Recently, detectors of gravitational waves recorded two unusual mergers of black holes. Analysis showed that in each pair, one of the black holes wasn't a simple one, but a 'prefabricated' one — it itself formed from the collision of two other holes in the past. It's like playing with a construction set: instead of only picking individual bricks, you sometimes use already assembled blocks.

The idea of black holes goes back to the calculations of Karl Schwarzschild, and the first attempt to catch gravitational waves was made by Joseph Weber half a century ago. Today we know that galaxies, whose classification was pioneered by Edwin Hubble, often harbor supermassive black holes and gas disks at their centers — ideal places for the birth of 'prefabricated' black holes.

Gravitational waves carry information through space at the speed of light, and we've learned to read it. To see such mergers also in ordinary light, we'll need spectroscopy — a method that allows us to split light into colors and find lines of hydrogen from heated gas. This will be helped by the Hubble telescope and future observatories. Thus, a new chapter in astronomy will open: unified vision in gravity and light.