A quark star is a hypothetical type of compact star, exotic star, where extremely high core temperature and pressure have forced Nucleon to form quark matter, a continuous state of matter consisting of free .
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
black hole.
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 the unsolved problems in physics.
Numerical simulations of the physics inside suggests that the core of these stars could contain deconfined quark-gluon plasma.
History
The analysis about quark stars was first proposed in 1965 by Soviet physicists
Dmitri Ivanenko and D. F. Kurdgelaidze.
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.
Formation
It is hypothesized that when the neutron-degenerate matter, which makes up
, is put under sufficient pressure from the star's own
gravity or the initial
supernova creating it, the individual
break down into their constituent
(
and
), 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.
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.
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 , as strange quarks are, relatively speaking, a very heavy type of quark particle.[ in ]
Strange stars
Stars made of strange quark matter are known as strange stars. These form a distinct subtype of quark stars.
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.
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 10
12 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.
Observed overdense neutron stars
At least under the assumptions mentioned above, the probability of a given neutron star being a quark star is low, 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.
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 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;
Another star, XTE J1739-285,[Shiga, David; "Fastest spinning star may have exotic heart" , New Scientist, 2007 February 20] 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.
It was reported in 2008 that observations of supernovae SN 2006gy, SN 2005gj and SN 2005ap also suggest the existence of quark stars.[Chadha, Kulvinder Singh; "Second Supernovae Point to Quark Stars" , Astronomy Now Online, 2008 June 04] It has been suggested that the collapsed core of supernova SN 1987A may be a quark star.[Parsons, Paul; "Quark star may hold secret to early universe" , New Scientist, 2009 February 18]
In 2015, Zi-Gao Dai et al. from Nanjing University suggested that Supernova ASASSN-15lh is a newborn strange quark star.
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.
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:
-
Robert L. Jaffe 1977, suggested a tetraquark state with strangeness (qs).
-
Robert L. Jaffe 1977 suggested the H dibaryon, 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 (QQ).
-
In 1987, a pentaquark state was first proposed with a charm anti-quark (qqqs).
-
Pentaquark state with an antistrange quark and four light quarks consisting of up- and down-quarks only (qqqq).
-
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).
-
pentaquark++ and antiparticle −−.
-
Doubly strange pentaquark (ssdd), member of the light pentaquark antidecuplet.
-
Charmed pentaquark Θc(3100) (uudd) state was detected by the H1 collaboration.
-
Tetraquark particles might form inside neutron stars and under other extreme conditions. In 2008, 2013 and 2014 the tetraquark particle of Z(4430), was discovered and investigated in laboratories on Earth.
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See also
Sources and further reading
External links
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Neutron Star/Quark Star Interior (image to print)
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Whitfield, John; "Quark star glimmers", Nature, 2002 April 11
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"Debate sparked on quark stars", CERN Courier 42, #5, June 2002, page 13
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Beck, Paul; "Wish Upon a Quark Star", Popular Science, June 2002
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Krivoruchenko, M. I.; "Strange, quark, and metastable neutron stars" , JETP Letters, vol. 46, no. 1, 10 July 1987, pages 3–6 (page 6: Perhaps a 1,700-year-old quark star in SNR MSH 15–52)
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Rothstein, Dave; "Curious About Astronomy: What process would bring about a quark star?", question #445, January 2003
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Nemiroff, Robert; Bonnell, Jerry; "RX J185635-375: Candidate Quark Star", Astronomy Picture of the Day, NASA Goddard Space Flight Center, 2002 April 14
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Anderson, Mark K.: Quarks or Quirky Neutron Stars?, Wired News, 2002 April 19
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Boyce, Kevin; Still, Martin; "What is the news about a possible Strange Quark Star?", Ask an Astrophysicist, NASA Goddard Space Flight Center, 2002 April 12
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Marquit, Miranda; "Seeing 'Strange' Stars", physorg.com, 2006 February 8
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"Quark Stars Could Produce Biggest Bang", spacedaily.com, 2006 June 7
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Niebergal, Brian: "Meissner Effect in Strange Quark Stars", Computational Astro-Physics Calgary Alberta, University of Calgary
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Bryner, Jeanna; "Quark Stars Involved in New Theory of Brightest Supernovae", Space.com, 2008 June 3 (The first-ever evidence of a neutron star collapsing into a quark star is announced)
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Cramer, John G.: "Quark Stars, Alternate View Column AV-114", Analog Science Fiction & Fact Magazine, November 2002
