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An exotic star is a hypothetical compact star composed of exotic matter (something not made of electrons, protons, neutrons, or muons), and balanced against gravitational collapse by degeneracy pressure or other quantum properties.

Types of exotic stars include

Of the various types of exotic star proposed, the most well evidenced and understood is the quark star, although its existence is not confirmed.

Quark stars and strange stars

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Main articles: Quark star and Strange star

A quark star is a hypothesized object that results from the decomposition of neutrons into their constituent quarks under extremely intense gravitational pressure balanced by electrical repulsion and degeneracy pressure.[1][2] Such a star would be smaller and more dense than a neutron star, and may survive in this new state indefinitely, if no extra mass is added. Quark stars that contain strange matter are called strange stars.[3] Such a star, first proposed by Edward Witten, would consist of confined quarks, essentialy a giant nucleon.[4]

Based on observations released by the Chandra X-Ray Observatory on 10 April 2002, two objects, named RX J1856.5−3754 and 3C 58, were suggested as quark star candidates. The former appeared to be much smaller and the latter much colder than expected for a neutron star, suggesting that they were composed of material denser than neutronium. However, these observations were met with skepticism by researchers who said the results were not conclusive.[who?] After further analysis, RX J1856.5−3754 was excluded from the list of quark star candidates.[5]

Electroweak stars

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An electroweak star is a hypothetical type of exotic star in which the gravitational collapse of the star is prevented by radiation pressure resulting from electroweak burning; that is, the energy released by the conversion of quarks into leptons through the electroweak force. This proposed process might occur in a volume at the star's core approximately the size of an apple, containing about two Earth masses, and reaching temperatures on the order of 1015 K (1 PK).[6][7] Electroweak stars could be identified through the equal number of neutrinos emitted of all three generations, taking into account neutrino oscillation.[6]

Preon stars

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A preon star is a proposed type of compact star made of preons, a group of hypothetical subatomic particles. Preon stars would be expected to have huge densities, exceeding 1023 kg/m3. They may have greater densities than quark stars, and they would be heavier but smaller than white dwarfs and neutron stars.[8]

Boson stars

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A boson star is a hypothetical astronomical object formed out of particles called bosons. Conventional stars are formed from mostly protons and electrons, which are fermions, but also contain a large proportion of helium-4 nuclei, which are bosons, and smaller amounts of various heavier nuclei, which can be either. For this type of star to exist, there must be a stable type of boson with self-repulsive interaction; one possible candidate particle[9] is the still-hypothetical "axion" (which is also a candidate for the not-yet-detected "non-baryonic dark matter" particles, which appear to compose roughly 25% of the mass of the Universe). It is theorized[10] that unlike normal stars (which emit radiation due to gravitational pressure and nuclear fusion), boson stars would be transparent and invisible. The immense gravity of a compact boson star would bend light around the object, creating an empty region resembling the shadow of a black hole's event horizon. Like a black hole, a boson star would absorb ordinary matter from its surroundings, but because of the transparency, matter (which would probably heat up and emit radiation) would be visible at its center. Rotating boson star models are also possible. Unlike black holes these have quantized angular momentum, and their energy density profiles are torus-shaped, which can be understood as a result of deformation due to centrifugal forces.[11]

There is no significant evidence that such stars exist. However, it may become possible to detect them by the gravitational radiation emitted by a pair of co-orbiting boson stars.[12][13] GW190521, thought to be the most energetic black hole merger ever recorded, may be the head-on collision of two boson stars.[14] In addition, gravitational wave signals from compact binary boson star mergers can be degenerate with those from black hole mergers, suggesting that some gravitational wave observations interpreted as originating in a black hole binary could really originate in a boson star binary.[15] The invisible companion to a Sun-like star identified by Gaia mission could be a black hole or either a boson star or an exotic star of other types.[16][17]

Boson stars may have formed through gravitational collapse during the primordial stages of the Big Bang.[18] At least in theory, a supermassive boson star could exist at the core of a galaxy, which may explain many of the observed properties of active galactic cores.[19] However, more recent general-relativistic magnetohydrodynamic simulations, combined with imaging performed by the Event Horizon Telescope, is believed to have largely ruled out the possibility that Sagittarius A*, the supermassive object at the center of the Milky Way, could be a boson star.[20]

Bound states in cosmological bosonic fields have also been proposed as an alternative to dark matter.[21] The dark matter haloes surrounding most galaxies might be viewed as enormous "boson stars."[22]

Compact boson stars and boson shells are often modelled using massive bosonic fields, such as complex scalar fields and U(1) gauge fields, coupled to gravity. The presence of a positive or negative cosmological constant in the theory facilitates a study of these objects in de Sitter and anti-de Sitter spaces.[23][24][25][26][27]

By changing the potential associated with the matter model, different families of boson star models can be obtained. The so-called solitonic potential, which introduces a degenerate vacuum state at a finite value of the field amplitude, can be used to construct boson star models so compact that they possess a pair of photon orbits, one of which is stable.[28] Because they trap light, such boson stars could mimic much of the observational phenomenology of black holes.

