Although neutron stars are extremely dense, they are still gaseous and, consequently, behave like fluids.
Gravity pulls on their fluid matter, elongating it.
This tidal distortion, though slight, can have a momentous effect on the stars' orbits. In 1994, Stuart Shapiro and Fred Rasio, then physicists at Cornell University, predicted that when the separation distance between neutron stars is approximately three times their radii, the stars are unable to maintain separate orbits. They are ripped apart by the tides, forcing them to merge. Astronomers knew that tidal instabilities alone were enough to cause a merger. But Swesty's group suspected that gravitational radiation was underrated as a factor during the stars' final orbits.