Quark star

Last updated

A quark star is a hypothetical type of compact, exotic star, where extremely high core temperature and pressure have forced nuclear particles to form quark matter, a continuous state of matter consisting of free quarks.

Contents

Background

Some massive stars collapse to form neutron stars at the end of their life cycle, as has been both observed and explained theoretically. Under the extreme temperatures and pressures inside neutron stars, the neutrons are normally kept apart by a degeneracy pressure, stabilizing the star and hindering further gravitational collapse. However, it is hypothesized that under even more extreme temperature and pressure, the degeneracy pressure of the neutrons is overcome, and the neutrons are forced to merge and dissolve into their constituent quarks, creating an ultra-dense phase of quark matter based on densely packed quarks. In this state, a new equilibrium is supposed to emerge, as a new degeneracy pressure between the quarks, as well as repulsive electromagnetic forces, will occur and hinder total gravitational collapse.

If these ideas are correct, quark stars might occur, and be observable, somewhere in the universe. Such a scenario is seen as scientifically plausible, but it has been impossible to prove both observationally and experimentally, because the very extreme conditions needed for stabilizing quark matter cannot be created in any laboratory nor has it been observed directly in nature. The stability of quark matter, and hence the existence of quark stars, is for these reasons among the unsolved problems in physics.

If quark stars can form, then the most likely place to find quark star matter would be inside neutron stars that exceed the internal pressure needed for quark degeneracy – the point at which neutrons break down into a form of dense quark matter. They could also form if a massive star collapses at the end of its life, provided that it is possible for a star to be large enough to collapse beyond a neutron star but not large enough to form a black hole.

If they exist, quark stars would resemble and be easily mistaken for neutron stars: they would form in the death of a massive star in a Type II supernova, be extremely dense and small, and possess a very high gravitational field. They would also lack some features of neutron stars, unless they also contained a shell of neutron matter, because free quarks are not expected to have properties matching degenerate neutron matter. For example, they might be radio-silent, or have atypical sizes, electromagnetic fields, or surface temperatures, compared to neutron stars.

History

The analysis about quark stars was first proposed in 1965 by Soviet physicists D. D. Ivanenko and D. F. Kurdgelaidze. [1] [2] Their existence has not been confirmed.

The equation of state of quark matter is uncertain, as is the transition point between neutron-degenerate matter and quark matter. Theoretical uncertainties have precluded making predictions from first principles. Experimentally, the behaviour of quark matter is being actively studied with particle colliders, but this can only produce very hot (above 1012  K) quark–gluon plasma blobs the size of atomic nuclei, which decay immediately after formation. The conditions inside compact stars with extremely high densities and temperatures well below 1012  K cannot be recreated artificially, as there are no known methods to produce, store or study "cold" quark matter directly as it would be found inside quark stars. The theory predicts quark matter to possess some peculiar characteristics under these conditions.[ citation needed ]

Formation

Mass-radius relations for models of a neutron star with no exotic states (red) and a quark star (blue). TOV solution neutron quark star mass radius diagram.png
Mass–radius relations for models of a neutron star with no exotic states (red) and a quark star (blue).

It is hypothesized that when the neutron-degenerate matter, which makes up neutron stars, is put under sufficient pressure from the star's own gravity or the initial supernova creating it, the individual neutrons break down into their constituent quarks (up quarks and down quarks), forming what is known as quark matter. This conversion may be confined to the neutron star's center or it might transform the entire star, depending on the physical circumstances. Such a star is known as a quark star. [4] [5]

Stability and strange quark matter

Ordinary quark matter consisting of up and down quarks has a very high Fermi energy compared to ordinary atomic matter and is stable only under extreme temperatures and/or pressures. This suggests that the only stable quark stars will be neutron stars with a quark matter core, while quark stars consisting entirely of ordinary quark matter will be highly unstable and re-arrange spontaneously. [6] [7]

