of the quark matter is true, then some neutron stars could actually be strange stars, built entirely of strange matter [@Al86; @Ha86]. For a review of strange star properties, see [@Ch98].

Several mechanisms have been proposed for the formation of quark stars. Quark stars are expected to form during the collapse of the core of a massive star after the supernova explosion, as a result of a first or second order phase transition, resulting in deconfined quark matter [@Da]. The proto-neutron star core, or the neutron star core, is a favorable environment for the conversion of ordinary matter into strange quark matter [@ChDa]. Another possibility is that some neutron stars in low-mass X-ray binaries can accrete sufficient mass to undergo a phase transition to become strange stars [@Ch96]. This mechanism has also been proposed as a source of radiation emission for cosmological $\gamma $-ray bursts [@Ch96]. Some basic properties of strange stars like mass, radius, collapse and nucleation of quark matter in neutron stars cores have been also studied [@Gl95; @Gl97; @ChHa; @HaCh00; @HaCh02; @Ha04].

The structure of a realistic strange star is very complicated, but its basic properties can be described as follows [@Al86]. Beta-equilibrated strange quark - star 