Strangeon Stars: A Compact Star Alternative

• Strangeon stars are hypothetical self-bound compact stars composed of localized multi-quark clusters (strangeons) that differ from conventional neutron and quark stars.
• Models using Lennard–Jones and linked-bag EOS frameworks capture their high stiffness, predicting unique mass–radius relations, oscillation modes, and tidal behaviors.
• Their global solid nature offers explanations for pulsar glitches, starquakes, and merger phenomena, though the microphysical details remain model-dependent.

Strangeon stars (SnSs) are hypothetical self-bound compact stars composed of strangeons—localized multi-quark clusters with approximate $u,d,s$ flavor symmetry—rather than nucleonic matter or fully deconfined strange quark matter. In the strangeon framework, bulk dense matter is a form of three-flavor “gigantic nucleus,” and, after sufficient cooling, may become globally solid rather than retaining only a thin solid crust over a fluid core. This makes SnSs conceptually distinct from both conventional neutron stars and conventional strange quark stars, while preserving the broader idea that compact stars probe nonperturbative QCD at supranuclear density (Lai et al., 2017, Lu et al., 2017, Xia et al., 3 Nov 2025).

1. Concept and nomenclature

The term strangeon was introduced as a contraction of “strange nucleon,” intended to emphasize an analogy with nucleons as effective constituents of ordinary nuclei. In this picture, stable micro-nuclei are effectively two-flavored, whereas stable macro-nuclei under gravitational compression may become three-flavored if light-flavor symmetry is restored. Strangeons are therefore not ordinary hyperons and not free quarks, but localized multi-quark clusters containing $u$, $d$, and $s$ quarks; suggested quark numbers include $N_{\rm q}=6, 9, 12,$ and $18$, and the exact cluster structure remains uncertain (Lu et al., 2017, Yuan et al., 2017).

A central distinction in the literature is between strangeon matter and strange quark matter. Both contain nonzero strangeness, but strange quark matter is modeled as deconfined bulk quark matter, whereas strangeon matter keeps quarks localized inside composite clusters. This difference is not merely terminological: it changes the assumed degrees of freedom, the stiffness of the equation of state (EOS), the surface boundary condition, and the plausibility of global solidity. In review formulations, “strange matter” is sometimes used as an umbrella term that includes both strange quark matter and strangeon matter, with strangeon stars identified as the clustered, strongly coupled branch of that broader category (Xia et al., 3 Nov 2025).

The strangeon hypothesis is motivated by the claim that compact-star interiors occupy a nonperturbative QCD regime in which weak-coupling quark matter is not guaranteed. One programmatic argument estimates an internal energy scale of order $0.5~\mathrm{GeV}$ from inter-quark spacing at a few times nuclear density and interprets this as high enough for three-flavor participation but possibly too low for complete deconfinement. A later perspective expresses the same idea in baryonic terms, estimating typical pulsar internal energies as $\epsilon \sim 10^2~\mathrm{MeV}$, again placing the problem in the strong-coupling regime where clustering rather than deconfinement may be favored (Lu et al., 2017, Qi et al., 18 Jul 2025).

2. Microphysics and equations of state

The most widely used strangeon-matter EOS in recent SnS work is a Lennard–Jones model, intended to encode nonrelativistic cluster dynamics plus strong short-range repulsion. In one common formulation, the inter-strangeon potential is

$$u(r)=4\epsilon\left[\left(\frac{\sigma}{r}\right)^{12}-\left(\frac{\sigma}{r}\right)^6\right],$$

with corresponding zero-temperature energy density and pressure

$$\rho=2\epsilon\left(A_{12}\sigma^{12}n^{5}-A_6\sigma^{6}n^3\right)+nN_qm_q, \qquad P=4\epsilon\left(2A_{12}\sigma^{12}n^{5}-A_6\sigma^{6}n^3\right),$$

where $u$0, $u$1, and $u$2 is the strangeon number density. This EOS is self-bound, since $u$3 occurs at finite density, and it becomes very stiff at high density because the repulsive $u$4 term dominates (Gao et al., 2021, Li et al., 2022, Zhang et al., 2023).

A complementary, more microscopic construction is the linked bag model, which extends MIT-bag ideas from isolated hadrons to dense matter built from linked quark clusters. In that framework, strangeons are color-singlet $u$5-quark clusters with $u$6, the bag constant is made density dependent, and the model is first calibrated to nuclear saturation properties before being continued to three-flavor clustered matter. A principal result is that the energy per baryon decreases as the number of valence quarks per strangeon increases, while the EOS becomes stiffer rather than softer. For $u$7, representative models give $u$8, $u$9, and $d$0, with wider parameter space allowing $d$1 and $d$2 (Miao et al., 2020).

Across these models, several features recur. Strangeon matter is treated as self-bound by the strong interaction, with a finite surface density at zero pressure. In the linked-bag study, the zero-pressure energy density is $d$3, corresponding to about $d$4 times nuclear saturation mass density, while the 2017 programmatic papers emphasize that self-binding allows low-mass compact objects down to $d$5 and even “strangeon planets” (Miao et al., 2020, Lu et al., 2017).

