Xing-Jiang Zhu

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.

In the dense family of old stars that astronomers call the cluster NGC 6791, a lonely radio beacon has been spotted — the pulsar PSR J1922+3745. Such objects are born when a massive star explodes and its core collapses into a tiny sphere the size of a city, which spins and emits radio waves like a rotating lighthouse beam.

Globular clusters, dense swarms of hundreds of thousands of stars, hide millisecond pulsars. These are dead stars spinning at breakneck speed: one rotation every few milliseconds, like a kitchen blender on max.