It has been shown that the high Fermi energy making ordinary quark matter unstable at low temperatures and pressures can be lowered substantially by the transformation of a sufficient number of up and down quarks into strange quarks, as strange quarks are, relatively speaking, a very heavy type of quark particle. [6] This kind of quark matter is known specifically as strange quark matter and it is speculated and subject to current scientific investigation whether it might in fact be stable under the conditions of interstellar space (i.e. near zero external pressure and temperature). If this is the case (known as the Bodmer–Witten assumption), quark stars made entirely of quark matter would be stable if they quickly transform into strange quark matter. [8]

Strange stars

Stars made of strange quark matter are known as strange stars. These form a distinct subtype of quark stars. [8]

Theoretical investigations have revealed that quark stars might not only be produced from neutron stars and powerful supernovas, they could also be created in the early cosmic phase separations following the Big Bang. [6] If these primordial quark stars transform into strange quark matter before the external temperature and pressure conditions of the early Universe makes them unstable, they might turn out stable, if the Bodmer–Witten assumption holds true. Such primordial strange stars could survive to this day. [6]

Characteristics

Quark stars have some special characteristics that separate them from ordinary neutron stars. Under the physical conditions found inside neutron stars, with extremely high densities but temperatures well below 1012 K, quark matter is predicted to exhibit some peculiar characteristics. It is expected to behave as a Fermi liquid and enter a so-called color-flavor-locked (CFL) phase of color superconductivity, where "color" refers to the six "charges" exhibited in the strong interaction, instead of the two charges (positive and negative) in electromagnetism. At slightly lower densities, corresponding to higher layers closer to the surface of the compact star, the quark matter will behave as a non-CFL quark liquid, a phase that is even more mysterious than CFL and might include color conductivity and/or several additional yet undiscovered phases. None of these extreme conditions can currently be recreated in laboratories so nothing can be inferred about these phases from direct experiments. [9]

If the conversion of neutron-degenerate matter to (strange) quark matter is total, a quark star can to some extent be imagined as a single gigantic hadron. But this "hadron" will be bound by gravity, rather than by the strong force that binds ordinary hadrons.[ citation needed ]

Observed overdense neutron stars

At least under the assumptions mentioned above, the probability of a given neutron star being a quark star is low,[ citation needed ] so in the Milky Way there would only be a small population of quark stars. If it is correct, however, that overdense neutron stars can turn into quark stars, that makes the possible number of quark stars higher than was originally thought, as observers would be looking for the wrong type of star.[ citation needed ]

A neutron star without deconfinement to quarks and higher densities cannot have a rotational period shorter than a millisecond; even with the unimaginable gravity of such a condensed object the centripetal force of faster rotation would eject matter from the surface, so detection of a pulsar of millisecond or less period would be strong evidence of a quark star.

Observations released by the Chandra X-ray Observatory on April 10, 2002, detected two possible quark stars, designated RX J1856.5−3754 and 3C 58, which had previously been thought to be neutron stars. Based on the known laws of physics, the former appeared much smaller and the latter much colder than it should be, suggesting that they are composed of material denser than neutron-degenerate matter. However, these observations are met with skepticism by researchers who say the results were not conclusive; [10] and since the late 2000s, the possibility that RX J1856 is a quark star has been excluded.

Another star, XTE J1739-285, [11] has been observed by a team led by Philip Kaaret of the University of Iowa and reported as a possible quark star candidate.

In 2006, You-Ling Yue et al., from Peking University, suggested that PSR B0943+10 may in fact be a low-mass quark star. [12]

It was reported in 2008 that observations of supernovae SN 2006gy, SN 2005gj and SN 2005ap also suggest the existence of quark stars. [13] It has been suggested that the collapsed core of supernova SN 1987A may be a quark star. [14] [15]

In 2015, Zi-Gao Dai et al. from Nanjing University suggested that Supernova ASASSN-15lh is a newborn strange quark star. [16]

In 2022 it was suggested that GW190425, which likely formed as a merger between two neutron stars giving off gravitational waves in the process, could be a quark star. [17]

Other hypothesized quark formations

Apart from ordinary quark matter and strange quark matter, other types of quark-gluon plasma might hypothetically occur or be formed inside neutron stars and quark stars. This includes the following, some of which has been observed and studied in laboratories:

See also

Related Research Articles

<span class="mw-page-title-main">Hadron</span> Composite subatomic particle

In particle physics, a hadron is a composite subatomic particle made of two or more quarks held together by the strong interaction. They are analogous to molecules that are held together by the electric force. Most of the mass of ordinary matter comes from two hadrons: the proton and the neutron, while most of the mass of the protons and neutrons is in turn due to the binding energy of their constituent quarks, due to the strong force.

