Mirror Stars: Diverse Hidden-Sector Objects

• Mirror stars are astrophysical objects that emerge in diverse contexts, including hidden-sector dark matter, gravitational lensing, and higher-dimensional theories.
• Research highlights include two-temperature emission systems in hidden-sector stars and distinct mass–radius relations in mirror neutron stars, emphasizing their unique structural properties.
• Observational strategies such as Gaia-based surveys, spectral analysis, and gravitational wave signatures are critical for identifying and differentiating mirror stars from conventional objects.

Searching arXiv for recent and relevant papers on mirror stars and related usages of the term. Using the arXiv search tool to retrieve papers on mirror stars, mirror neutron stars, electromagnetic signatures, and topological-star usage. Relevant arXiv records include "Mirror Neutron Stars" (Hippert et al., 2021), "Mirror Neutron Stars: How QCD can be used to study dark matter through gravitational waves" (Hippert et al., 2022), "Signatures of Mirror Stars" (Curtin et al., 2019), "How To Discover Mirror Stars" (Curtin et al., 2019), "How to search for Mirror Stars with Gaia" (Howe et al., 2021), "Electromagnetic Signatures of Mirror Stars" (Armstrong et al., 2023), "Generalized Predictions for the Electromagnetic Signatures of Mirror Stars" (Cabral et al., 31 Mar 2026), and the 5D/topological-star papers (Bronnikov et al., 3 Mar 2025, Bronnikov et al., 19 Jul 2025, Bronnikov et al., 28 Mar 2026). “Mirror stars” is a polysemous term in recent literature. In dark-sector phenomenology it usually denotes stars or compact remnants composed wholly or partly of mirror matter, typically in dissipative dark-matter or Mirror Twin Higgs frameworks, and includes mirror neutron stars, mirror-dark-matter-admixed compact stars, and oscillation-generated mixed stars (Curtin et al., 2019). In other contexts the same phrase has been used for higher-dimensional horizonless objects with reflecting surfaces (“topological stars”), for strongly lensed star-cluster macroimages across a critical curve, and indirectly through the stellar “mirror principle” describing anticorrelated core contraction and envelope expansion in low-mass stars (Bronnikov et al., 19 Jul 2025).

1. Terminological scope

The term spans several distinct research programs rather than a single astrophysical class.

Usage of “mirror stars”	Meaning	Representative papers
Hidden-sector stars	Stars made of dissipative dark matter with mirror electromagnetism and nuclear physics	(Curtin et al., 2019, Curtin et al., 2019, Armstrong et al., 2023)
Mirror neutron stars / mixed compact stars	Compact objects made of mirror nuclear matter or admixed with mirror dark matter	(Hippert et al., 2021, Hippert et al., 2022, Issifu et al., 1 Jul 2026)
Oscillation-induced mixed stars / antimatter-cored mirror stars	Objects generated by $n\!-\!n'$ or $\bar n'\!-\!\bar n$ transitions	(Berezhiani et al., 2020, Berezhiani, 2021)
Lensed “mirror stars”	Opposite-parity macroimages of a star cluster across a critical curve	(Zackrisson et al., 20 Jan 2026)
Higher-dimensional mirror or topological stars	5D compact objects with a reflecting boundary instead of a horizon	(Bronnikov et al., 3 Mar 2025, Bronnikov et al., 28 Mar 2026)

This multiplicity matters because observational signatures, governing equations, and even the ontological status of the “star” differ sharply across usages. In the dark-sector literature the object is a genuine stellar or compact body with hidden microphysics. In the lensing literature it is not a star at all but a pair of macroimages. In the 5D literature it is a geometric compact object whose surface reflects signals. In stellar-evolution theory, the related “mirror principle” is a structural response pattern rather than a distinct object (Hekker et al., 2020).

