Exotic star

An exotic star is a hypothetical compact star composed of something other than electrons, protons, neutrons, and muons; and balanced against gravitational collapse by degeneracy pressure or other quantum properties. These include quark stars (composed of quarks) and perhaps strange stars (based upon strange quark matter, a condensate of up, down and strange quarks), as well as speculative preon stars (composed of preons, a hypothetical particle and "building block" of quarks, if quarks prove to be decomposable into component sub-particles). Of the various types of exotic star proposed, the kind we have best understanding of, and evidence for, is the quark star.

Exotic stars are largely theoretical, because it is difficult to test in detail how such forms of matter may behave, and (prior to the fledgling technology of gravitational-wave astronomy) we also lack satisfactory means of detection of cosmic objects that do not radiate electromagnetically or through known particles, or ways to verify and distinguish novel cosmic objects from other known objects. For preon stars (and similar) we also lack knowledge about whether quarks are indeed composed of some kind of sub-particle, and therefore whether preons physically exist.

However candidates for such objects are occasionally identified as such, based on indirect evidence gained from properties we can observe, when the object exhibits properties suggesting such a nature.

Quark stars and strange stars

Main article: Quark star

A quark star is a hypothesized object that results from the decomposition of neutrons into their constituent up and down quarks 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 very large nucleon. Quark stars that contain strange matter are called strange stars.

Based on observations released by the Chandra X-Ray Observatory on April 10, 2002, two objects, designated RX J1856.5-3754 and 3C58, were suggested as quark star candidates. The former appeared much smaller and the latter much colder than expected for neutron stars, 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.. After further analysis, RX J1856.5-3754 was excluded from the list of quark star candidates.

Electroweak stars

Main article: Electroweak star

An electroweak star is a theoretical type of exotic star, whereby the gravitational collapse of the star is prevented by radiation pressure resulting from electroweak burning, that is, the energy released by conversion of quarks to leptons through the electroweak force. This process occurs in a volume at the star's core approximately the size of an apple, containing about two Earth masses.[1]

The stage of life of a star that produces an electroweak star is theorized to occur after a supernova collapse. Electroweak stars are denser than quark stars, and may form when quark degeneracy pressure is no longer able to withstand gravitational attraction, but may still be withstood by electroweak burning radiation pressure.[2] This phase of a star's life may last upwards of 10 million years.[1][2][3][4]

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 neutron stars although they are smaller and, therefore, less massive than white dwarfs and neutron stars.[5] 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[6] 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 the Higgs boson, quark-gluon plasma and evidence related to physics beyond the Standard Model.

In general relativity, if the star collapses to a size smaller than its Schwarzschild radius, an event horizon will exist at that radius and the star will become a black hole. Thus, the size of a preon star may vary from around 1 metre with an absolute mass of 100 earths to the size of a pea with a mass roughly equal to the Moon.

Boson stars

A boson star is a hypothetical astronomical object that is formed out of particles called bosons (conventional stars are formed out of fermions). For this type of star to exist, there must be a stable type of boson with self-repulsive interaction; one possible candidate particle[7] is the still-hypothetical "axion" (which is also a candidate for the not-yet detected "non-baryonic Dark Matter" particles that appear to compose roughly 25% of the matter in the Universe).

As of 2016 there is no significant evidence that such a star exists. However, it may become possible to detect them by the gravitational radiation emitted by a pair of co-orbiting boson stars.[8][9]

Boson stars may have been formed through gravitational collapse during the primordial stages of the big bang.[10] At least in theory, a supermassive boson star could exist at the core of a galaxy, which might explain many of the observed properties of active galactic cores.[11]

Boson stars have also been proposed as a candidate dark matter object,[12] and it has been hypothesized that the "Dark Matter Halos" surrounding most galaxies might be viewed as enormous "Boson Stars." [13]

Planck star

A Planck star is a hypothetical astronomical object where the energy density is around the Planck density (5.15500 × 1096 kg/m3). In 2014 Carlo Rovelli and Francesca Vidotto proposed that there is a Planck star inside a black hole.[14] This theory, if correct, would resolve the black hole firewall and black hole information paradox. This idea is based on loop quantum gravity.

See also

References

  1. 1 2 D. Shiga (4 January 2010). "Exotic stars may mimic big bang". New Scientist. Retrieved 2010-02-18.
  2. 1 2 "Theorists Propose a New Way to Shine – And a New Kind of Star: 'Electroweak'". ScienceDaily. 15 December 2009. Retrieved 2009-12-16.
  3. ↑ Tudor Vieru (15 December 2009). "New Type of Cosmic Objects: Electroweak Stars". Softpedia. Retrieved 2009-12-16.
  4. ↑
  5. ↑ Hannson, J; F. Sandin (9 June 2005). "Preon stars: a new class of cosmic compact objects". Physics Letters B. 616 (1–2): 1–7. arXiv:astro-ph/0410417Freely accessible. Bibcode:2005PhLB..616....1H. doi:10.1016/j.physletb.2005.04.034. Retrieved 8 September 2011.
  6. ↑ Wilkins, Alasdair (9 December 2010). "Stars so weird that they make black holes look boring". io9. Retrieved 12 September 2015.
  7. ↑ Kolb, Edward W.; Tkachev, Igor I. (8 November 1993). "Axion Miniclusters and Bose Stars". Physical Review Letters. 71 (19): 3051. arXiv:hep-ph/9303313Freely accessible. doi:10.1103/PhysRevLett.71.3051.
  8. ↑ Schutz, Bernard F. (2003). Gravity from the ground up (3rd ed.). Cambridge University Press. p. 143. ISBN 0-521-45506-5.
  9. ↑ Palenzuela, C.; Lehner, L.; Liebling, S. L. (2008). "Orbital dynamics of binary boson star systems". Physical Review D. 77 (4): 044036. arXiv:0706.2435Freely accessible. Bibcode:2008PhRvD..77d4036P. doi:10.1103/PhysRevD.77.044036.
  10. ↑ Madsen, Mark S.; Liddle, Andrew R. (1990). "The cosmological formation of boson stars". Physics Letters B. 251 (4): 507. Bibcode:1990PhLB..251..507M. doi:10.1016/0370-2693(90)90788-8.
  11. ↑ Torres, Diego F.; Capozziello, S.; Lambiase, G. (2000). "Supermassive boson star at the galactic center?". Physical Review D. 62 (10): 104012. arXiv:astro-ph/0004064Freely accessible. Bibcode:2000PhRvD..62j4012T. doi:10.1103/PhysRevD.62.104012.
  12. ↑ Sharma, R.; Karmakar, S.; Mukherjee, S. "Boson star and dark matter". arXiv:0812.3470Freely accessible.
  13. ↑ Lee, Jae-weon; Koh, In-guy (1996). "Galactic Halos As Boson Stars". Physical Review D. 53 (4): 2236. arXiv:hep-ph/9507385Freely accessible. doi:10.1103/PhysRevD.53.2236.
  14. ↑ Rovelli, Carlo; Vidotto, Francesca (2014). "Planck stars". International Journal of Modern Physics D. 23 (12). arXiv:1401.6562Freely accessible. Bibcode:2014IJMPD..2342026R. doi:10.1142/S0218271814420267.
  15. ↑ Small, dark, and heavy: But is it a black hole?, Matt Visser, Carlos Barcelo, Stefano Liberati, Sebastiano Sonego, February 2009
  16. ↑ How Quantum Effects Could Create Black Stars, Not Holes
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