Observed black holes. Illustration of the masses of black holes detected by X-ray studies (purple) and by the LIGO detector (blue). The LIGO (Laser Interferometer Gravitational-wave Observatory) detectors in the USA record the gravity waves produced by the merger of large black holes. The observatory has definitively detected four sets of black hole mergers, with a third one (less certain) shown with a dotted border. The X-ray study detections are of black holes of less than 20 solar masses. The GW150914 merger of black holes of around 36 and 29 solar masses, produced a black hole of over 60 solar masses. The GW151226 merger of black holes of some 14 and 8 solar masses, formed a black hole of over 20 solar masses. At right are the mergers GW170104 and GW170814 that formed black holes of around 50 solar masses.