# How do neutron star binaries form?

Do neutron star binary systems come from previously active-star binaries, where where both stars have gone supernova and left behind neutron stars that are still in orbit? Or do they form when two previously unbound neutron stars approach each other and fall into orbit?

• Neutron stars are very rare. They need supernovas. The closest known is 400ly away. The chance for 2 NS to meet and build a binary system is very small. Note, the pairing requires also a third star, to get away kinetical energy from the system (or the NSs would avoid each other on hyperbolic orbit). – peterh Aug 19 at 14:01