§ Preon stars (speculative material composed of preons, 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 the quark 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 of gravitational-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.
A quark star is a hypothesized object that results from the decomposition of neutrons into their constituent up and downquarks under gravitational pressure. It is expected to be smaller and denser than a neutron star, and may survive in this new state indefinitely, if no extra mass is added. Effectively, it is a single, very large hadron. Quark stars that contain strange matter are called strange stars.
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.[3]
Electroweak stars
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.[4] 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 1015K (1 PK).[5][6]
The stage of life of a star that produces an electroweak star is theorized to occur after a supernova collapse. Electroweak stars are predicted to be denser than quark stars, and may form when gravitational attraction can no longer be withstood by quark degeneracy pressure, but can still be withstood by electroweak-burning radiation pressure.[7]
This phase of a star's life may last upwards of 10 million years.[6][7][8][9] This phase of a star's life may last upwards of 10 million years.[6]
The energy output of an electroweak star is limited by the quark supply rate, which is dictated by gravitational collapse. Each interaction converts nine quarks into three anti-leptons, violating conservation of baryon and lepton number while preserving B−L, generating around 300 GeV per interaction. The energy diffuses out of the star as a mixture of neutrinos and photons. Electroweak stars could be identified through the equal number of neutrinos emitted of all three generations, taking into account neutrino oscillation.[5]
Preon stars
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.[10] Preon stars could originate from supernova explosions or the Big Bang. Such objects could be detected in principle through gravitational lensing of gamma rays. Preon stars are a potential candidate for dark matter. However, current observations[11] from particle 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 (the Large Hadron Collider) is not designed specifically for this and its research program is directed towards other areas, such as studying the Higgs boson, quark–gluon plasma and evidence related to physics beyond the Standard Model.[clarification needed]
Boson stars
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[12]
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[13]
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. Simulations suggest that rotating boson stars would be torus, or "doughnut-shaped", as centrifugal forces would give the bosonic matter that form.
As of 2024[update], 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,[14][15] and GW190521, thought to be the most energetic black hole merging, may be the head-on collision of two boson stars.[16]
Boson stars may have formed through gravitational collapse during the primordial stages of the Big Bang.[17]
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.[18]
Boson stars have also been proposed as candidate dark matter objects,[19]
and it has been hypothesized that the dark matter haloes surrounding most galaxies might be viewed as enormous "boson stars."[20]
Boson stars composed of elementary particles with spin-1 have been labelled Proca stars.[26]
Braaten, Mohapatra, and Zhang (2016) have theorized that a new type dense axion star may exist in which gravity is balanced by the mean-field pressure of the axion Bose–Einstein condensate.[27]
The possibility that dense axion stars exist has been challenged by other work that does not support this claim.[28]
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.[30]
^
Quantum effects may prevent true black holes from forming and give rise instead to dense entities called black stars.[2]
References
^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.