Boson stars composed of elementary particles with spin-1 have been labelled Proca stars.[29]

Planck stars

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Main article: Planck star

In loop quantum gravity, a Planck star is a hypothetically possible astronomical object that is created when the energy density of a collapsing star reaches the Planck energy density. Under these conditions, assuming gravity and spacetime are quantized, there arises a repulsive "force" derived from Heisenberg's uncertainty principle. In other words, if gravity and spacetime are quantized, the accumulation of mass-energy inside the Planck star cannot collapse beyond this limit to form a gravitational singularity because it would violate the uncertainty principle for spacetime itself.[30]

Q-stars

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Q-stars are hypothetical objects that originate from supernovae or the big bang. They are theorized to be massive enough to bend space-time to a degree such that some, but not all light could escape from its surface. These are predicted to be denser than neutron stars or even quark stars.[31]

Dark stars

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In Newtonian mechanics, objects dense enough to trap any emitted light are called dark stars,[32] as opposed to black holes in general relativity. However, the same name is used for hypothetical ancient "stars" which derived energy from dark matter.[33] Quantum effects may prevent true black holes from forming and give rise instead to dense entities called black stars.[34]

See also

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References

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Sources

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Exotic star

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An exotic star is a hypothetical compact object composed of particles other than electrons, protons, and neutrons, balanced against by degeneracy pressure from some exotic form of . These objects differ from conventional stars, white dwarfs, neutron stars, and black holes by relying on unconventional states or quantum effects for stability, often achieving extreme compactness with radii comparable to their . Exotic stars encompass a variety of theoretical models, each proposing distinct compositions and support mechanisms:
  • Quark stars or strange stars: Composed of deconfined up, down, and strange quarks in a state of strange quark matter, potentially the absolute ground state of baryonic matter more stable than nuclear matter; they may form from the collapse of neutron stars exceeding the Tolman-Oppenheimer-Volkoff limit.
  • Boson stars: Self-gravitating solitons formed from complex scalar (bosonic) fields, stabilized by the Heisenberg uncertainty principle rather than degeneracy pressure, with masses ranging from planetary scales to thousands of solar masses.
  • Hybrid stars: Neutron stars harboring a core of quark matter or a mixed hadron-quark phase, bridging standard neutron stars and fully exotic quark objects.
  • Other variants, such as electroweak stars (supported by electroweak interactions at extreme densities) or preon stars (made of hypothetical preons), explore even more speculative physics beyond the Standard Model.
These structures challenge traditional models and are invoked to explain astrophysical anomalies, including compact objects in the black hole-neutron star observed via , such as in GW190814. No exotic stars have been confirmed observationally as of 2025, though they remain candidates for such events. Their detection could reveal new physics, such as quark deconfinement or bosonic candidates, through signatures in echoes, pulsar timing, or multimessenger astronomy. Ongoing observations from facilities like LIGO-Virgo-KAGRA (LVK) and future missions aim to distinguish exotic stars from black holes or neutron stars by measuring compactness, tidal deformability, and post-merger remnants.

Fundamentals

Definition and Characteristics

Exotic stars are hypothetical compact stellar objects formed as remnants of massive stars, distinguished from conventional main-sequence stars and by their composition of or support from exotic fields or forces, rather than degeneracy pressure from electrons or s. These objects maintain against through degeneracy pressure from exotic fermionic matter, such as deconfined quarks, or quantum effects from bosonic fields, enabling extreme compactness in some models not achievable in standard baryonic matter configurations. For compact fermionic exotic stars, such as quark stars, key characteristics include radii typically ranging from 5 to 10 km—smaller than the 10–15 km radii of neutron stars—and central densities approaching or exceeding nuclear saturation density, on the order of 10^{17} to 10^{18} kg/m³. This high density arises from the tightly bound nature of exotic matter states, which can be self-bound without a surface discontinuity in density, unlike neutron stars that transition to lower-density crusts. Mass-radius relations for such exotic stars often permit greater stability at higher masses compared to neutron stars, potentially exceeding the Tolman–Oppenheimer–Volkoff mass limit of approximately 2–3 solar masses for conventional neutron star equations of state, depending on the specific exotic matter model. Due to their smaller sizes, compact exotic stars could support faster rotation rates and stronger than , leading to distinct observational signatures such as higher spin frequencies or more intense magnetospheric activity. For instance, models predict rotational periods potentially below 1 millisecond, contrasting with the ~1.4 millisecond minimum observed in pulsars. The concept of exotic stars emerged in the late 1960s, with Kaup (1968) proposing self-gravitating configurations of scalar fields (boson stars), followed by Itoh (1970), who calculated the hydrostatic structure of hypothetical , and was expanded in the 1980s through models of stable matter by (1984).

