# Astronomy:Exotic star

Short description: Hypothetical compact star made of exotic matter

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. Exotic stars include quark stars (composed of quarks) and perhaps strange stars (composed of strange quark matter, a condensate of up, down and strange quarks), as well as speculative § Preon stars (composed of preons, which are hypothetical particles and "building blocks" of quarks, 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.

Exotic stars are largely theoretical – 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 cosmic objects that do not radiate electromagnetically or through known particles. It is not yet possible to verify novel cosmic objects of this nature by distinguishing them from known objects. Candidates for such objects are occasionally identified based on indirect evidence gained from observable properties.

## Quark stars and strange stars

Main pages: Astronomy:Quark star and Astronomy:Strange 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 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. After further analysis, RX J1856.5-3754 was excluded from the list of quark star candidates.[1]

## Electroweak stars

Main page: Astronomy:Electroweak star

An electroweak star is a theoretical 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 process occurs in a volume at the star's core approximately the size of an apple and containing about two Earth masses.[2]

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 gravitational attraction can no longer be withstood by quark degeneracy pressure, but can still be withstood by electroweak-burning radiation pressure.[3] This phase of a star's life may last upwards of 10 million years.[2][3][4][5]

## Boson stars

A boson star is a hypothetical astronomical object formed out of particles called bosons (conventional stars are formed from mostly protons, which are fermions, but also consist of Helium-4 nuclei, which comprise bosons). For this type of star to exist, there must be a stable type of boson with self-repulsive interaction; one possible candidate particle[6] 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[7] 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 2018), 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,[8][9] and GW190521, thought to be the most energetic black hole merging, may be the head-on collision of two boson stars.[10]

Boson stars may have formed through gravitational collapse during the primordial stages of the Big Bang.[11] 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.[12]

Boson stars have also been proposed as candidate dark matter objects,[13] and it has been hypothesized that the dark matter haloes surrounding most galaxies might be viewed as enormous "boson stars."[14]

The compact boson stars and boson shells are often studied involving fields like the massive (or massless) complex scalar fields, the U(1) gauge field and gravity with conical potential. 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.[15][16][17][18][19]

Boson stars composed of elementary particles with spin-1 have been labelled Proca stars.[20]

Braaten, Mohapatra, and Zhang 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.[21] The possibility that dense axion stars exist has been challenged by other work that does not support this claim.[22]

## Planck stars

Main page: Astronomy:Planck star

In loop quantum gravity, a Planck star is a theoretically 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.[23]

