Double neutron stars are pairs of ultra-dense remnants of exploded massive stars. They orbit each other and gradually spiral closer until they merge in a powerful flash — a source of gravitational waves. But where do such pairs actually form? One possibility is inside star clusters, where stars are plentiful and interact closely.
Astronomers took motion data for 11 such pairs and 167 globular clusters (dense, ancient 'balls' of stars) and 2967 open clusters (younger, looser groups). Then they 'ran the tape backward' in time, like rewinding a film.
But open clusters are far more interesting.
Especially lucky is the double pulsar J0737-3039—a pair of neutron stars that emit radio pulses like two lighthouses. Its past trajectory intersects the young open cluster Theia 58: the probability of being born there is about 14%. It's like finding an old photograph that shows where you come from.
So some of these pairs may start their lives in relatively small, young stellar families.
The double pulsar J0737-3039 is the only known pair where both components are visible as pulsars; they orbit each other in just 2.4 hours.
In Carl Sagan's novel 'Contact', the heroine travels through a system of black holes, but neutron star mergers in reality are also 'wormholes' for gravitational waves.