Neutronium is a hypothetical substance made purely of neutrons. The word was coined by scientist Andreas von Antropoff in 1926 for the hypothetical "element of atomic number zero" that he placed at the head of the periodic table. However, the meaning of the term has changed over time, and from the last half of the 20th century onward it has been also used to refer to extremely dense substances resembling the neutron-degenerate matter theorized to exist in the cores of neutron stars; hereinafter "degenerate neutronium" will refer to this.

<span class="mw-page-title-main">Neutron star</span> Collapsed core of a massive star

A neutron star is the collapsed core of a massive supergiant star, which had a total mass of between 10 and 25 solar masses (M☉), possibly more if the star was especially metal-rich. Except for black holes, neutron stars are the smallest and densest known class of stellar objects. Neutron stars have a radius on the order of 10 kilometers (6 mi) and a mass of about 1.4 M☉. They result from the supernova explosion of a massive star, combined with gravitational collapse, that compresses the core past white dwarf star density to that of atomic nuclei.

<span class="mw-page-title-main">Black dwarf</span> Theoretical stellar remnant

A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently to no longer emit significant heat or light. Because the time required for a white dwarf to reach this state is calculated to be longer than the current age of the universe, no black dwarfs are expected to exist in the universe at the present time. The temperature of the coolest white dwarfs is one observational limit on the universe's age.

Degenerate matter occurs when the Pauli exclusion principle significantly alters a state of matter at low temperature. The term is used in astrophysics to refer to dense stellar objects such as white dwarfs and neutron stars, where thermal pressure alone is not enough to avoid gravitational collapse. The term also applies to metals in the Fermi gas approximation.

In astronomy, the term compact object refers collectively to white dwarfs, neutron stars, and black holes. It could also include exotic stars if such hypothetical, dense bodies are confirmed to exist. All compact objects have a high mass relative to their radius, giving them a very high density, compared to ordinary atomic matter.

A strange star is a hypothetical compact astronomical object, a quark star made of strange quark matter.

<span class="mw-page-title-main">Gravitational collapse</span> Contraction of an astronomical object due to the influence of its gravity

Gravitational collapse is the contraction of an astronomical object due to the influence of its own gravity, which tends to draw matter inward toward the center of gravity. Gravitational collapse is a fundamental mechanism for structure formation in the universe. Over time an initial, relatively smooth distribution of matter, after sufficient accretion, may collapse to form pockets of higher density, such as stars or black holes.

Quark matter or QCD matter refers to any of a number of hypothetical phases of matter whose degrees of freedom include quarks and gluons, of which the prominent example is quark-gluon plasma. Several series of conferences in 2019, 2020, and 2021 were devoted to this topic.

An exotic star is a hypothetical compact star composed of exotic matter, and balanced against gravitational collapse by degeneracy pressure or other quantum properties.

Color superconductivity is a phenomenon where matter carries color charge without loss, on analogy to the way conventional superconductors can carry electric charge without loss. Color superconductivity is predicted to occur in quark matter if the baryon density is sufficiently high (i.e., well above the density and energies of an atomic nucleus) and the temperature is not too high (well below 1012 kelvins). Color superconducting phases are to be contrasted with the normal phase of quark matter, which is just a weakly interacting Fermi liquid of quarks.

A quark-nova is the hypothetical violent explosion resulting from the conversion of a neutron star to a quark star. Analogous to a supernova heralding the birth of a neutron star, a quark nova signals the creation of a quark star. The term quark-novae was coined in 2002 by Dr. Rachid Ouyed and Drs. J. Dey and M. Dey.