2. Hidden-sector mirror stars as dissipative dark objects

In the hidden-sector sense, mirror stars arise when dark matter contains analogues of Standard Model electromagnetism and nuclear physics, so that dark matter can radiate, cool, collapse, and undergo stellar evolution. Neutral Naturalness, especially Mirror Twin Higgs, provides a standard motivation: the hidden sector is related to the visible one by a discrete symmetry and contains mirror electrons, mirror nuclei, and a massless dark photon $A_D^\mu$, making mirror stellar structure and fusion possible (Curtin et al., 2019). This literature treats mirror stars as a generic consequence of hidden sectors with long-range electromagnetic interactions and nuclear binding, not as a one-off construction (Curtin et al., 2019).

A central portal is photon–dark-photon kinetic mixing,

$$\mathcal L_\mathit{mix} = -\frac{1}{4} F_{\mu\nu} F^{\mu\nu} -\frac{1}{4} F_{D\mu\nu}F_D^{\mu\nu} -\frac{\epsilon}{2} F_{\mu\nu} F_D^{\mu\nu} + \cdots$$

or equivalently

$$\mathcal{L} \supset \frac{1}{2}\,\epsilon\,F_{\mu\nu}F_D^{\mu\nu},$$

which gives mirror particles a tiny effective SM millicharge and allows capture of ordinary interstellar gas by a mirror star (Curtin et al., 2019). The relevant scattering is Rutherford-like,

$$\frac{d\sigma}{dE_R} = \frac{2\pi \epsilon^2 \alpha^2 Z_1^2 Z_2^2}{m_T v^2 E_R^2},$$

so even very small $\epsilon$ can accumulate SM matter over stellar lifetimes (Curtin et al., 2019).

The captured material sinks to the center and forms an SM “nugget.” In the early signal calculations, the nugget reaches an equilibrium temperature $$T_{\rm eq} \sim 4\times 10^3 - 7\times 10^3\,\mathrm{K}$$, often near $10^4$ K, and sits deep in the core with $R_{\rm nugget} \sim 10^{-3} - 10^{-2}\,R_\star$ (Curtin et al., 2019). Optical/near-IR emission comes from the nugget, whereas a separate X-ray signal can arise when mirror-core photons convert into SM photons in the captured material, with benchmark mirror-core temperatures $\bar n'\!-\!\bar n$0 and characteristic photon energies $\bar n'\!-\!\bar n$1 (Curtin et al., 2019). The distinctive phenomenological claim is therefore a two-temperature system: a faint visible nugget at $\bar n'\!-\!\bar n$2 K and a much hotter hidden core.

Search methodology has evolved from analytic scalings to survey-ready pipelines. Gaia-based strategies use parallaxes, colors, and broad-band photometry to identify anomalously faint white-dwarf-like objects, then cross-match to Pan-STARRS, DENIS, ALLWISE, and 2MASS to build pseudo-spectra (Howe et al., 2021). In a demonstration search restricted to 100 pc, the optically thin signal region was taken as

$\bar n'\!-\!\bar n$3

with quality cuts such as parallax_over_error > 10 and astrometric_excess_noise < 1; after the stated cuts, 14 candidates remained in the optically thin signal region, and one source survived the final Gaia color-consistency cut as the strongest candidate (Howe et al., 2021).

The spectral modeling has also bifurcated into optically thin and optically thick nugget regimes. For optically thin nuggets, Cloudy calculations show a narrow HR-diagram locus with

$\bar n'\!-\!\bar n$4

nebular continua with Balmer and Paschen jumps, and unusual emission-line ratios, notably elevated $\bar n'\!-\!\bar n$5-to-metal ratios relative to planetary nebulae and spectra unlike white-dwarf photospheres (Armstrong et al., 2023). For optically thick nuggets, the structure is solved with stellar-structure equations, Rosseland mean opacities, a photospheric condition

$\bar n'\!-\!\bar n$6

and luminosity balance

$\bar n'\!-\!\bar n$7

These solutions occupy distinct regions of $\bar n'\!-\!\bar n$8 space and can, in the stated analysis, be distinguished from ordinary stars in HR diagrams and temperature–surface-gravity diagrams using astrometric and spectroscopic catalogues (Cabral et al., 31 Mar 2026).

