Source Classes

The merger of two black holes spiralling together — the most common gravitational-wave source detected, and usually a purely gravitational event with no light. The first ever direct detection, in 2015, was of such a merger over a billion light-years away.

The merger of two neutron stars — a gravitational-wave source that also lights up across the electromagnetic spectrum, producing a short gamma-ray burst and a kilonova. The 2017 event GW170817 was seen in both gravitational waves and light, founding multi-messenger astronomy with gravitational waves.

The merger of a black hole with a neutron star — the third class of compact-binary merger, first confidently detected in gravitational waves in early 2020. Whether it produces light depends on whether the black hole tears the neutron star apart before swallowing it.