QCD - Motivated quark stars in the light of recent astrophysical observations

Are there quark stars in nature? We review the question in the light of mod ern astrophysical observations. Quark stars have properties which are very similar to those of neutron stars. For example they are now known to have t he same kind of cooling properties. Their masses may also be similar. On th e other hand, gamma ray bursts (GRB), the brightest phenomenon observable a t present in the sky, could possibly be conversion of normal or neutron mat ter on the surface of a quark star. The best observational evidence for the existence of quark stars seems to b e some compact objects, the X-Ray burst source 4U 1820-30, the X-ray pulsar Her X-l, the star 4U 1728-34 and SAX J-1808.4-3658, this last one being th e stablest and fastest rotating hard X-ray pulsar known to date. The mass o f at least these four objects is high and their radius is low, placing them close to the black hole line, in the mass-radius (M-R) plot.