3. Mirror neutron stars

A more compact realization is the mirror neutron star: a degenerate remnant composed of mirror nuclear matter and motivated particularly by Mirror Twin Higgs models (Hippert et al., 2021). In that framework the mirror Higgs vacuum expectation value is larger than the SM value, with phenomenologically interesting values typically

$\bar n'\!-\!\bar n$9

so mirror fermions and gauge bosons are heavier and the mirror QCD scale is shifted (Hippert et al., 2022). The mirror confinement scale is fitted as

$A_D^\mu$0

and mirror pions satisfy roughly

$A_D^\mu$1

This rescaling alters the nuclear equation of state from crust to core (Hippert et al., 2021).

The stellar modeling uses a realistic neutron-star EoS rescaled to the mirror sector. The crust is based on the Baym–Pethick–Sutherland model; the core is described by relativistic mean-field nuclear matter with local charge neutrality and beta equilibrium; and the full compact-star structure follows the Tolman–Oppenheimer–Volkoff equations (Hippert et al., 2022). The resulting mirror neutron stars are smaller and denser than ordinary neutron stars. For $A_D^\mu$2, one study finds masses roughly

$A_D^\mu$3

radii roughly

$A_D^\mu$4

and compactness below

$A_D^\mu$5

(Hippert et al., 2021). A robust scaling reported for the mass-radius sequence is

$A_D^\mu$6

or equivalently that mirror-star masses and radii scale approximately with the mirror baryon mass in a steep manner (Hippert et al., 2021).

Because these objects are neutral under SM interactions, their principal observables are gravitational. The literature emphasizes smaller tidal deformabilities, altered mass-radius curves, and distinctive inspiral signatures in gravitational waves, together with possible constraints from binary pulsars (Hippert et al., 2022). Approximate universality survives: the $A_D^\mu$7-Love-$A_D^\mu$8 relations remain universal at the $A_D^\mu$9 level, and binary Love relations remain accurate to better than $$\mathcal L_\mathit{mix} = -\frac{1}{4} F_{\mu\nu} F^{\mu\nu} -\frac{1}{4} F_{D\mu\nu}F_D^{\mu\nu} -\frac{\epsilon}{2} F_{\mu\nu} F_D^{\mu\nu} + \cdots$$0 in the quoted analysis (Hippert et al., 2021). This suggests that standard neutron-star inference machinery can be adapted to mirror neutron stars, even though the sources would be electromagnetically dark.

4. Oscillation-driven mixed stars, antistars, and antimatter cores

A distinct compact-star literature does not posit entire mirror stars formed ab initio, but instead generates mixed objects through neutron–mirror-neutron conversion. The minimal interaction is a mass mixing,

$$\mathcal L_\mathit{mix} = -\frac{1}{4} F_{\mu\nu} F^{\mu\nu} -\frac{1}{4} F_{D\mu\nu}F_D^{\mu\nu} -\frac{\epsilon}{2} F_{\mu\nu} F_D^{\mu\nu} + \cdots$$1

or equivalently $$\mathcal L_\mathit{mix} = -\frac{1}{4} F_{\mu\nu} F^{\mu\nu} -\frac{1}{4} F_{D\mu\nu}F_D^{\mu\nu} -\frac{\epsilon}{2} F_{\mu\nu} F_D^{\mu\nu} + \cdots$$2, which violates separate baryon numbers $$\mathcal L_\mathit{mix} = -\frac{1}{4} F_{\mu\nu} F^{\mu\nu} -\frac{1}{4} F_{D\mu\nu}F_D^{\mu\nu} -\frac{\epsilon}{2} F_{\mu\nu} F_D^{\mu\nu} + \cdots$$3 and $$\mathcal L_\mathit{mix} = -\frac{1}{4} F_{\mu\nu} F^{\mu\nu} -\frac{1}{4} F_{D\mu\nu}F_D^{\mu\nu} -\frac{\epsilon}{2} F_{\mu\nu} F_D^{\mu\nu} + \cdots$$4 but preserves $$\mathcal L_\mathit{mix} = -\frac{1}{4} F_{\mu\nu} F^{\mu\nu} -\frac{1}{4} F_{D\mu\nu}F_D^{\mu\nu} -\frac{\epsilon}{2} F_{\mu\nu} F_D^{\mu\nu} + \cdots$$5 (Berezhiani, 2021). In one formulation the mirror fraction $$\mathcal L_\mathit{mix} = -\frac{1}{4} F_{\mu\nu} F^{\mu\nu} -\frac{1}{4} F_{D\mu\nu}F_D^{\mu\nu} -\frac{\epsilon}{2} F_{\mu\nu} F_D^{\mu\nu} + \cdots$$6 obeys a rate equation

