We perform an { \mathcal { O } } ( \alpha _ { s } ^ { 2 } ) perturbative calculation of the equation of state of cold but dense QCD matter with two massless and one massive quark flavor , finding that perturbation theory converges reasonably well for quark chemical potentials above 1 GeV .
Using a running coupling constant and strange quark mass , and allowing for further non-perturbative effects , our results point to a narrow range where absolutely stable strange quark matter may exist .
Absent stable strange quark matter , our findings suggest that quark matter in compact star cores becomes confined to hadrons only slightly above the density of atomic nuclei .
Finally , we show that equations of state including quark matter lead to hybrid star masses up to M \sim 2 M _ { \odot } , in agreement with current observations .
For strange stars , we find maximal masses of M \sim 2.75 M _ { \odot } and conclude that confirmed observations of compact stars with M > 2 M _ { \odot } would strongly favor the existence of stable strange quark matter .