Somemassive stars collapse to formneutron stars at the end of theirlife 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 adegeneracy pressure, stabilizing the star and hindering further gravitational collapse.[2] However, it is hypothesized that under even more extreme temperature and pressure, the degeneracy pressure of the neutrons is overcome, and theneutrons are forced to merge and dissolve into their constituent quarks, creating an ultra-densephase ofquark 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 repulsiveelectromagnetic forces, will occur and hindertotal 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 has not been proven observationally or experimentally; the very extreme conditions needed for stabilizing quark matter cannot be created in any laboratory and has not been observed directly in nature. The stability of quark matter, and hence the existence of quark stars, is for these reasons among theunsolved problems in physics.
Numerical simulations of the physics insideneutron stars suggests that the core of these stars could contain deconfined quark-gluon plasma.[3][4][5] The idea is that in the core the internal pressure needed forquark degeneracy – the point at whichneutrons break down into a form of dense quark matter.
The analysis about quark stars was first proposed in 1965 by Soviet physicistsD. D. Ivanenko andD. F. Kurdgelaidze.[6][7] Their existence has not been confirmed.
Theequation of state ofquark matter is uncertain, as is the transition point between neutron-degenerate matter and quark matter.[8] Theoretical uncertainties have precluded making predictions fromfirst principles. Experimentally, the behaviour of quark matter is being actively studied with particle colliders, but this can only produce very hot (above 1012K)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 1012K 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]
Mass–radius relations for models of a neutron star with no exotic states (red) and a quark star (blue)[9]
It is hypothesized that when theneutron-degenerate matter, which makes upneutron stars, is put under sufficient pressure from the star's owngravity or the initialsupernova creating it, the individualneutrons break down into their constituentquarks (up quarks anddown 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.[10][11]
Ordinary quark matter consisting of up and down quarks has a very highFermi 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.[12][13]
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 intostrange quarks, as strange quarks are, relatively speaking, a very heavy type of quark particle.[12] This kind of quark matter is known specifically asstrange 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.[14]
Stars made ofstrange quark matter are known as strange stars. These form a distinct subtype of quark stars.[14]
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 earlycosmic phase separations following theBig Bang.[12] 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.[12]
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 aFermi liquid and enter a so-called color-flavor-locked (CFL) phase ofcolor superconductivity, where "color" refers to the six "charges" exhibited in thestrong interaction, instead of the two charges (positive and negative) inelectromagnetism. 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.[15]
A comparison of the size of quark star candidateRX J1856, based on data from theChandra X-Ray Observatory, to existing models of the theoretical minimum and maximum diameters of neutron and quark 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 centrifugal 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 theChandra X-ray Observatory on April 10, 2002, detected two possible quark stars, designatedRX J1856.5−3754 and3C 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 thanneutron-degenerate matter. However, these observations are met with skepticism by researchers who say the results were not conclusive;[16] and since the late 2000s, the possibility thatRX J1856 is a quark star has been excluded.
Another star,XTE J1739-285,[17] has been observed by a team led by Philip Kaaret of theUniversity of Iowa and reported as a possible quark star candidate.
It was reported in 2008 that observations of supernovaeSN 2006gy,SN 2005gj andSN 2005ap also suggest the existence of quark stars.[19] It has been suggested that the collapsed core of supernovaSN 1987A may be a quark star.[20][21]
In 2015, Zi-Gao Dai et al. from Nanjing University suggested that SupernovaASASSN-15lh is a newborn strange quark star.[22]
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.[23]
Apart from ordinary quark matter and strange quark matter, other types ofquark-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:
Robert L. Jaffe 1977, suggested afour-quark state with strangeness (qsqs).
Robert L. Jaffe 1977 suggested the Hdibaryon, a six-quark state with equal numbers of up-, down-, and strange quarks (represented as uuddss or udsuds).
Bound multi-quark systems with heavy quarks (QQqq).
In 1987, apentaquark state was first proposed with a charm anti-quark (qqqsc).
Pentaquark state with an antistrange quark and four light quarks consisting of up- and down-quarks only (qqqqs).
Light pentaquarks are grouped within an antidecuplet, the lightest candidate, Θ+, which can also be described by the diquark model of Robert L. Jaffe and Wilczek (QCD).
Doubly strange pentaquark (ssddu), member of the light pentaquark antidecuplet.
Charmed pentaquark Θc(3100) (uuddc) state was detected by the H1 collaboration.[24]
Tetraquark particles might form inside neutron stars and under other extreme conditions. In 2008, 2013 and 2014 the tetraquark particle ofZ(4430), was discovered and investigated in laboratories onEarth.[25]
^F. Douchin, P. Haensel,A unified equation of state of dense matter and neutron star structure, "Astron. Astrophys." 380, 151 (2001).
^Shapiro, Stuart L.; Teukolsky, Saul A. (2008).Black Holes, White Dwarfs and Neutron Stars: The Physics of Compact Objects. Wiley.ISBN978-0471873167.
^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.ISBN978-3-540-42340-9.
^abWeber, Fridolin; Kettner, Christiane; Weigel, Manfred K.; Glendenning, Norman K. (1995)."Strange-matter Stars".Archived from the original on 2022-03-22. Retrieved2020-03-26. inKumar, 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.
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.ISBN978-3-540-42340-9.
