtained from the internal perturbation problem coupled to the background stellar structure (Zhang et al., 2024). The Poincaré-covariant contact-interaction study uses

$4.3\times10^{11}\,\mathrm{g/cm^3}$5

and explicitly accounts for nonzero surface energy density in bare self-bound stars (Zhang et al., 22 Apr 2026). For $4.3\times10^{11}\,\mathrm{g/cm^3}$6 stars it quotes $4.3\times10^{11}\,\mathrm{g/cm^3}$7 for $4.3\times10^{11}\,\mathrm{g/cm^3}$8, $4.3\times10^{11}\,\mathrm{g/cm^3}$9, and $E \sim 10^{18}-10^{19}\,\mathrm{V/cm}$0 for $E \sim 10^{18}-10^{19}\,\mathrm{V/cm}$1, $E \sim 10^{18}-10^{19}\,\mathrm{V/cm}$2, $E \sim 10^{18}-10^{19}\,\mathrm{V/cm}$3, describing both as fully compatible with GW170817 constraints (Zhang et al., 22 Apr 2026).

Low-mass strange objects exhibit especially distinctive tidal properties. For bare strange quark planets and dwarfs, one study finds $E \sim 10^{18}-10^{19}\,\mathrm{V/cm}$4 and the analytic scalings

$E \sim 10^{18}-10^{19}\,\mathrm{V/cm}$5

adding that bare strange quark planets are effectively rigid to tides and have extremely small tidal deformabilities (Wang et al., 2021). The same work states that, for a typical $E \sim 10^{18}-10^{19}\,\mathrm{V/cm}$6 compact star, the tidal deformability of a strange dwarf is about $E \sim 10^{18}-10^{19}\,\mathrm{V/cm}$7 times less than that of a normal white dwarf, and that the distinction between strange planets and normal planets is even more pronounced (Wang et al., 2021).

Oscillation spectra provide a complementary diagnostic. In the Newtonian Cowling treatment of strange stars with a strangelet crust, the $E \sim 10^{18}-10^{19}\,\mathrm{V/cm}$8 spheroidal $E \sim 10^{18}-10^{19}\,\mathrm{V/cm}$9-mode and $\mathcal{E}=E^2/(8\pi)$0-mode frequencies are calculated for a two-component core-plus-crust model (Asbell et al., 2017). For homogeneous strange quark stars, $\mathcal{E}=E^2/(8\pi)$1-modes lie in the $\mathcal{E}=E^2/(8\pi)$2–$\mathcal{E}=E^2/(8\pi)$3 kHz range and $\mathcal{E}=E^2/(8\pi)$4-modes in the $\mathcal{E}=E^2/(8\pi)$5–$\mathcal{E}=E^2/(8\pi)$6 kHz range, higher than for typical neutron stars, while the addition of a realistic strangelet crust changes the spheroidal frequencies only very slightly, by a few Hz for the $\mathcal{E}=E^2/(8\pi)$7-mode and up to about $\mathcal{E}=E^2/(8\pi)$8 for the $\mathcal{E}=E^2/(8\pi)$9-modes (Asbell et al., 2017). The paper stresses that Newtonian gravity overestimates the frequencies compared with relativistic calculations (Asbell et al., 2017).

Fully general-relativistic simulations have extended the analysis from isolated stars to mergers. A novel smooth-crust prescription was introduced to regularize the enthalpy discontinuity at the surface, using a very thin crust with mass $10^{18}$00, thickness $10^{18}$01 m, and only $10^{18}$02 effect on tidal deformability (Zhu et al., 2021). The oscillation frequencies of isolated simulated stars agree with perturbative predictions, with the $10^{18}$03 mode within $10^{18}$04–$10^{18}$05 of linear perturbation theory (Zhu et al., 2021). In equal-mass $10^{18}$06 binary mergers, the inspiral and post-merger frequencies follow the same quasi-universal relations derived from hadronic stars when expressed in terms of the tidal deformability $10^{18}$07, including

$10^{18}$08

but not when expressed in terms of the average compactness (Zhu et al., 2021). This is a direct statement that universal relations framed in $10^{18}$09 can mask the self-bound nature of strange stars.