$$\mathcal L_\mathit{mix} = -\frac{1}{4} F_{\mu\nu} F^{\mu\nu} -\frac{1}{4} F_{D\mu\nu}F_D^{\mu\nu} -\frac{\epsilon}{2} F_{\mu\nu} F_D^{\mu\nu} + \cdots$$7

and an initially ordinary neutron star can gradually become a two-fluid mixed star containing both ordinary and mirror matter (Berezhiani et al., 2020).

The most compact analytic result in this framework is the asymptotic $$\mathcal L_\mathit{mix} = -\frac{1}{4} F_{\mu\nu} F^{\mu\nu} -\frac{1}{4} F_{D\mu\nu}F_D^{\mu\nu} -\frac{\epsilon}{2} F_{\mu\nu} F_D^{\mu\nu} + \cdots$$8 “maximally mixed star.” If both sectors obey the same EoS, then

$$\mathcal L_\mathit{mix} = -\frac{1}{4} F_{\mu\nu} F^{\mu\nu} -\frac{1}{4} F_{D\mu\nu}F_D^{\mu\nu} -\frac{\epsilon}{2} F_{\mu\nu} F_D^{\mu\nu} + \cdots$$9

so

$$\mathcal{L} \supset \frac{1}{2}\,\epsilon\,F_{\mu\nu}F_D^{\mu\nu},$$0

This leads directly to the prediction of compact-star “twins” with the same mass but different radii, and to the possibility that sufficiently massive stars collapse to black holes as mirror conversion proceeds (Berezhiani et al., 2020). Early-time growth of the mirror fraction is approximately linear,

$$\mathcal{L} \supset \frac{1}{2}\,\epsilon\,F_{\mu\nu}F_D^{\mu\nu},$$1

with a characteristic timescale

$$\mathcal{L} \supset \frac{1}{2}\,\epsilon\,F_{\mu\nu}F_D^{\mu\nu},$$2

in the quoted estimate (Berezhiani et al., 2020).

Berezhiani extended the same logic to the reverse process inside mirror neutron stars (Berezhiani, 2021). If the mirror baryon asymmetry is negative, mirror compact stars are anti-mirror neutron stars, and $$\mathcal{L} \supset \frac{1}{2}\,\epsilon\,F_{\mu\nu}F_D^{\mu\nu},$$3 transitions generate ordinary antimatter gravitationally trapped inside the mirror star interior. Accreted interstellar gas can then annihilate on the antimatter core, producing gamma rays with spectra concentrated below the nucleon mass scale and no high-energy tail beyond $$\mathcal{L} \supset \frac{1}{2}\,\epsilon\,F_{\mu\nu}F_D^{\mu\nu},$$4 GeV. The proposal was explicitly linked to 14 point-like gamma-ray candidates in the 4FGL catalogue with spectra consistent with baryon–antibaryon annihilation and not obviously associated with known gamma-ray source classes (Berezhiani, 2021). The same paper suggested that mergers of such mirror neutron stars could eject antimatter-rich material and perhaps contribute to cosmic antihelium and heavier antinuclei, with an antihelium fraction at the level $$\mathcal{L} \supset \frac{1}{2}\,\epsilon\,F_{\mu\nu}F_D^{\mu\nu},$$5 relative to helium; the paper itself presents this connection as speculative (Berezhiani, 2021).