Theoretical Basis

Exotic stars rely on unconventional forms of matter to maintain against , distinct from the supporting ordinary neutron stars. These include degenerate quark matter, where s form a Fermi sea under extreme densities, providing degeneracy pressure via the ; scalar fields, which can lead to Bose-Einstein condensation in bosonic configurations; preon composites, hypothetical sub- particles forming even denser fermionic matter; and electroweak symmetry restoration phases, where high temperatures and densities (~10^{12} K and nuclear densities) dissolve the Higgs , allowing weakly interacting particles to dominate the core. The stability of these structures hinges on their (EOS), which relates PP to ϵ\epsilon and must be sufficiently stiff to counteract . For degenerate quark matter, the MIT bag model provides a simple phenomenological EOS, treating quarks as non-interacting fermions confined by a constant BB representing the QCD vacuum : P=13(ϵ−4B),P = \frac{1}{3} (\epsilon - 4B), with the bag constant BB typically in the range 50–100 MeV/fm³, ensuring positive up to critical densities where phase transitions may occur. This EOS exemplifies how can support masses exceeding the Tolman-Oppenheimer-Volkoff (TOV) limit for neutron stars (~2–3 M⊙M_\odot), as softer nuclear EOS lead to collapse while stiffer exotic ones permit larger radii and masses. The general framework for stability involves solving the relativistic TOV equation for spherically symmetric, static configurations: dPdr=−G(ϵ+P)(m(r)+4πr3P)r2(1−2Gm(r)/r),\frac{dP}{dr} = -\frac{G (\epsilon + P)(m(r) + 4\pi r^3 P)}{r^2 (1 - 2Gm(r)/r)}, coupled with the mass continuity equation dm/dr=4πr2ϵdm/dr = 4\pi r^2 \epsilon (in units where c=1c=1), where a stiffer EOS (higher PP for given ϵ\epsilon) delays the onset of instability by reducing the central pressure gradient. Formation pathways for exotic stars generally involve the remnants of core-collapse supernovae from massive progenitors (>8 M⊙M_\odot), where the proto-compact object reaches densities beyond ~10^{15} g/cm³, triggering a phase transition from hadronic to exotic matter under the immense pressure of implosion. In this process, the collapsing core may convert neutron-rich matter into a quark phase or other exotic states if the EOS permits, stabilizing at a new equilibrium without forming a black hole. Quantum and relativistic effects are central: for fermionic exotics like quark or preon matter, the Pauli exclusion principle enforces a minimum momentum spread, yielding degeneracy pressure P∝(ℏc)(ϵ)4/3/m2P \propto (\hbar c) (\epsilon)^{4/3}/m^2 that scales with density; conversely, for bosonic scalar fields, Bose-Einstein condensation allows macroscopic occupation of the ground state, supporting the star via uncertainty-principle-induced gradients rather than exclusion, potentially forming stable solitons up to ~10 M⊙M_\odot.

Hadronic and Substructure Stars

Quark Stars and Strange Stars

Quark stars are hypothetical compact objects composed of deconfined matter, arising when the central density of a surpasses the deconfinement threshold, approximately 5×10175 \times 10^{17} kg/m³, allowing quarks to transition from hadronic confinement to a free state. This idea was initially explored by Itoh in 1970, who analyzed the of such stars using the MIT bag model to ensure stability by introducing a bag constant BB that mimics confinement pressure. In this model, the balance between quark degeneracy pressure and the bag energy density prevents collapse, enabling stable configurations at densities several times that of nuclear saturation (ρ0≈2.8×1017\rho_0 \approx 2.8 \times 10^{17} kg/m³). Strange stars form a specialized class of quark stars, characterized by strange quark matter (SQM) containing nearly equal fractions of up, down, and s to achieve weak equilibrium and minimize energy. The Bodmer-Witten conjecture, first proposed by Bodmer in 1971 and elaborated by in 1984, hypothesizes that SQM represents the true of baryonic , with greater than ordinary nuclei (56Fe^{56}\mathrm{Fe}), implying that any contact with normal could trigger rapid conversion via strangeness production. This stability arises because the 's inclusion equalizes Fermi levels across flavors, reducing the overall energy per compared to two-flavor quark . Key properties of strange stars include average densities around 101810^{18} kg/m³, enabling extreme compactness with maximum masses of approximately 1.5–2 M⊙M_\odot and radii of 7–10 km, yielding higher mass-to-radius ratios than typical stars. Unlike neutron stars, they lack a thick crust, featuring instead a thin surface (~100 fm) where the quark matter sharply interfaces with the vacuum, supported by and potentially generating intense up to 101710^{17} V/cm; this could lead to the ejection of strangelets, hypothetical nuggets of SQM. The equation of state () for SQM, derived in the MIT bag model for massless quarks, takes the causal linear form P=13(ε−4B),P = \frac{1}{3} (\varepsilon - 4B), where PP denotes pressure, ε\varepsilon is energy density, and BB (typically 50–100 MeV/fm³) encapsulates non-perturbative QCD effects; hyperon-like softening from finite strange quark mass further stiffens the EOS at lower densities. Theoretically, strange stars hold implications for high-energy astrophysics, as the phase transition from hadronic to SQM in a neutron star core could liberate ∼1053\sim 10^{53} erg of energy, powering short gamma-ray bursts via explosive decompression or strangelet emission. Similarly, instabilities in the thin crust, such as starquakes, might produce fast radio bursts through sudden magnetic field reconfiguration or plasma oscillations. These scenarios align with the Tolman-Oppenheimer-Volkoff structure equations, providing a compact alternative to neutron star models while respecting observed pulsar timing constraints.