<span class="mw-page-title-main">Nuclear matter</span> System of interacting nucleons

Nuclear matter is an idealized system of interacting nucleons that exists in several phases of exotic matter that, as of yet, are not fully established. It is not matter in an atomic nucleus, but a hypothetical substance consisting of a huge number of protons and neutrons held together by only nuclear forces and no Coulomb forces. Volume and the number of particles are infinite, but the ratio is finite. Infinite volume implies no surface effects and translational invariance.

The following outline is provided as an overview of and topical guide to black holes:

In particle physics, hexaquarks, alternatively known as sexaquarks, are a large family of hypothetical particles, each particle consisting of six quarks or antiquarks of any flavours. Six constituent quarks in any of several combinations could yield a colour charge of zero; for example a hexaquark might contain either six quarks, resembling two baryons bound together, or three quarks and three antiquarks. Once formed, dibaryons are predicted to be fairly stable by the standards of particle physics.

Strange matter is quark matter containing strange quarks. In extreme environments, strange matter is hypothesized to occur in the core of neutron stars, or, more speculatively, as isolated droplets that may vary in size from femtometers (strangelets) to kilometers, as in the hypothetical strange stars. At high enough density, strange matter is expected to be color superconducting.

A strangelet is a hypothetical particle consisting of a bound state of roughly equal numbers of up, down, and strange quarks. An equivalent description is that a strangelet is a small fragment of strange matter, small enough to be considered a particle. The size of an object composed of strange matter could, theoretically, range from a few femtometers across to arbitrarily large. Once the size becomes macroscopic, such an object is usually called a strange star. The term "strangelet" originates with Edward Farhi and Robert Jaffe in 1984. Strangelets can convert matter to strange matter on contact. Strangelets have been suggested as a dark matter candidate.

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 and reaching temperatures on the order of 1015 K.

<span class="mw-page-title-main">Nuclear pasta</span> Theoretical matter within neutron stars

In astrophysics and nuclear physics, nuclear pasta is a theoretical type of degenerate matter that is postulated to exist within the crusts of neutron stars. If it exists, nuclear pasta would be the strongest material in the universe. Between the surface of a neutron star and the quark–gluon plasma at the core, at matter densities of 1014 g/cm3, nuclear attraction and Coulomb repulsion forces are of comparable magnitude. The competition between the forces leads to the formation of a variety of complex structures assembled from neutrons and protons. Astrophysicists call these types of structures nuclear pasta because the geometry of the structures resembles various types of pasta.

