We discuss cold dense QCD by examining constraints from neutron stars , nuclear experiments , and QCD calculations at low and high baryon density .
The two solar mass constraint and suggestive small radii ( \sim 10 - 13 km ) of neutron stars constrain the strength of hadron-quark matter phase transitions .
Assuming the adiabatic continuity from hadronic to quark matter , we use a schematic quark model for hadron physics and examine the size of medium coupling constants .
We find that to baryon density n _ { B } \sim 10 n _ { 0 } ( n _ { 0 } : nuclear saturation density ) , the model coupling constants should be as large as in the vacuum , indicating that gluons remain non-perturbative even after the quark matter formation .