Radial oscillations have also been proposed as a dissipation channel specific to strange quark matter objects. One recent study assumes a critical surface density

$10^{18}$10

below which strange quark matter becomes unstable and decays into photons, hadrons, and leptons (Jałocha et al., 13 Jul 2025). For small oscillation amplitudes $10^{18}$11, it gives the total energy to be radiated for small constant-density objects as

$10^{18}$12

and quotes $10^{18}$13 for a $10^{18}$14 star with $10^{18}$15, with energy emitted on timescales of order $10^{18}$16 ms (Jałocha et al., 13 Jul 2025). The same work argues that larger amplitudes may lead to fragmentation or dissolution of the surface layers (Jałocha et al., 13 Jul 2025).

5. Formation, evolution, and astrophysical environments

Several channels for strange-star formation have been proposed. In low-mass X-ray binaries, one study assumes that quark deconfinement occurs when the neutron-star core density reaches $10^{18}$17, where $10^{18}$18, corresponding to a critical density of $10^{18}$19 (Zhu et al., 2013). With a standard equation of state, the corresponding critical gravitational mass is $10^{18}$20, implying that a typical $10^{18}$21 neutron star must accrete $10^{18}$22 before conversion (Zhu et al., 2013). Population synthesis in that work gives a conversion fraction of about $10^{18}$23–$10^{18}$24 of LMXBs, depending strongly on accretion efficiency and initial neutron-star mass, and an estimated Galactic birthrate of isolated strange stars or submillisecond pulsars of $10^{18}$25–$10^{18}$26 per Myr if the conversion disrupts the binary (Zhu et al., 2013).

The two-families scenario embeds strange-star formation in binary evolution more broadly. There, below a critical gravitational mass $10^{18}$27 only hadronic stars exist; in the range $10^{18}$28 hadronic stars and strange quark stars coexist; and above $10^{18}$29 all compact objects are strange quark stars, with $10^{18}$30–$10^{18}$31 and $10^{18}$32–$10^{18}$33 (Wiktorowicz et al., 2017). Large-scale population synthesis identifies accretion from a secondary onto a neutron star as the main channel for strange-star formation in binaries, accounting for $10^{18}$34–$10^{18}$35 of cases, while strange quark stars make up $10^{18}$36–$10^{18}$37 of all compact objects in binaries and $10^{18}$38–$10^{18}$39 in LMXBs (Wiktorowicz et al., 2017). Double strange-quark-star systems are rare, with mergers at a rate of $10^{18}$40 Gyr$10^{18}$41 in the Milky Way (Wiktorowicz et al., 2017).

A more recent binary-supernova scenario considers a compact system containing a neutron-star companion and an evolved carbon-oxygen or Wolf-Rayet star. Three-dimensional SPH simulations show that fallback onto the newborn neutron star and hypercritical accretion onto the companion can raise the central density enough to trigger deconfinement (Becerra et al., 29 Jul 2025). That work states that the accretion rates reach $10^{18}$42–$10^{18}$43, and uses the strangeness threshold $10^{18}$44–$10^{18}$45 as the deconfinement criterion (Becerra et al., 29 Jul 2025). It further argues that the conversion can release $10^{18}$46–$10^{18}$47 erg and may leave NS–SQS or SQS–SQS binaries (Becerra et al., 29 Jul 2025).

Thermal evolution from proto-strange stars to cold stable strange stars has also been modeled. A self-consistent thermodynamic treatment based on the baryon density-dependent quark mass model follows the star through four stages: neutrino-trapped birth, early deleptonization, maximum heating/neutrino transparency, and final cooling to $10^{18}$48 after about $10^{18}$49 years (Chen et al., 16 Jan 2026). The maximum mass and radius decrease monotonically along this sequence, from $10^{18}$50, $10^{18}$51 in the first stage to $10^{18}$52, $10^{18}$53 for the cold stable strange star (Chen et al., 16 Jan 2026). The same study reports core temperatures below $10^{18}$54 MeV at birth, below $10^{18}$55 MeV in the second stage, below $10^{18}$56 MeV during maximum heating, and $10^{18}$57 MeV in the final state, while noting that cold strange-star configurations are consistent with HESS J1731-347, PSR J1231-1411, PSR J0030+0451, PSR J0348+0432, and PSR J0740+6620 (Chen et al., 16 Jan 2026).

