Schematic phase diagram of matter at extreme density and temperature. The vertical axis represents temperature. The horizontal axis is the quark chemical potential . As increases at low temperature, the system passes from vacuum to nuclear matter at . At an unknown higher chemical potential there is a transition to quark matter, which, at sufficiently high density, is expected to be in the color-flavor-locked (CFL) phase. Neutron star matter is in the lower part of the phase diagram, ranging up to an unknown maximum density at the center of the star.