The strangeon microphysics remains model dependent. Besides Lennard–Jones and linked-bag descriptions, the literature includes self-bound polytropes, corresponding-state constructions, and H-dibaryon/Yukawa-type models. Review treatments explicitly note that no first-principles QCD derivation of the strangeon EOS currently exists, and that the effective inter-strangeon interaction, the preferred quark number per strangeon, and the condensed-matter phase structure remain open (Xia et al., 3 Nov 2025).

3. Structure, rotation, tidal response, and oscillations

Static strangeon stars are usually modeled with the TOV equations. In the linked-bag study and in the rotational and oscillation analyses based on the Lennard–Jones EOS, SnSs are consistently found to be compact, self-bound objects with high maximum masses. Some EOS realizations support $d$6, whereas the more conservative linked-bag calibration yields maximum masses around $d$7. The variation is therefore model dependent rather than a single universal prediction (Li et al., 2022, Miao et al., 2020).

Slow-rotation calculations in the Hartle–Thorne expansion show that the self-bound surface materially alters perturbative stellar structure. Because strangeon stars have finite surface density, the rotational and tidal perturbation equations require explicit surface jump corrections. Within the Lennard–Jones model, strangeon stars are more homogeneous than hadronic neutron stars, remain closer to the incompressible-fluid limit in $d$8, and satisfy the standard I–Love–Q relations to percent-level accuracy despite their unusual stiffness. GW170817-like tidal bounds disfavor the stiffest low-$d$9 models, notably LX2430 and LX2450, and for the parameter space considered one infers $s$0 under $s$1 (Gao et al., 2021).

The oscillation spectrum is another distinctive feature. For nonrotating strangeon stars in the relativistic Cowling approximation, the quadrupolar nonradial $s$2-mode frequency lies in the range

$s$3

substantially above the $s$4 kHz values quoted for the comparison neutron-star and quark-star EOSs in the same study. Radial oscillations also display self-bound behavior: as the central density approaches the minimum self-bound density, the fundamental radial frequency diverges, unlike in ordinary neutron-star sequences (Li et al., 2022).

A further structural implication follows from a dimensionless rescaling of the Lennard–Jones model. After defining $s$5, the stellar structure becomes effectively a one-parameter family. In that rescaled space, strangeon stars are generically ultracompact enough to satisfy

$s$6

so that a photon sphere exists while Buchdahl’s limit is respected. The same framework yields a GW170817-compatible minimum echo frequency of about $s$7 kHz in the empirical parameter space, with $s$8 Hz possible only near the extended theoretical limit (Zhang et al., 2023).

4. Global solidity, glitches, and starquakes

One of the most distinctive strangeon-star claims is that, once cooled below a sufficiently low temperature, the entire star may become solid rather than merely developing a solid crust. This idea underlies the strangeon-star interpretation of pulsar glitches, precession, mountain-building, and a variety of starquake-powered transients. The global-solid picture is repeatedly contrasted with the standard neutron-star analogy of a fluid core plus thin solid crust; one paper encapsulates the difference by comparing a solid strangeon star to a “cooked egg” and an ordinary neutron star to a “raw egg” (Chen et al., 2023, Lu et al., 2017).

In the glitch model developed for solid strangeon stars, the moment of inertia is written as

$s$9

where $N_{\rm q}=6, 9, 12,$0 encodes oblateness and $N_{\rm q}=6, 9, 12,$1 represents matter redistribution by plastic flow. A glitch is then produced by

$N_{\rm q}=6, 9, 12,$2

with post-glitch recovery controlled by the recoverable elastic component. The phenomenological recovery law

$N_{\rm q}=6, 9, 12,$3

with $N_{\rm q}=6, 9, 12,$4, is used to unify Crab-like high-$N_{\rm q}=6, 9, 12,$5 and Vela-like low-$N_{\rm q}=6, 9, 12,$6 glitches as different mixtures of elastic motion and irreversible plastic flow. For bulk-invariable glitches, the released energy is estimated as

$N_{\rm q}=6, 9, 12,$7

small enough to explain why ordinary radio-pulsar glitches often show little radiative output (Lai et al., 2017).

A separate relativistic treatment models solid stress through pressure anisotropy. With

$N_{\rm q}=6, 9, 12,$8

the anisotropic hydrostatic equilibrium equation acquires the extra force term $N_{\rm q}=6, 9, 12,$9, and the releasable free energy is estimated globally from the binding-energy difference between anisotropic and isotropic equilibrium models at fixed baryonic content: $18$0 For two heuristic anisotropy prescriptions,

$18$1

the calculated free energy can exceed $18$2 when the local anisotropy satisfies $18$3 over much of the star. The quoted range $18$4 overlaps the observed energies of SGR giant flares and is proposed as an alternative to magnetar-strength magnetic dissipation (Chen et al., 2023).

