Credit: Image courtesy of LIGO, NSF, Aurore Simonnet (Sonoma State U.)
During the initial stage of the merger known as the inspiral, the black holes orbit a common center of mass gradually coming closer to one another. Then the merger proper occurs and most of the gravitational waves are emitted. The red and blue lines at the bottom of the image represent the gravitational signal associated with a black hole merger. Following the merger proper, the now single black hole undergoes oscillations referred to as the ringdown.