Preon Stars

Preon stars represent a hypothetical extension of compact stellar objects, composed of formed from , which are proposed sub-quark constituents of quarks and leptons. The preon hypothesis emerged in the as an attempt to address limitations in the by positing that quarks and leptons are composite structures bound by a new confining force at energy scales around 1 TeV. This concept was initially formalized in the Pati-Salam , where preons serve as the fundamental building blocks unifying quarks and leptons under an SU(4) × SU(2)_L × SU(2)_R gauge symmetry. Subsequent theoretical models in the late 1970s and 1980s developed specific schemes, such as the rishon model proposed by Haim Harari in 1979, which constructs all fermions from two types of preons (T and V rishons) with charges of ±1 and 0, respectively, to reproduce the observed particle spectrum without introducing new quantum numbers. These models predict preon confinement analogous to (QCD), where preons bind into stable composites at high densities, potentially forming in astrophysical environments exceeding 10^{26} kg/m³—far beyond the quark deconfinement threshold in or stars. Building briefly on concepts, preon stars would arise if pushes matter to energies where s further deconfine into preons. The notion of preon stars as distinct cosmic objects was first systematically explored in theoretical literature around the early 2000s, drawing from 1980s composite preon models to propose stable configurations resistant to black hole formation. In a seminal 2005 study, Fredrik Sandin modeled preon stars as self-gravitating spheres of interacting fermionic preons governed by a bag model equation of state (EOS), given by ρc2=3p+4B\rho c^2 = 3p + 4B, where BB is the bag constant (typically B1/4≈100−1000B^{1/4} \approx 100-1000 GeV) representing confinement energy. This EOS is stiffer than that of quark matter due to the higher density regime and additional bag pressure, supporting stability through preon degeneracy and confinement pressures that balance gravity via positive radial oscillation frequencies in the Oppenheimer-Volkoff framework. Typical properties of preon stars include central densities ≥1023\geq 10^{23} g/cm³ (≈1026\approx 10^{26} kg/m³), radii on the order of 0.1–1 m, and masses up to approximately 102710^{27} kg (roughly 500 masses or 0.001 solar masses), with the maximum configuration approaching but not exceeding the to avert collapse. These objects could form primordially from early density fluctuations or as remnants of massive star collapses beyond the limit, potentially serving as candidates or sources of ultra-high-energy cosmic rays through evaporation or interactions. Later models, such as a three-layer structure with a preon core, quark-gluon plasma mantle, and hadronic envelope, predict similar scales but incorporate negative pressures (ρ=−p\rho = -p) in outer layers for quasi-equilibrium stability. Despite their theoretical appeal, stars remain highly speculative due to the absence of experimental evidence for s, with searches at energies up to several TeV (e.g., at the LHC) yielding no signatures of compositeness, constraining binding scales to >10 TeV in most models as of 2025. Their viability is intrinsically linked to grand unified theories beyond the , where interactions might resolve hierarchy problems, but current null results from precision electroweak measurements and flavor physics further limit parameter space. Ongoing astrophysical searches for compact or anomalous gravitational signals offer potential indirect probes, though no confirmed detections exist.

Q-Stars

Q-stars represent a class of hypothetical compact objects modeled as stable non-topological solitons emerging from the quantum chromodynamics (QCD) vacuum in the Friedberg-Lee-Sirlin model, where the conserved baryon number arises as a Noether charge from the scalar field configuration rather than from degenerate fermionic matter. Proposed by Friedberg, Lee, and Pang in the 1980s, these structures are supported by spatial variations in the QCD vacuum energy density, modeled by a scalar field that mimics the chiral condensate, creating a self-gravitating soliton without relying on Pauli exclusion pressure from quarks or other fermions. In this framework, the interior consists of deconfined quark matter confined by the soliton profile, distinct from traditional degenerate matter models. Key properties of Q-stars include a typical radius of approximately 5 km and a mass around 1 (M⊙), comparable to observed stars but achieved through bosonic field dynamics rather than fermionic degeneracy. The density profile exhibits a bag-like interior with higher central transitioning to the perturbative QCD exterior, ensuring a smooth matching without sharp boundaries. Stability against radial perturbations and collapse is maintained by the configuration, which prevents decay into lower-energy states, even under general relativistic effects. The mathematical model employs a phenomenological Lagrangian with a scalar field σ coupled to quarks, leading to an equation of state where pressure balances the vacuum energy gradient. Q-stars may form through the gravitational collapse of a neutron star when nuclear matter transitions to the soliton phase at extreme densities, or primordially during the early universe from density fluctuations in the quark-gluon plasma. Observationally, they could mimic neutron stars in pulsar timing and gravitational wave signals due to similar mass-radius relations, potentially explaining compact remnants without invoking exotic fermionic equations of state. Compared to standard quark stars, Q-stars offer advantages such as the absence of surface instabilities from sharp density drops and a inherently self-bound structure due to the solitonic profile, avoiding the need for external pressure confinement. As of 2025, gravitational wave detections from LIGO/Virgo/KAGRA provide no direct evidence but constrain possible mass-radius relations for such exotic objects.

