The LIGO event GW170817 demonstrated that double neutron star mergers take place in galaxies.
All our knowledge about double neutron stars comes for the twenty double neutron stars that have so far been discovered in our galaxy as binary radio pulsars. The first such system was discovered in 1974 by Joseph Taylor and Russel Hulse, who were awarded the 1993 Physics Nobel Prize for the discovery that the orbital period of this system decays at a rate exactly as predicted by Einstein’s General Relativity Theory for orbital decay due to the emission of gravitational waves.
Our ideas about how close double neutron stars form, date from the 1970s and will be reviewed here. Based on what we know about the double neutron star population in our Galaxy and on models of their formation, predictions can be made about the merger event rate expected for LIGO. The fact that LIGO already observed one such event means that either we have been very lucky or that for some unknown reason the actual merger rate is higher than predicted so far.