A more radical stellar-evolution application was proposed in which $$\mathcal{L} \supset \frac{1}{2}\,\epsilon\,F_{\mu\nu}F_D^{\mu\nu},$$6 oscillations inside ordinary stars, sourced mainly by

$$\mathcal{L} \supset \frac{1}{2}\,\epsilon\,F_{\mu\nu}F_D^{\mu\nu},$$7

reshape late evolution, white-dwarf and neutron-star progenitor limits, and Type II-P versus II-L supernova phenomenology (Tan, 2019). That work claims that a reduced effective Chandrasekhar limit by a factor $$\mathcal{L} \supset \frac{1}{2}\,\epsilon\,F_{\mu\nu}F_D^{\mu\nu},$$8 explains the $$\mathcal{L} \supset \frac{1}{2}\,\epsilon\,F_{\mu\nu}F_D^{\mu\nu},$$9 threshold between white-dwarf formation and core collapse, and interprets pulsations through

$$\frac{d\sigma}{dE_R} = \frac{2\pi \epsilon^2 \alpha^2 Z_1^2 Z_2^2}{m_T v^2 E_R^2},$$0

A plausible reading is that this paper should be treated as a highly nonstandard reinterpretation rather than a consensus model, since it repeatedly frames the support from observations as the author’s interpretation (Tan, 2019).

5. Mirror-dark-matter-admixed compact stars

A large adjacent literature studies compact stars with a mirror-dark-matter component treated as a second fluid or, more recently, through a self-consistent single-fluid mean-field framework. In the two-fluid strange-star model with the standard MIT bag EoS, the total density and pressure are

$$\frac{d\sigma}{dE_R} = \frac{2\pi \epsilon^2 \alpha^2 Z_1^2 Z_2^2}{m_T v^2 E_R^2},$$1

and the dark mass fraction is

$$\frac{d\sigma}{dE_R} = \frac{2\pi \epsilon^2 \alpha^2 Z_1^2 Z_2^2}{m_T v^2 E_R^2},$$2

The stated conclusion is that pure strange stars cannot simultaneously satisfy the PSR J0740+6620 mass, PSR J0030+0451 NICER constraints, and GW170817 tidal deformability, whereas strange stars in GW170817 must have a mirror-dark-matter core with

$$\frac{d\sigma}{dE_R} = \frac{2\pi \epsilon^2 \alpha^2 Z_1^2 Z_2^2}{m_T v^2 E_R^2},$$3

(Yang et al., 2021).

The color-flavor locked extension of this idea treats the dark component as mirror CFL strange quark matter obeying the same EoS as the visible sector (Yang et al., 2024). A key geometric distinction is that $$\frac{d\sigma}{dE_R} = \frac{2\pi \epsilon^2 \alpha^2 Z_1^2 Z_2^2}{m_T v^2 E_R^2},$$4 gives a core, $$\frac{d\sigma}{dE_R} = \frac{2\pi \epsilon^2 \alpha^2 Z_1^2 Z_2^2}{m_T v^2 E_R^2},$$5 a halo, and $$\frac{d\sigma}{dE_R} = \frac{2\pi \epsilon^2 \alpha^2 Z_1^2 Z_2^2}{m_T v^2 E_R^2},$$6 a pure invisible mirror star. For pure CFL strange stars, satisfying HESS J1731-347, GW190814, and GW170817 simultaneously requires pairing gaps above about 200 MeV; with mirror dark matter, the quoted lower bounds are relaxed. For example, when $$\frac{d\sigma}{dE_R} = \frac{2\pi \epsilon^2 \alpha^2 Z_1^2 Z_2^2}{m_T v^2 E_R^2},$$7, the allowed range

$$\frac{d\sigma}{dE_R} = \frac{2\pi \epsilon^2 \alpha^2 Z_1^2 Z_2^2}{m_T v^2 E_R^2},$$8

permits a minimum pairing gap below $$\frac{d\sigma}{dE_R} = \frac{2\pi \epsilon^2 \alpha^2 Z_1^2 Z_2^2}{m_T v^2 E_R^2},$$9 MeV, i.e. one half of the strange quark mass $\epsilon$0 MeV used in that paper (Yang et al., 2024).

