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, 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).
Another star, XTE J1739-285, has been observed by a team led by Philip Kaaret of the University of Iowa and reported as a possible candidate.
It remains to be seen how the question of quark star or strange star existence will play out.
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.
Read more about this topic: Quark Star
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