Quark clusters in quark stars and possible astrophysical implications

A quark-cluster state, rather than the color-super-conductivity state, may appear in matter with low-temperature but high density, since the phase transition of chiral symmetry broken and that of color-confinement could not occur simultaneously. Such quark clusters might be stable in strange quark matter. Quark stars would have temperatures to be low enough to freeze by residual color interaction between the clusters, and become then solid ones. The charge numbers and the sizes of quark clusters, as well as the residual interactions in-between are investigated. The solid state properties of quark stars are constrained by observations.