Force-Dominated Stars

Electroweak Stars

Electroweak stars represent a hypothetical class of compact stellar objects where extreme leads to core conditions that restore electroweak symmetry, unifying the electromagnetic and weak nuclear forces within the framework. In this regime, the core temperature surpasses the electroweak scale of approximately 100 GeV, causing the Higgs to vanish and enabling non-perturbative processes that violate (B) and (L) number conservation. Proposed by , Lue, Starkman, and Stojkovic in 2009, these stars arise as an intermediate evolutionary stage for massive stellar remnants, following the exhaustion of matter support and preceding formation. The central core of an electroweak star achieves densities exceeding 10^{28} kg/m³, far beyond those of or stars, where the transitions into a hot plasma of quarks, leptons, and gauge bosons. At these conditions, quarks effectively "melt" into a deconfined state, and the electroweak interactions dominate, with energy released through sphaleron-mediated transitions that convert heavy quarks into lighter leptons and neutrinos, each process liberating roughly 300 GeV per particle. This energy generation balances gravitational pressure, preventing immediate , while the plasma's opacity to neutrinos traps , contributing to the core's . The structure ties directly to extensions of at higher densities, potentially evolving from the of a core. Key properties include a total radius of approximately 8 km and a mass around 1.3 solar masses, with the electroweak core itself spanning only a few centimeters amid a surrounding shell of less extreme matter. Support begins with Fermi degeneracy pressure from leptons and neutrinos in the outer layers, transitioning inward to electroweak-dominated dynamics where the equation of state approximates ε ≈ 3P, incorporating chiral symmetry restoration in the quark-gluon plasma and pressure from perturbative electroweak interactions at high temperature. Sphaleron processes play a crucial role in the equation of state by enforcing equilibrium through B + L violation, effectively resetting chemical potentials to zero and sustaining the plasma state. Electroweak stars exhibit quasi-stability, persisting for over 10 million years due to the continuous energy input from electroweak burning, which could significantly delay black hole formation in progenitors with masses up to several solar masses. This longevity arises from the balance between energy release rates—enormous in the core but moderated at the surface by gravitational redshift and neutrino scattering—and the Tolman-Oppenheimer-Volkoff equilibrium conditions. Theoretically, these objects mirror the electroweak phase transition in the early universe, where similar high-temperature symmetry restoration shaped primordial matter distributions, suggesting electroweak stars as astrophysical laboratories for Standard Model processes at unattainable accelerator energies.

Boson Stars

Boson stars are hypothetical compact objects formed by self-gravitating configurations of , representing equilibrium solutions to the coupled Einstein-Klein-Gordon equations for a minimally coupled to . These structures were first proposed in the through numerical studies of spherically symmetric eigenstates, with . Kaup introducing the of a "Klein-Gordon geon" in 1968 and Remo Ruffini and Silvano Bonazzola extending the analysis to quantized in 1969. Unlike fermionic stars supported by degeneracy pressure, boson stars rely on the balance between the attractive gravitational force and the dispersive, quantum mechanical nature of the , allowing multiple bosons to occupy the same without Pauli exclusion. These objects exhibit a wide range of physical properties depending on the boson's mass and self-interaction potential, manifesting in either diffuse, extended forms or compact configurations akin to neutron stars. Their masses span from planetary scales (∼10^{-6} M_⊙ for heavy bosons) to supermassive regimes (up to ∼10^5 M_⊙ for lighter fields), with radii varying from kilometers for compact boson stars to astronomical units for dilute ones, and no inherent upper limit on central density as in degenerate matter stars. For instance, in the absence of self-interactions, the equilibrium is maintained through the Heisenberg uncertainty principle, which provides an effective pressure against gravitational collapse. The mathematical framework governing boson stars derives from the sourced by the stress-energy tensor of a complex φ, combined with the Klein-Gordon equation in curved : □ϕ=dVdϕ,\square \phi = \frac{dV}{d\phi}, where □ is the covariant d'Alembertian operator and V(φ) is the , typically V = m_φ² |φ|² / 2 for a free massive field with boson mass m_φ. Equilibrium configurations satisfy a generalized , balancing the field's (from time dependence), gradient energy (from spatial variations), (if self-interacting), and . Stationary solutions often assume a time dependence φ ∝ e^{-iωt} ψ(r), reducing the system to a nonlinear eigenvalue problem solved numerically or via series expansions. Stability analyses reveal that boson stars possess a maximum beyond which perturbations lead to either dispersal or , estimated as M_max ≈ M_Pl² / m_φ, where M_Pl is the Planck (∼1.22 × 10^{19} GeV/c²), scaling inversely with the boson . This limit arises from the competition between gravitational attraction and the scalar field's dispersion; for excited states with nodes in the radial profile, stability windows narrow, and spinning boson stars incorporate via azimuthal dependence, enabling Kerr-like metrics without horizons. Configurations below this mass threshold are dynamically stable against small perturbations, as confirmed by linear and numerical evolutions. In astrophysical contexts, boson stars serve as candidates for ultralight , particularly stars formed from QCD axions or axion-like particles, where the 's mass (∼10^{-5} to 10^{-22} eV) yields structures from asteroid-sized miniclusters to scales. They also act as black hole mimics, producing similar signatures in mergers or shadows in imaging, yet distinguishable by the absence of an and potential scalar radiation.

