Massive Compact Stars as Quark Stars
  High massive compact stars have been reported recently in the literature, providing strong constraints on the properties of the ultradense matter beyond the saturation nuclear density. In view of these results, the calculations of quark star or hybrid star equilibrium structure must be compatible with the provided observational data. But, since the used equations of state describing quark matter are in general too soft, in comparison with the equation of states used to describe the hadronic or nuclear matter, the calculated quark star models presented in the literature are in general not suitable to explain the stability of high compact massive objects. 
Hilario Rodrigues, Sergio B. Duarte and Jos Carlos T. de Oliveira
astro-ph.HE nucl-th