  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 (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 nB ~ 10n0 (n0: 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.