Extreme Density and Alternative Stars

Planck Stars

Planck stars represent a theoretical construct within , where the collapse of a massive star or reaches the Planck density of approximately 5×10965 \times 10^{96} kg/m³, at which point quantum-gravitational effects generate a repulsive pressure that halts further compression and induces a bounce, forming a stable, ultra-dense core devoid of singularities. This idea, introduced by and Francesca Vidotto in 2014, posits that the bounce occurs rapidly in the local of the collapsing matter—on the order of microseconds—but appears prolonged externally due to extreme , potentially spanning billions of years. Unlike classical predictions of infinite-density singularities, the Planck star maintains a finite structure governed by quantum corrections to . The physical properties of a Planck star include an initial radius comparable to the Planck length of 1.6×10−351.6 \times 10^{-35} m, though its effective radius expands relative to the shrinking of the parent as erodes the exterior. The total mass remains conserved from the progenitor , with the core's stability ensured by the balance between gravitational attraction and quantum pressure, preventing collapse beyond the bounce point. This configuration draws on extreme equations of state at Planck scales, where dominates over classical matter behaviors. Theoretically, the bounce mechanism relies on an effective metric derived from , which modifies the spacetime dynamics near the Planck regime to avoid curvature divergences. As the evaporates through , the Planck star's persistence leads to a modified evaporation timeline, slower than semiclassical expectations, as the core does not radiate efficiently until the horizon approaches its surface, potentially triggering a rapid energy release. This framework circumvents the by preserving within the bouncing core, which could transition into a , releasing encoded data without loss. Among the implications, Planck stars offer a pathway to resolve tensions with in strong , including avoidance of firewall paradoxes through the early onset of quantum effects that disrupt semiclassical horizon assumptions. Observational prospects include detectable signatures in signals from mergers, where deviations from Kerr templates—such as scale-dependent modifications—could reveal the presence of Planckian cores. Recent advancements since 2020 have linked this model to asymptotic safety programs in , deriving explicit metrics for Planck stars via running gravitational couplings that enhance stability and predict refined bounce dynamics.

Dark Stars

Dark stars are hypothetical first-generation stars in the early , powered primarily by heating from the of weakly interacting massive particles (WIMPs) serving as candidates, rather than by in their cores. This concept was proposed by Freese et al. in , who described these objects as a distinct phase of preceding conventional Population III stars. These stars exhibit extreme properties due to the sustained heating, which prevents premature collapse and allows prolonged growth. They can reach enormous radii of up to several AU (≈ 2×1032 \times 10^3 R⊙R_\odot), while maintaining low effective surface temperatures around 2000–5000 K, making them cool and dim compared to fusion-powered stars. Their lifetimes extend to approximately 10910^9 years, far longer than typical massive stars, as the provides a steady energy source that slows contraction. Eventually, as the density depletes through and outward , the heating diminishes, leading to rapid collapse and the formation of black holes with masses up to 10510^5–10610^6 solar masses (M⊙M_\odot). The underlying mechanism relies on the of WIMPs captured within the protostellar gas cloud, where the high density in the early enhances the process. The annihilation rate per unit volume, Γ\Gamma, is given by Γ=⟨σv⟩ρDM22mDM,\Gamma = \frac{\langle \sigma v \rangle \rho_{\rm DM}^2}{2 m_{\rm DM}}, where ⟨σv⟩\langle \sigma v \rangle is the thermally averaged annihilation cross-section times , ρDM\rho_{\rm DM} is the density, and mDMm_{\rm DM} is the WIMP mass. This rate deposits energy at approximately 103810^{38} erg/s, sufficient to balance gravitational contraction and support the star's structure against collapse. WIMPs scatter off baryons in the cloud, becoming gravitationally bound and increasing the local density, which quadratically boosts the annihilation efficiency. Dark stars are theorized to form in minihalos at redshifts z∼20z \sim 20–30, during the cosmic dawn when dark matter halos of 10510^5–10610^6 M⊙M_\odot first assemble. Their large final masses position them as potential seeds for supermassive black holes observed at high redshifts, providing a pathway to explain the rapid growth of these objects without invoking unconventional accretion physics. Recent models since 2020 have extended the original framework by incorporating self-interacting (SIDM), where WIMPs with additional short-range interactions enhance annihilation rates and allow dark stars to form in halos up to 10810^8 M⊙M_\odot. Other updates explore the role of primordial black holes as a fraction of , which could seed denser cores and alter the heating dynamics in these protostars. As of 2023, observations from the (JWST) have identified potential supermassive dark star candidates at high redshifts (z ≈ 10–15), such as JADES-GS-z14-0, exhibiting extended sizes, low temperatures, and high luminosities consistent with dark star models.

