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Quark star is a star composed of quark matter.
Quark star or strange star is a hypothetical type of exotic star composed of quark matter, or strange matter. Quark stars 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 color force). Quark matter/strange matter is one candidate for the theoretical Dark matter that is a feature of several cosmological theories.
A quark star may be formed from a neutron star through a process called quark deconfinement. This process may produce a quark nova. The resultant star should have free quarks in its interior. The deconfinement process should release immense amounts of energy, perhaps being the most energetic explosions in existence. It may be that gamma ray bursts are indeed quark-novae. A quark star lies between neutron stars and black holes in terms of both mass and density, and if sufficient additional matter is added to a quark star, it will collapse into a black hole.
Neutron stars with masses of 1.5-1.8 solar masses with rapid spin are theoretically the best candidates for conversion. This amounts to 1% of the projected neutron star population. An extrapolation based on this indicates that up to 2 quark-novae may occur in the observable universe each day.
Theoretically quark stars may be radio quiet, so radio-quiet neutron stars may be quark stars.
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 (Jaikumar et al. 2006). However, the team 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, published in the Physical Review D 73, 114016 (2006), 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.
Observed overdense neutron stars.
Quark stars and strange stars are largely theoretical at this point, 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 neutronium. However, these observations have been under attack by researchers who say the results were not conclusive; it remains to be seen how the question of quark star or strange star existence will play out. Recently a third star, XTE J1739-285 , has been observed by a team led by Philip Kaaret of the University of Iowa, and also reported as a possible candidate.
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