This list of black holes (and stars considered probable candidates) is organized by mass (including black holes of undetermined mass); some items in this list are galaxies or star clusters that are believed to be organized around a black hole. Messier and New General Catalogue designations are given where possible.
As of February 2019[update], 10 mergers of binary black holes have been observed. In each case two black holes merged to a larger black hole. In addition, one neutron star merger has been observed (GW170817), forming a black hole. In addition, over 30 alerts have been issued since April 2019, of black hole merger candidates.
In addition, the signal of several binary black holes merging into a single black hole and in so doing producing gravitational waves have been observed by the LIGO instrument. These are listed above in the section Black holes detected by gravitational wave signals.
As of 2014, there are 5 triple black hole systems known.