Credit: NASA, ESA/Hubble, and A. Feild/STScI. Release date: 23 March 2017.

This illustration shows how two supermassive black holes merged to form a single black hole which was then ejected from its parent galaxy.

Panel 1: Two galaxies are interacting and finally merging with each other. The supermassive black holes in their centres are attracted to each other.

Panel 2: As soon as the supermassive black holes get close they start orbiting each other, in the process creating strong gravitational waves.

Panel 3: As they radiate away gravitational energy the black holes move closer to each other over time and finally merge.

Panel 4: If the two black holes do not have the same mass and rotation rate, they emit gravitational waves more strongly along one direction. When the two black holes finally collide, they stop producing gravitational waves and the newly merged black hole then recoils in the opposite direction to the strongest gravitational waves and is shot out of its parent galaxy.

https://www.spacetelescope.org/images/heic1706b/

From the Hubble Space Telescope website. https://www.spacetelescope.org/