References

  1. ↑ Ivanenko, Dmitri D.; Kurdgelaidze, D. F. (1965). "Hypothesis concerning quark stars". Astrophysics. 1 (4): 251–252. Bibcode:1965Ap......1..251I. doi:10.1007/BF01042830. S2CID   119657479.
  2. ↑ Ivanenko, Dmitri D.; Kurdgelaidze, D. F. (1969). "Remarks on quark stars". Lettere al Nuovo Cimento. 2: 13–16. Bibcode:1969NCimL...2...13I. doi:10.1007/BF02753988. S2CID   120712416.
  3. ↑ F. Douchin, P. Haensel, A unified equation of state of dense matter and neutron star structure, "Astron. Astrophys." 380, 151 (2001).
  4. ↑ Shapiro, Stuart L.; Teukolsky, Saul A. (2008). Black Holes, White Dwarfs and Neutron Stars: The Physics of Compact Objects. Wiley. ISBN   978-0471873167.
  5. ↑ Blaschke, David; Sedrakian, Armen; Glendenning, Norman K., eds. (2001). Physics of Neutron Star Interiors. Lecture Notes in Physics. Vol. 578. Springer-Verlag. doi:10.1007/3-540-44578-1. ISBN   978-3-540-42340-9.
  6. 1 2 3 4 Witten, Edward (1984). "Cosmic separation of phases". Physical Review D. 30 (2): 272–285. Bibcode:1984PhRvD..30..272W. doi:10.1103/PhysRevD.30.272.
  7. ↑ Farhi, Edward; Jaffe, Robert L. (1984). "Strange matter". Physical Review D. 30 (11): 2379. Bibcode:1984PhRvD..30.2379F. doi:10.1103/PhysRevD.30.2379.
  8. 1 2 Weber, Fridolin; Kettner, Christiane; Weigel, Manfred K.; Glendenning, Norman K. (1995). "Strange-matter Stars". Archived from the original on 2022-03-22. Retrieved 2020-03-26. in Kumar, Shiva; Madsen, Jes; Panagiotou, Apostolos D.; Vassiliadis, G. (eds.). International Symposium on Strangeness and Quark Matter, Kolymbari, Greece, 1-5 Sep 1994. Singapore: World Scientific. pp. 308–317.
  9. ↑ Alford, Mark G.; Schmitt, Andreas; Rajagopal, Krishna; Schäfer, Thomas (2008). "Color superconductivity in dense quark matter". Reviews of Modern Physics. 80 (4): 1455–1515. arXiv: 0709.4635 . Bibcode:2008RvMP...80.1455A. doi:10.1103/RevModPhys.80.1455. S2CID   14117263.
  10. ↑ Trümper, Joachim E.; Burwitz, Vadim; Haberl, Frank W.; Zavlin, Vyatcheslav E. (June 2004). "The puzzles of RX J1856.5-3754: neutron star or quark star?". Nuclear Physics B: Proceedings Supplements. 132: 560–565. arXiv: astro-ph/0312600 . Bibcode:2004NuPhS.132..560T. CiteSeerX   10.1.1.314.7466 . doi:10.1016/j.nuclphysbps.2004.04.094. S2CID   425112.
  11. ↑ Shiga, David; "Fastest spinning star may have exotic heart" Archived 2012-08-25 at the Wayback Machine , New Scientist, 2007 February 20
  12. ↑ Yue, You-Ling; Cui, Xiao-Hong; Xu, Ren-Xin (2006). "Is PSR B0943+10 a low-mass quark star?". Astrophysical Journal. 649 (2): L95–L98. arXiv: astro-ph/0603468 . Bibcode:2006ApJ...649L..95Y. doi:10.1086/508421. S2CID   18183996.
  13. ↑ Chadha, Kulvinder Singh; "Second Supernovae Point to Quark Stars" Archived 2010-01-25 at the Wayback Machine , Astronomy Now Online, 2008 June 04
  14. ↑ Chan; Cheng; Harko; Lau; Lin; Suen; Tian (2009). "Could the compact remnant of SN 1987A be a quark star?". Astrophysical Journal. 695 (1): 732–746. arXiv: 0902.0653 . Bibcode:2009ApJ...695..732C. doi:10.1088/0004-637X/695/1/732. S2CID   14402008.
  15. ↑ Parsons, Paul; "Quark star may hold secret to early universe" Archived 2015-03-18 at the Wayback Machine , New Scientist, 2009 February 18
  16. ↑ Dai, Zi-Gao; Wang, Shan-Qin; Wang, J. S.; Wang, Ling-Jun; Yu, Yun-Wei (2015-08-31). "The Most Luminous Supernova ASASSN-15lh: Signature of a Newborn Rapidly-Rotating Strange Quark Star". The Astrophysical Journal. 817 (2): 132. arXiv: 1508.07745 . Bibcode:2016ApJ...817..132D. doi: 10.3847/0004-637X/817/2/132 . S2CID   54823427.
  17. ↑ "Strange quark star may have formed from a lucky cosmic merger". Space.com . 16 September 2022.
  18. ↑ H1 Collaboration; Aktas, A.; Andreev, V.; Anthonis, T.; Asmone, A.; Babaev, A.; et al. (2004). "Evidence for a narrow anti-charmed baryon state of mass". Physics Letters B. 588 (1–2): 17–28. arXiv: hep-ex/0403017 . Bibcode:2004PhLB..588...17A. doi:10.1016/j.physletb.2004.03.012. S2CID   119375207.{{cite journal}}: CS1 maint: numeric names: authors list (link)
  19. ↑ Koberlein, Brian (10 April 2014). "How CERN's discovery of exotic particles may affect astrophysics". Universe Today. Archived from the original on 14 April 2014. Retrieved 14 April 2014./

Sources and further reading

This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.