## References

1. Truemper, J. E.; Burwitz, V.; Haberl, F.; Zavlin, V. E. (June 2004). "The puzzles of RX J1856.5-3754: neutron star or quark star?". Nuclear Physics B: Proceedings Supplements 132: 560–565. doi:10.1016/j.nuclphysbps.2004.04.094. Bibcode2004NuPhS.132..560T.
2. Shiga, D. (4 January 2010). "Exotic stars may mimic Big Bang". New Scientist.
3.
4. Vieru, Tudor (15 December 2009). "New Type of Cosmic Objects: Electroweak Stars". Softpedia.
5. "Astronomers Predict New Class of 'Electroweak' Star". Technology Review. 10 December 2009.
6. Kolb, Edward W.; Tkachev, Igor I. (29 March 1993). "Axion Miniclusters and Bose Stars". Physical Review Letters 71 (19): 3051–3054. doi:10.1103/PhysRevLett.71.3051. PMID 10054845. Bibcode1993PhRvL..71.3051K.
7. 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: 29.
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. doi:10.1103/PhysRevD.77.044036. Bibcode2008PhRvD..77d4036P.
10. Bustillo, Juan Calderón; Sanchis-Gual, Nicolas; Torres-Forné, Alejandro; Font, José A.; Vajpeyi, Avi; Smith, Rory; Herdeiro, Carlos; Radu, Eugen et al. (2021). "GW190521 as a Merger of Proca Stars: A Potential New Vector Boson of 8.7×10−13 eV". Physical Review Letters 126 (8): 081101. doi:10.1103/PhysRevLett.126.081101. PMID 33709746.
11. Madsen, Mark S.; Liddle, Andrew R. (1990). "The cosmological formation of boson stars". Physics Letters B 251 (4): 507. doi:10.1016/0370-2693(90)90788-8. Bibcode1990PhLB..251..507M.
12. Torres, Diego F.; Capozziello, S.; Lambiase, G. (2000). "A supermassive boson star at the galactic center?". Physical Review D 62 (10): 104012. doi:10.1103/PhysRevD.62.104012. Bibcode2000PhRvD..62j4012T.
13. Sharma, R.; Karmakar, S.; Mukherjee, S. (2008). "Boson star and dark matter". arXiv:0812.3470 [gr-qc].
14. Lee, Jae-weon; Koh, In-guy (1996). "Galactic Halos As Boson Stars". Physical Review D 53 (4): 2236–2239. doi:10.1103/PhysRevD.53.2236. PMID 10020213. Bibcode1996PhRvD..53.2236L.
15. Kumar, S.; Kulshreshtha, U.; Kulshreshtha, D. S. (2016). "Charged compact boson stars and shells in the presence of a cosmological constant". Physical Review D 94 (12): 125023. doi:10.1103/PhysRevD.94.125023. Bibcode2016PhRvD..94l5023K.
16. Kumar, S.; Kulshreshtha, U.; Kulshreshtha, D. S. (2016). "Charged compact boson stars and shells in the presence of a cosmological constant". Physical Review D 93 (10): 101501. doi:10.1103/PhysRevD.93.101501. Bibcode2016PhRvD..93j1501K.
17. Kleihaus, B.; Kunz, J.; Lammerzahl, C.; List, M. (2010). "Boson Shells Harbouring Charged Black Holes". Physical Review D 82 (10): 104050. doi:10.1103/PhysRevD.82.104050. Bibcode2010PhRvD..82j4050K.
18. Hartmann, B.; Kleihaus, B.; Kunz, J.; Schaffer, I. (2013). "Compact (A)dS Boson Stars and Shells". Physical Review D 88 (12): 124033. doi:10.1103/PhysRevD.88.124033. Bibcode2013PhRvD..88l4033H.
19. Kumar, S.; Kulshreshtha, U.; Kulshreshtha, D. S.; Kahlen, S.; Kunz, J. (2017). "Some new results on charged compact boson stars". Physics Letters B 772: 615–620. doi:10.1016/j.physletb.2017.07.041. Bibcode2017PhLB..772..615K.
20. Brito, Richard; Cardoso, Vitor; Herdeiro, Carlos A.R.; Radu, Eugen (January 2016). "Proca stars: Gravitating Bose–Einstein condensates of massive spin 1 particles" (in en). Physics Letters B 752: 291–295. doi:10.1016/j.physletb.2015.11.051. Bibcode2016PhLB..752..291B. Retrieved 25 July 2021.
21. Braaten, Eric; Mohapatra, Abhishek; Zhang, Hong (2016). "Dense Axion Stars". Physical Review Letters 117 (12): 121801. doi:10.1103/PhysRevLett.117.121801. PMID 27689265. Bibcode2016PhRvL.117l1801B. Retrieved 26 September 2018.
22. Visinelli, Luca; Baum, Sebastian; Redondo, Javier; Freese, Katherine; Wilczek, Frank (2018). "Dilute and Dense Axion Stars". Physics Letters B 777: 64–72. doi:10.1016/j.physletb.2017.12.010. Bibcode2018PhLB..777...64V.
23. Rovelli, Carlo; Vidotto, Francesca (2014). "Planck stars". International Journal of Modern Physics D 23 (12): 1442026. doi:10.1142/S0218271814420267. Bibcode2014IJMPD..2342026R.
24. Small, dark, and heavy: But is it a black hole? . Visser, Matt; Barcelo, Carlos; Liberati, Stefano; Sonego, Sebastiano (February 2009)
25. How Quantum Effects Could Create Black Stars, Not Holes