6. Observational probes, multimessenger signatures, and current constraints

The chief observational difficulty is that strange stars and neutron stars can have similar masses and compactness. Several papers explicitly state that distinguishing them is extremely difficult using traditional mass-radius measurements, spin, or cooling alone (Zhu et al., 2021, Wang et al., 2021). The general-relativistic merger simulations sharpen this point: quark-star and hadronic-star binaries with comparable tidal deformabilities produce very similar inspiral gravitational-wave signals, even when their radii differ by over $10^{18}$58 km, and become distinguishable only if independent radius or compactness information is available (Zhu et al., 2021).

One proposed avenue is the search for strange quark planets. Because SQM objects are self-bound and extraordinarily dense, the tidal disruption radius is much smaller than for normal planets. In the strange-planet merger study, the disruption radius is written as

$10^{18}$59

which for a strange planet with $10^{18}$60 becomes $10^{18}$61 cm (Geng et al., 2015). The same work gives the GW power

$10^{18}$62

and the strain amplitude for a circularized binary,

$10^{18}$63

reporting $10^{18}$64 at $10^{18}$65 kpc for a strange planet of mass $10^{18}$66 near disruption (Geng et al., 2015). A related observational study proposes that pulsar planets with orbital periods $10^{18}$67 s are strong SQM candidates because normal planets could not survive so close to the host (Wang et al., 2021).

Electromagnetic manifestations have likewise been discussed. Bare stars can exceed the Eddington limit and may have pair-creation-driven surface emission (Weber et al., 2012). Accretion-induced or spontaneous conversion of a neutron star into a strange star has been associated with large energy releases: the Dyson-Schwinger study gives

$10^{18}$68

and quotes $10^{18}$69 for a typical $10^{18}$70 neutron star and $10^{18}$71 for the heaviest stars considered (Chen et al., 2016). The two-families population-synthesis work links such transitions to GRB-like phenomena not associated with supernovae or double-neutron-star mergers (Wiktorowicz et al., 2017). The recent review also notes proposals connecting strange-star crust collapse and binary-merger or conversion events with short gamma-ray bursts and fast radio bursts (Zhang et al., 2024).

Current observational constraints do not yet compel a unique interpretation. The Poincaré-covariant contact-interaction EOSs are reported to match pulsar mass measurements, GW170817, and NICER-related constraints for suitable parameter choices (Zhang et al., 22 Apr 2026). The evolutionary study argues that cold strange-star sequences can reproduce observed masses and radii of several compact objects that are difficult to explain in standard neutron-star models (Chen et al., 16 Jan 2026). A Bayesian ranking analysis based on NICER mass-radius measurements goes further, stating that for PSR J0614-3329 the measured $10^{18}$72 and $10^{18}$73 km provide a strong case for strange quark stars over physically motivated neutron-star models compatible with such a low radius (Shirke et al., 4 Aug 2025). That study reports that many nucleonic EOSs are decisively disfavored relative to a broad class of strange-star EOSs, although some softer or crossover models remain comparatively competitive (Shirke et al., 4 Aug 2025).

Controversies remain. The 4D Einstein-Gauss-Bonnet paper itself notes that the claim that the resulting 4D theory is of pure graviton has been cast in doubt on several grounds (Banerjee et al., 2020). In merger phenomenology, the fate of ejected strange quark matter remains unsettled: fully relativistic simulations find that dynamical ejecta are $10^{18}$74 less for strange-star binaries than for hadronic ones, with much smaller high-velocity and high-entropy tails, but also conclude that predicting kilonova or nucleosynthetic signatures requires an accurate model for evaporation of ejected quarks into nucleons (Zhu et al., 2021). This suggests that the principal challenge is no longer just constructing viable strange-star models, but identifying observables that are simultaneously robust against EOS degeneracy and sensitive to self-bound quark matter.