Finite-temperature and rotating configurations have also been studied. In a two-fluid model of rotating proto-neutron stars, baryonic matter is treated with a DDME2 relativistic mean-field EoS, the dark sector mirrors the visible one, and only gravity couples the sectors (Issifu et al., 28 Jul 2025). A 5% dark component makes stars more compact, raises polar redshift, lowers the threshold for rotational instabilities, and shifts representative nonrotating maximum masses from about $\epsilon$1 to $\epsilon$2 in the early neutrino-trapped stage and from about $\epsilon$3 to $\epsilon$4 in the cold stage (Issifu et al., 28 Jul 2025).

The latest single-fluid treatment replaces fixed-density prescriptions by a global dark fraction

$\epsilon$5

and a contact vector current–current interaction

$\epsilon$6

which shifts the chemical potentials as

$\epsilon$7

The local relation $\epsilon$8 is imposed self-consistently throughout the star (Issifu et al., 1 Jul 2026). In this framework the EOS softens, incompressibility $\epsilon$9 decreases by about $$T_{\rm eq} \sim 4\times 10^3 - 7\times 10^3\,\mathrm{K}$$0–$$T_{\rm eq} \sim 4\times 10^3 - 7\times 10^3\,\mathrm{K}$$1, radii shrink by about $$T_{\rm eq} \sim 4\times 10^3 - 7\times 10^3\,\mathrm{K}$$2–$$T_{\rm eq} \sim 4\times 10^3 - 7\times 10^3\,\mathrm{K}$$3 km, and maximum masses drop systematically; at $$T_{\rm eq} \sim 4\times 10^3 - 7\times 10^3\,\mathrm{K}$$4, for example, $$T_{\rm eq} \sim 4\times 10^3 - 7\times 10^3\,\mathrm{K}$$5 decreases from $$T_{\rm eq} \sim 4\times 10^3 - 7\times 10^3\,\mathrm{K}$$6 to $$T_{\rm eq} \sim 4\times 10^3 - 7\times 10^3\,\mathrm{K}$$7 for NL3$$T_{\rm eq} \sim 4\times 10^3 - 7\times 10^3\,\mathrm{K}$$8, from $$T_{\rm eq} \sim 4\times 10^3 - 7\times 10^3\,\mathrm{K}$$9 to $10^4$0 for FSU2R, and from $10^4$1 to $10^4$2 for DDME2 (Issifu et al., 1 Jul 2026). The same study reports that the direct Urca threshold shifts to higher densities, modifying rapid-cooling onset in an EOS-dependent manner (Issifu et al., 1 Jul 2026).

6. Other astrophysical meanings: lensed mirror images and the stellar mirror principle

Outside dark-matter phenomenology, “mirror stars” can denote strongly lensed star-cluster macroimages. Near a fold caustic, a star cluster can produce two bright mirror-flipped macroimages of opposite parity across a critical curve. Without microlensing they should share the same spectral energy distribution, but the symmetry can break because the two macroimages, separated by about $10^4$3–$10^4$4 arcsec, are microlensed by different compact objects in the foreground cluster lens (Zackrisson et al., 20 Jan 2026). The color diagnostic used is

$10^4$5

with a practical detectability criterion

$10^4$6

For a standard Kroupa IMF, detectable mismatches are expected only for cluster masses $10^4$7 and ages $10^4$8 Myr, with the best candidates at $10^4$9–$R_{\rm nugget} \sim 10^{-3} - 10^{-2}\,R_\star$0 and $R_{\rm nugget} \sim 10^{-3} - 10^{-2}\,R_\star$1–$R_{\rm nugget} \sim 10^{-3} - 10^{-2}\,R_\star$2 Myr (Zackrisson et al., 20 Jan 2026). In Pop III-like top-heavy IMFs extending to $R_{\rm nugget} \sim 10^{-3} - 10^{-2}\,R_\star$3–$R_{\rm nugget} \sim 10^{-3} - 10^{-2}\,R_\star$4, the favored range shifts to $R_{\rm nugget} \sim 10^{-3} - 10^{-2}\,R_\star$5–$R_{\rm nugget} \sim 10^{-3} - 10^{-2}\,R_\star$6 at ages about $R_{\rm nugget} \sim 10^{-3} - 10^{-2}\,R_\star$7–$R_{\rm nugget} \sim 10^{-3} - 10^{-2}\,R_\star$8 Myr, and a $R_{\rm nugget} \sim 10^{-3} - 10^{-2}\,R_\star$9 cluster can be more than 2 magnitudes brighter in F444W than a standard-IMF cluster of similar F115W and F200W brightness because of stars in the $\bar n'\!-\!\bar n$00–$\bar n'\!-\!\bar n$01 range reaching very luminous cool end stages (Zackrisson et al., 20 Jan 2026). Here the term describes lensing geometry, not hidden-sector stellar physics.