Thorne–Żytkow Objects

Thorne–Żytkow objects (TZOs) are hypothetical hybrid stars consisting of a core embedded within the envelope of a massive star. These exotic systems were first proposed by Kip S. Thorne and Anna N. Żytkow in 1977, who modeled their equilibrium structure as stars with degenerate neutron cores surrounded by non-degenerate envelopes. Formation occurs through the merger of a with a massive companion star during a common-envelope phase, where the neutron star spirals inward and accretes material from the envelope, stabilizing at the center without disrupting the outer layers. These objects exhibit properties resembling red supergiants but with distinctive internal dynamics driven by the core. The convective facilitates unique , including r-process pathways that produce heavy elements, leading to anomalous surface abundances such as elevated and levels, alongside relatively low iron content compared to typical supergiants. The 's heat and material mixing in the sustain the star's luminosity for up to 10^5 years, mimicking an star while hiding the compact core. Detection of TZOs relies primarily on spectroscopic analysis of spectral lines revealing these abundance anomalies, as the objects appear optically similar to normal red supergiants. A prominent candidate is in the , identified in 2014 for its enhanced , , and abundances, though subsequent studies have questioned its classification due to distance and variability concerns. Challenges include distinguishing these signatures from those of extreme stars or dust-obscured objects, compounded by the rarity of mergers in observed populations. TZOs remain stable during their lifetime but are expected to evolve by ejecting their envelopes through intensified mass loss or dynamical instability, potentially exposing the and leading to a or . Recent studies, including analyses of astrometric data, have suggested additional candidates among luminous red supergiants in the by cross-referencing proper motions and luminosities with predicted TZO traits. This builds briefly on the stability of isolated neutron stars, providing a compact core capable of withstanding the envelope's pressure.

Observational Prospects

Detection Methods

Multi-messenger astronomy plays a crucial role in detecting exotic stars, particularly through signals from mergers involving these compact objects. Detectors such as and Virgo have observed binary mergers that provide constraints on the equation of state () at extreme densities, which can distinguish exotic stars from standard neutron stars if their EOS deviates significantly. For instance, the binary neutron star merger imposed tight limits on the EOS, ruling out overly stiff configurations that might otherwise support certain exotic structures, with tidal deformability measurements indicating radii below approximately 13 km for a 1.4 object. These observations suggest that future mergers detected by upgraded facilities like LIGO-Virgo-KAGRA could reveal signatures of boson stars or quark matter stars through distinct waveform patterns during inspiral and ringdown phases. Electromagnetic signatures offer another pathway for identification, often manifesting as bursts or anomalous pulsations due to surface instabilities unique to exotic compositions. Quark stars, for example, may exhibit enhanced emission from thin, bare surfaces lacking a crust, leading to bursts triggered by reconfiguration or accretion-induced quakes. Early candidates like RX J1856.5-3754 were proposed based on their isolated spectra, but later analyses favor a standard interpretation with a radius around 16 km. Cooling curves for electroweak or boson stars also differ, showing slower emission or altered photon luminosities compared to , observable via long-term monitoring with telescopes like . Specific instrumental techniques further refine these detections, including mass-radius measurements from the Interior Composition Explorer (NICER) mission, which probes via pulse profile modeling of X-ray hotspots. NICER's data on high-mass pulsars, such as PSR J0740+6620 (mass ~2.08 M⊙), yield a radius of 12.92_{-1.13}^{+2.09} km (68% as of 2024), implying ~12.5 km for a 1.4 M⊙ object and constraining models where exotic phases like quark matter could fit within observed bounds or predict smaller radii for twin-star scenarios. Fast radio bursts (FRBs) have been theoretically linked to crustal fractures in , where intermittent collapses release coherent radio emission; repeating FRBs like those from FRB 180916 could arise from planetary interactions or on a strange star surface. Numerical simulations complement these efforts by predicting observable signals, such as gravitational waveforms from star binaries modeled via codes like GRChombo, which generate high-fidelity templates for up to 20 orbits before merger to aid parameter estimation in data. For dark stars, the (JWST) has identified potential candidates in early universe galaxies through , revealing luminous, bloated objects up to 10^6 solar masses powered by annihilation rather than fusion, as seen in JADES survey data; 2025 analyses have confirmed additional candidates at z ≈ 10–15. Post-2020 advances, including the Vera C. Rubin Observatory's Legacy Survey of Space and Time, promise to detect transients from exotic star disruptions or mergers via wide-field imaging of millions of events annually, filling gaps in real-time multi-wavelength follow-up.