5. Formation, thermal evolution, and merger phenomenology

In supernova birth scenarios, strangeon-star formation is described as strangeonization, schematically

$18$5

analogous in spirit to neutronization. A nascent strangeon star is assumed to begin with internal energy of order $18$6, including a potentially important pion component, and to cool primarily through neutrino emission. The SN1987A study argues that a liquid–solid phase transition at

$18$7

can reproduce a neutrino burst lasting a few to a few tens of seconds, followed by a sharp cutoff once the star solidifies and its heat capacity collapses. In the favored fits, the isothermal case prefers $18$8 MeV and the non-isothermal case $18$9 MeV (Yuan et al., 2017).

Binary mergers supply a second major formation channel for extreme strangeon-star states. In the original merger paper, a strangeon EOS with $0.5~\mathrm{GeV}$0 gives a canonical tidal deformability

$0.5~\mathrm{GeV}$1

comfortably within the GW170817 upper limits quoted there. For a $0.5~\mathrm{GeV}$2 binary, the remnant is taken to be a hyper-massive strangeon star of mass $0.5~\mathrm{GeV}$3, and the electromagnetic counterpart is modeled as a strangeon kilonova: unstable strangeon nuggets in the ejecta power an early blue component, while remnant spin-down powers a later red component. In that same framework, latent heat released as the remnant solidifies at $0.5~\mathrm{GeV}$4 MeV yields an X-ray plateau lasting $0.5~\mathrm{GeV}$5 s (Lai et al., 2017).

Rotational support further enlarges the post-merger parameter space. In the slowly rotating approximation with the Lennard–Jones EOS, the maximum stable baryonic mass for the adopted strangeon model is

$0.5~\mathrm{GeV}$6

compared with $0.5~\mathrm{GeV}$7 for the comparison AP4 neutron-star EOS. Along constant-baryon-mass sequences, rigid rotation raises the maximum gravitational mass by about $0.5~\mathrm{GeV}$8 above $0.5~\mathrm{GeV}$9, versus $\epsilon \sim 10^2~\mathrm{MeV}$0 for neutron stars. The resulting spin-down-induced contraction releases gravitational energy that can, with efficiency $\epsilon \sim 10^2~\mathrm{MeV}$1, contribute to short-GRB X-ray plateaus. Fits to several Swift short bursts find $\epsilon \sim 10^2~\mathrm{MeV}$2, $\epsilon \sim 10^2~\mathrm{MeV}$3 ms, and $\epsilon \sim 10^2~\mathrm{MeV}$4, implying lower dipole fields than pure magnetar-powered plateau models (Yang et al., 2024).

6. Hybrid configurations, observational tests, and open problems

The pure strangeon-star picture has also been generalized to hybrid strangeon stars, in which a self-bound strangeon mantle surrounds a core of strange quark matter, especially in the color-flavor-locked phase. The transition is treated as a Maxwell construction determined by

$\epsilon \sim 10^2~\mathrm{MeV}$5

and the study concludes that a broad strangeon–SQM mixed phase is not favored once charge neutrality is imposed. Unpaired quark cores generally fail to support $\epsilon \sim 10^2~\mathrm{MeV}$6 while remaining radially stable, whereas CFL cores can satisfy the mass, radius, and GW170817 tidal constraints (Zhang et al., 2023).

Several observational tests recur across the strangeon-star literature. High pulsar masses, especially $\epsilon \sim 10^2~\mathrm{MeV}$7, are repeatedly cited as especially favorable to stiff strangeon EOSs; self-binding implies a distinctive low-mass branch, potentially down to $\epsilon \sim 10^2~\mathrm{MeV}$8; sub-millisecond pulsars would be regarded as strong evidence because solid strangeon stars are argued not to suffer the usual fluid $\epsilon \sim 10^2~\mathrm{MeV}$9-mode limitation; and precise measurements of the moment of inertia of PSR J0737−3039A, NICER radius constraints, and future multimessenger data are all presented as discriminants (Lu et al., 2017, Miao et al., 2020, Gao et al., 2021).

The principal controversies remain microphysical rather than phenomenological. Programmatic reviews are explicit that the strangeon hypothesis lacks a first-principles QCD derivation; the Lennard–Jones, linked-bag, corresponding-state, and polytropic EOSs are effective models; strangeon size and composition are uncertain; and several calculations use fluid or anisotropic-fluid approximations for matter that is physically intended to be solid. The anisotropy model used for starquake energetics is heuristic, the strangeon-to-quark transition parameters in hybrid stars are phenomenological, and many claimed astrophysical successes remain suggestive rather than unique. Current support for SnSs therefore lies in their ability to offer a coherent explanation of stiff EOS behavior, self-bound surfaces, global-solid phenomenology, and certain transient energetics, rather than in decisive empirical confirmation (Lai et al., 2017, Chen et al., 2023, Xia et al., 3 Nov 2025).

Within that landscape, strangeon stars occupy a specific niche: they are neither ordinary neutron stars nor conventional strange quark stars, but clustered three-flavor compact stars whose defining claims are self-binding, high stiffness, finite-density surfaces, and possible global solidity. Whether dense matter actually selects that phase remains open, but the hypothesis has developed into a technically articulated alternative compact-star paradigm with concrete implications for mass–radius relations, glitches, supernova neutrinos, merger remnants, tidal deformability, oscillation modes, and burst energetics (Lu et al., 2017, Li et al., 2022, Yang et al., 2024).