Quark star
A quark star ( engl. also strange star to German " strange star") is a hypothetical compact object that could arise as a product of a supernova. In the quark star matter would be so densely packed together that neutrons lose their identity and quasi- free quarks would be present. The term (english strange star) is because the subject matter would also contain strange quarks and would thus be assigned to the so-called strange matter.
Formation
With the consumption of its nuclear fuel material by nuclear fusion, the matter of a star is very strongly compressed by gravity. Depending on the mass of the star is formed while a white dwarf, a neutron star, a ( hypothetical ) quark star or a black hole, sometimes accompanied by a supernova or hypernova.
For neutron stars, there is a mass limit, the Tolman -Oppenheimer - Volkoff limit, the value of which is according to current estimates between 1.5 and 3 solar masses. The excess of a neutron star that limit, it collapses and a black hole. Would be the further a neutron star mass limit approaches, the greater the assumed quark-gluon plasma ball inside it.
So far, there are no observations which show that the theoretically possible compression of neutron matter of an existing neutron star takes place at a quark star in the universe. But may have many neutron stars, at least in its interior, such a quark -gluon plasma.
Observation
The evidence of a quark star is considered difficult because its observable properties similar to those of distance of a neutron star. There were up to now two candidates for possible quark star discovered, one of which is already out:
RX J1856 - 3754 was discovered in 1992 by the X-ray satellite ROSAT, but only in 1996 it was because of its distance ( 180-420 light-years ) with the Hubble Space Telescope possible, the Pulsar also visually scan. Due to the total radiation a diameter of 11 km was calculated, which is too low a value, even for a neutron star. Later measurements have shown, however, that in this star shine only the polar caps. RX J1856 - 3754 also has a much larger diameter than 11 km and is thus no longer a candidate for a quark star.
The pulsar J0205 6449 in supernova remnant 3C58 is associated with a supernova that was observed in 1181 by Japanese and Chinese astronomers. Because of its great distance of about 10,000 light- years to Earth could not calculate its diameter, its luminosity, however, is 16- fold lower than the comparable young pulsars. This could be an indication that it is a quark star.
According to theoretical models, quark stars could form in low-mass X-ray binaries. These will be transferred from a companion matter to a neutron star. Thus, should a neutron star with a mass of 1.4 solar masses still accrete 0.5 solar masses in order to transform into a quark star. As a candidate for true 2S 0921-63. However, the mass measurements in X-ray binaries are always fraught with uncertainty, and a mass of 1.44 solar masses, which is typical of a neutron star, can not be excluded.