Potential Evidence and Challenges

One prominent candidate for a is the in the 3C 58, which exhibits an anomalously low cooling rate compared to standard models, potentially explained by the presence of matter that alters emission processes. observations indicate that its radius and surface temperature are consistent with a bare quark star, lacking a crust, though this interpretation remains debated due to uncertainties in atmospheric absorption. For dark stars, potential imprints include distortions in the () power spectrum from their early formation and annihilation-powered luminosity, as well as signatures in (JWST) observations of high-redshift galaxies that appear brighter and more massive than expected under standard cosmology. These JWST-detected early galaxies at z ≈ 10–15 may reflect dark star activity, where dark matter heating sustains massive before conventional Population III stars dominate, though distinguishing this from alternative models like modified star formation efficiency poses difficulties. Gravitational wave events provide indirect constraints on exotic star equations of state (EOS). The binary merger showed no clear exotic signatures, such as unusual tidal deformability, but its tidal parameters tightly limit the EOS stiffness, excluding many soft EOS models for or hybrid stars and implying a maximum mass below ~2.5 M⊙ in many models, consistent with observed masses up to 2.35 M⊙. Subsequent events like GW190425 have further refined these bounds, ruling out some star configurations while allowing others with stiffer EOS. Thorne–Żytkow object (TŻO) candidates exhibit chemical anomalies, such as enhanced , , and abundances in their atmospheres, attributed to from a core accreting material. The star in the was proposed as a TŻO based on these enhancements observed via , marking the first such candidate with a distinctive profile. However, follow-up analyses suggest these anomalies could arise from foreground contamination or standard evolution, rendering the evidence inconclusive without higher-resolution spectra. Detecting exotic stars faces significant challenges, including degeneracy with models where similar mass-radius relations and cooling curves can mimic exotic compositions under varying assumptions. The lack of direct imaging for sub-solar mass or ultra-compact objects, combined with theoretical uncertainties in the high-density —such as the transition to matter or condensation—complicates differentiation, as multiple families fit current multimessenger data. Future prospects include (ELT) to resolve chemical profiles in TŻO candidates among red supergiants, potentially confirming anomalous abundances at higher precision than current facilities. The (LISA) could detect boson stars through from extreme mass-ratio inspirals or oscillations, probing masses up to 10^5 solar masses where they mimic supermassive s. Post-2023 NANOGrav data, revealing a nano-Hz , offer hints for supermassive exotics like boson stars as alternatives to black hole binaries, though distinguishing sources requires multi-messenger follow-up.

References

  1. https://arxiv.org/abs/1810.07137
  2. https://arxiv.org/abs/astro-ph/0407155
  3. https://www.advancedsciencenews.com/a-quark-star-may-have-just-been-discovered/
  4. https://iopscience.iop.org/article/10.3847/1538-4357/abf355
  5. https://academic.oup.com/ptp/article/44/1/291/1896619
  6. https://doi.org/10.1103/PhysRev.172.1331
  7. https://journals.aps.org/prd/abstract/10.1103/PhysRevD.82.065017
  8. https://iopscience.iop.org/article/10.1088/1742-6596/861/1/012020
  9. https://www.sciencedirect.com/science/article/abs/pii/S221268642030039X
  10. https://ui.adsabs.harvard.edu/abs/2017Ap.....60..563A/abstract
  11. https://arxiv.org/abs/2404.00363
  12. https://doi.org/10.1086/164679
  13. https://doi.org/10.1103/PhysRevLett.77.1210
  14. https://arxiv.org/abs/astro-ph/0410417
  15. https://doi.org/10.1016/j.physletb.2005.04.034
  16. https://arxiv.org/abs/1209.3355
  17. https://arxiv.org/abs/0803.0223
  18. https://inspirehep.net/literature/780528
  19. https://doi.org/10.1103/PhysRevD.35.3648
  20. https://arxiv.org/abs/0912.0520
  21. https://doi.org/10.1142/S0218271814420267
  22. https://doi.org/10.1016/j.physletb.2014.11.020
  23. https://arxiv.org/abs/2506.17909
  24. https://arxiv.org/abs/2205.07088
  25. https://iopscience.iop.org/article/10.1086/590508
  26. https://iopscience.iop.org/article/10.1088/0004-637X/729/1/51
  27. https://academic.oup.com/mnras/article/483/3/3592/5232378
  28. https://arxiv.org/abs/2306.00938
  29. https://ui.adsabs.harvard.edu/abs/1977ApJ...212..832T/abstract
  30. https://iopscience.iop.org/article/10.3847/1538-4357/ad595e
  31. https://academic.oup.com/mnras/article/263/4/817/983270
  32. https://iopscience.iop.org/article/10.3847/1538-4357/abebe1
  33. https://academic.oup.com/mnrasl/article/443/1/L94/1054019
  34. https://academic.oup.com/mnras/article/479/3/3101/5047896
  35. https://iopscience.iop.org/article/10.3847/1538-4357/ad3ba6
  36. https://arxiv.org/abs/1711.03647
  37. https://arxiv.org/abs/2406.02715
  38. https://link.aps.org/doi/10.1103/PhysRevD.94.083010
  39. https://arxiv.org/abs/2406.14467
  40. https://www.sciencedirect.com/science/article/pii/S2666675821000771
  41. https://www.pnas.org/doi/10.1073/pnas.2305762120
  42. https://cns.utexas.edu/news/research/more-dark-star-candidates-found-jwst-data
  43. https://iopscience.iop.org/article/10.1088/1538-3873/acdb9a
  44. https://arxiv.org/pdf/1501.02394
  45. https://link.aps.org/doi/10.1103/PhysRevC.98.045804
  46. https://link.aps.org/doi/10.1103/PhysRevX.12.011058
  47. https://arxiv.org/abs/1406.0001
  48. https://arxiv.org/pdf/1806.07399
  49. https://arxiv.org/pdf/2311.15745
  50. https://arxiv.org/pdf/2502.05513
  51. https://arxiv.org/pdf/2507.15964
  52. https://arxiv.org/pdf/2502.04068
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