The existence of a two-solar mass neutron star constrains the   gravitational constant G_N at strong field
  In General Relativity there is a maximum mass allowed for neutron stars that, if exceeded, entails their collapse into a black hole. Its precise value depends on details of the nuclear matter equation of state about which we are much more certain thanks to recent progress in low-energy effective theories.   The discovery of a two-solar mass neutron star, near that maximum mass, when analyzed with modern equations of state, implies that Newton's gravitational constant in the star cannot exceed its value on Earth by more than 8% at 95% confidence level. This is a remarkable leap of ten orders of magnitude in the gravitational field intensity at which the constant has been constrained. 
Antonio Dobado, Felipe J. Llanes-Estrada (U. Complutense Madrid) and   Jose Antonio Oller (U. Murcia)
gr-qc hep-th nucl-th