preon stars (speculative material composed ofpreons, which are hypothetical particles and "building blocks" of quarks and leptons, should quarks be decomposable into component sub-particles).
Of the various types of exotic star proposed, the most well evidenced and understood is thequark star, although its existence is not confirmed.
Aquark star is a hypothesized object that results from the decomposition ofneutrons into their constituentquarks under extremely intense gravitational pressure balanced by electrical repulsion anddegeneracy pressure.[1][2] Such a star would be smaller and more dense than aneutron star, and may survive in this new state indefinitely, if no extra mass is added. Quark stars that containstrange matter are calledstrange stars.[3] Such a star, first proposed byEdward Witten, would consist of confined quarks, essentialy a giantnucleon.[4]
Based on observations released by theChandra X-Ray Observatory on 10 April 2002, two objects, namedRX J1856.5−3754 and3C 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 thanneutronium. 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]
Anelectroweak star is a hypothetical type of exotic star in which the gravitational collapse of the star is prevented byradiation pressure resulting fromelectroweak burning; that is, the energy released by the conversion ofquarks intoleptons through theelectroweak force. This proposed process might occur in a volume at the star's core approximately the size of anapple, containing about two Earth masses, and reachingtemperatures on the order of 1015K (1 PK).[6][7] Electroweak stars could be identified through the equal number of neutrinos emitted of all three generations, taking into accountneutrino oscillation.[6]
Apreon star is a proposed type of compact star made ofpreons, a group ofhypothetical subatomic particles. Preon stars would be expected to have hugedensities, exceeding 1023 kg/m3. They may have greater densities than quark stars, and they would be heavier but smaller thanwhite dwarfs and neutron stars.[8]
Aboson star is a hypotheticalastronomical object formed out of particles calledbosons. Conventionalstars are formed from mostly protons and electrons, which arefermions, but also contain a large proportion ofhelium-4 nuclei, which arebosons, 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'sevent 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 quantizedangular momentum, and their energy density profiles aretorus-shaped, which can be understood as a result of deformation due tocentrifugal 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 energeticblack 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 byGaia 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 ofactive galactic cores.[19] However, more recent general-relativisticmagnetohydrodynamic simulations, combined with imaging performed by theEvent Horizon Telescope, is believed to have largely ruled out the possibility thatSagittarius A*, the supermassive object at the center of theMilky Way, could be a boson star.[20]
Bound states in cosmological bosonic fields have also been proposed as an alternative todark matter.[21] Thedark matter haloes surrounding mostgalaxies might be viewed as enormous "boson stars."[22]
Compact boson stars and boson shells are often modelled using massive bosonic fields, such as complexscalar fields andU(1) gauge fields, coupled to gravity. The presence of a positive or negative cosmological constant in the theory facilitates a study of these objects inde Sitter andanti-de Sitter spaces.[23][24][25][26][27]
By changing thepotential associated with the matter model, different families of boson star models can be obtained. The so-calledsolitonic potential, which introduces a degeneratevacuum state at a finite value of the field amplitude, can be used to construct boson star models so compact that they possess a pair ofphoton 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 labelledProca stars.[29]
Q-stars are hypothetical objects that originate fromsupernovae 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 thanneutron stars or even quark stars.[31]
^Clark, Stuart (15 July 2017). "Holy moley! (Astronomers taking a first peek at our galaxy's black heart might be in for a big surprise)".New Scientist. p. 29.
Sandin, Fredrik (2007).Exotic Phases of Matter in Compact Stars. Department of Applied Physics and Mechanical Engineering / Division of Physics (Ph.D. thesis). Luleå, Sweden: Luleå Tekniska Universitet [Luleå University of Technology].ISSN1402-1544.
