A 2.1 Solar mass neutron star and dense matter.

Most neutron stars with well measured masses are near 1.4 Solar Masses.  However recently a much more massive neutron star has been found.  This has lots of implications for the equation of state of dense matter (pressure versus density) and for dense QCD.  The obervation is by Nice et al, and is on the preprint server as astro-ph/0508050, or click below
A 2.1 Solar Mass Pulsar from Relativistic Orbital Decay

A nice introduction to neutron star physics is the following article by J. Lattimer and M. Prakash,
The Physics of Neutron Stars

One implication of the neutron star maximum mass is an upper limit for the density of cold matter
The Ultimate Energy Density of Observable Cold Matter