The determination of neutron star masses is reviewed in light of a new measurement of 1.97 M _ { \odot } for PSR J1614-2230 and an estimate of 2.4 M _ { \odot } for the black widow pulsar .
Using a simple analytic model related to the so-called maximally compact equation of state , model-independent upper limits to thermodynamic properties in neutron stars , such as energy density , pressure , baryon number density and chemical potential , are established which depend upon the neutron star maximum mass .
Using the largest well-measured neutron star mass , 1.97 M _ { \odot } , it is possible to show that the energy density can never exceed about 2 GeV , the pressure about 1.3 GeV , and the baryon chemical potential about 2.1 GeV .
Further , if quark matter comprises a significant component of neutron star cores , these limits are reduced to 1.3 GeV , 0.9 GeV , and 1.5 GeV , respectively .
We also find the maximum binding energy of any neutron star is about 25 % of the rest mass .
Neutron matter properties and astrophysical constraints additionally imply an upper limit to the neutron star maximum mass of about 2.4 M _ { \odot } .
A measured mass of 2.4 M _ { \odot } would be incompatible with hybrid star models containing significant proportions of exotica in the form of hyperons , Bose condensates or quark matter .