The “mirror principle” in stellar evolution is another separate usage. In low-mass stars leaving the main sequence, the core contracts while the envelope expands; the paper formalizes this through a stationary point,

$\bar n'\!-\!\bar n$02

and especially a pivot defined by

$\bar n'\!-\!\bar n$03

The specific entropy is written as

$\bar n'\!-\!\bar n$04

and the mirror behavior corresponds to opposite signs of entropy change in interior and envelope (Hekker et al., 2020). During the red-giant-branch bump, however, this mirror temporarily disappears: between luminosity maximum and minimum there is no pivot, $\bar n'\!-\!\bar n$05 remains positive throughout the star, and “the star is fully contracting.” The mirror returns when the hydrogen-burning shell reaches the mean molecular weight discontinuity (Hekker et al., 2020). This use of “mirror” is therefore structural and thermodynamic rather than nominally taxonomic.

7. Higher-dimensional mirror stars and topological stars

In a separate 5D gravitational literature, “mirror stars” are horizonless compact objects whose would-be horizon is replaced by a perfectly reflecting surface after compactifying an extra dimension (Bronnikov et al., 3 Mar 2025). The basic mechanism is a time–extra-dimension swap in a five-dimensional black-hole-like geometry. In the T-Schwarzschild example,

$\bar n'\!-\!\bar n$06

the surface $\bar n'\!-\!\bar n$07 can become a mirror boundary rather than an event horizon when the fifth dimension is compact and spacelike (Bronnikov et al., 3 Mar 2025).

The best-developed branch is the magnetic 5D Einstein–Maxwell solution. The black-hole branch exists for

$\bar n'\!-\!\bar n$08

whereas the mirror-star branch exists for

$\bar n'\!-\!\bar n$09

and has a reflecting surface at $\bar n'\!-\!\bar n$10 where $\bar n'\!-\!\bar n$11 (Bronnikov et al., 19 Jul 2025). In this formulation the mirror star is less compact than a black hole of the same mass and observationally reflects low-energy particles and waves that do not resolve the compact fifth dimension. The perturbation problem reduces to a Schrödinger-type equation,

$\bar n'\!-\!\bar n$12

and the numerical stability result for magnetic mirror stars is

$\bar n'\!-\!\bar n$13

For the corresponding magnetic black holes, the stable region is

$\bar n'\!-\!\bar n$14

(Bronnikov et al., 19 Jul 2025).

Nonlinear electrodynamics generalizes this construction. A purely magnetic 5D Einstein-NED system with

$\bar n'\!-\!\bar n$15

admits both black-hole/black-string and mirror-star/topological-star branches, with the sign of an integration constant $\bar n'\!-\!\bar n$16 selecting the class: $\bar n'\!-\!\bar n$17 yields mirror stars and $\bar n'\!-\!\bar n$18 black holes (Bronnikov et al., 28 Mar 2026). The effective radial perturbation equation again assumes Schrödinger form, and the paper concludes that the whole obtained family of black holes is stable, while mirror-star solutions are stable only in a certain parameter range (Bronnikov et al., 28 Mar 2026). Both the Einstein–Maxwell and Einstein-NED analyses explicitly report disagreement with some earlier claims of full stability or different stability domains, so stability is a genuine point of contention in this subliterature (Bronnikov et al., 19 Jul 2025).