A quark star is a hypothetical type of compact exotic star composed of quark matter. These are ultra-dense phases of degenerate matter theorized to form inside particularly massive neutron stars.
It is theorized that when the neutron-degenerate matter which makes up a neutron star 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 quark (up quarks and down quarks), forming what is know as quark matter. This conversion might be confined to the neutron stars center or it might transform the entire star, depending on the physical circumstances. Such a star is known as a quark star.
It is speculated and subject to current scientific investigation if quark matter once formed, might in fact be stable under zero external pressure (ie. in interstellar space). If this is the case, quark stars made entirely of quark matter will be stable.
Quark stars might also be created in the early cosmic phase separations following the Big Bang. Under the assumption of stability under zero external pressure (known as the Bodmer-Witten assumption), such primordial stars might also be stable and could survive to this day. These stars are referred to as primordial quark stars.
In some circumstances it is theoretically possible, that some of the up and down quarks are transformed into strange quarks, when the neutron-degenerate matter is condensed. If this happens, the quark star will consist of strange quark matter - a subgroup of quark matter. Such a star is known as a strange star. As quark stars in general, strange stars might in theory also be created primordial.
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 the strong force that bounds ordinary hadrons.
Recent theoretical research has found mechanisms by which quark stars with "strange quark nuggets" may decrease the objects' electric fields and densities from previous theoretical expectations, causing such stars to appear very much like—nearly indistinguishable from—neutron stars. However, the team of Prashanth Jaikumar, Sanjay Reddy and Andrew W. Steiner made some fundamental assumptions, that led to uncertainties in their results large enough, that the case is not finally settled. More research, both observational and theoretical, remains to be done on strange stars in the future.
Other theoretical work contends that, "A sharp interface between quark matter and the vacuum would have very different properties from the surface of a neutron star"; and, addressing key parameters like surface tension and electrical forces that were neglected in the original study, the results show that as long as the surface tension is below a low critical value, the large strangelets are indeed unstable to fragmentation and strange stars naturally come with complex strangelet crusts, analogous to those of neutron stars.
Other theorized quark formations
- Jaffe 1977, suggested a four-quark state with strangeness (qsqs).
- 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 (QQqq).
- In 1987, a pentaquark 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, Ө+.
- This can also be described by the diquark model of Jaffe and Wilczek (QCD).
- Ө++ and antiparticle Ө−−.
- Doubly strange pentaquark (ssddu), member of the light pentaquark antidecuplet.
- Charmed pentaquark Өc(3100) (uuddc) state was detected by the H1 collaboration.
Observed overdense neutron stars
Statistically, the probability of a neutron star being a quark star is low, so in the Milky Way Galaxy there will only be a small population of quark stars (but, if it is correct that overdense neutron stars turn into quark stars, that makes the possible number of quark stars higher than was originally thought, as we would be looking for the wrong type of star). Quark stars and strange stars are entirely hypothetical as of 2011[update], but observations released by the Chandra X-Ray Observatory on April 10, 2002 detected two candidates, designated RX J1856.5-3754 and 3C58, 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; and since the late 2000s, the possibility that RX J1856 is a quark star has been excluded (see RX J1856.5-3754).
It was reported in 2008 that observations of supernovae SN2006gy, SN2005gj and SN2005ap also suggest the existence of quark stars. It has been suggested that the collapsed core of supernova SN1987A may be a quark star.
- Quantum chromodynamics
- Neutron stars – neutron matter – neutron-degenerate matter – neutron
- Tolman-Oppenheimer-Volkoff limit on the mass of a neutron star.
- Compact star
- Degenerate matter
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