§ 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.
Exotic stars are hypothetical – partly because it is difficult to test in detail how such forms of matter may behave, and partly because prior to the fledgling technology ofgravitational-wave astronomy, there was no satisfactory means of detecting compact astrophysical objects that do not radiate either electromagnetically or through known particles. While candidate objects are occasionally identified based on indirect evidence, it is not yet possible to distinguish their observational signatures from those of known objects.
Aquark star is a hypothesized object that results from the decomposition ofneutrons into their constituentup anddownquarks under gravitational pressure. It is expected to be smaller and denser than aneutron star, and may survive in this new state indefinitely, if no extra mass is added. Effectively, it is a single, very largehadron. Quark stars that containstrange matter are calledstrange stars.
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.[3]
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).[4][5] Electroweak stars could be identified through the equal number of neutrinos emitted of all three generations, taking into accountneutrino oscillation.[4]
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.[6] Preon stars could originate fromsupernova explosions or theBig Bang. Such objects could be detected in principle throughgravitational lensing ofgamma rays. Preon stars are a potential candidate fordark matter. However, current observations[7] fromparticle accelerators speak against the existence of preons, or at least do not prioritize their investigation, since the only particle detector presently able to explore very high energies (theLarge Hadron Collider) is not designed specifically for this and its research program is directed towards other areas, such as studying theHiggs boson,quark–gluon plasma and evidence related tophysics beyond the Standard Model.[clarification needed]
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[8]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[9]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. Simulations suggest that rotating boson stars would betorus-shaped, ascentrifugal forces would give the bosonic matter that form.
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.[10][11]GW190521, thought to be the most energeticblack hole merger ever recorded, may be the head-on collision of two boson stars.[12] 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.[13][14]
Boson stars may have formed through gravitational collapse during the primordial stages of the Big Bang.[15]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.[16]
Boson stars have also been proposed as candidatedark matter objects,[17]and it has been hypothesized that thedark matter haloes surrounding mostgalaxies might be viewed as enormous "boson stars."[18]
Boson stars composed of elementary particles with spin-1 have been labelledProca stars.[24]
Braaten, Mohapatra, and Zhang have theorized that a new type denseaxion-star may exist in which gravity is balanced by the mean-field pressure of the axionBose–Einstein condensate.[25]The possibility that dense axion stars exist has been challenged by other work that does not support this claim.[26]
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.[28]
^Visser, Matt; Barcelo, Carlos; Liberati, Stefano; Sonego, Sebastiano (February 2009). "Small, dark, and heavy: But is it a black hole?".arXiv:0902.0346v2 [gr-qc].Visser, Matt; Barcelo, Carlos; Liberati, Stefano; Sonego, Sebastiano (2009). "Small, dark, and heavy: But is it a black hole?".arXiv:0902.0346v2 [gr-qc].
^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.
