A quark star or strange star is a hypothetical type of exotic star composed of quark matter, or strange 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 due to the star's gravity, the individual neutrons break down into their constituent quarks – up quarks and down quarks. Some of these quarks may then become strange quarks and form strange matter. The star then becomes known as a "quark star" or "strange star", similar to a single gigantic hadron (but bound by gravity rather than the strong force). Quark matter/strange matter is one candidate for the theoretical dark matter that is a feature of several cosmological theories.
Strange star 
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 theory large enough that the case for it is not yet solid. 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.
It remains to be seen how the question of quark star or strange star existence will play out.
See also 
- 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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