The first gravitational-wave event detected from a black-hole merger was GW150914.
LIGO detected GW150914 on September 14, 2015, at its observatories in Hanford, Washington, and Livingston, Louisiana. The signal lasted about two-tenths of a second and had the characteristic rising “chirp” expected as two black holes spiraled together. The event was publicly announced on February 11, 2016, after extensive checks confirmed that the signal was not instrumental noise or a test injection.
The two original black holes had estimated masses of about 36 and 29 Suns. Their merger created a single black hole of roughly 62 solar masses; the missing mass, about three Suns, was converted into energy carried away by gravitational waves. The source was approximately 1.4 billion light-years from Earth.
GW150914 was both the first direct detection of gravitational waves and the first observed binary black-hole merger. It was named from “GW,” for gravitational wave, followed by the date of detection in the format 150914. Later events, including GW170817 and GW190521, expanded the catalog but did not replace its historical first.