Kyle Kremer

At the centers of many galaxies lurk supermassive black holes, surrounded by disks of gas, cosmic dust, and stars. Their collisions can spawn gravitational waves — ripples in spacetime.

In the dense core of the globular cluster NGC 6397, where stars jostle like commuters at rush hour, astronomers have spotted a peculiar pair. One star is a white dwarf with a surface temperature of 16,000 K, the scorched remnant of a star that has been cooling for billions of years, and circling it is an unseen companion.

Two heavy spheres circle not in a neat circle but in a highly flattened oval — now approaching, now flying apart. That's roughly how pairs of black holes on elongated orbits behave. Unlike calm pairs that gradually spiral together and merge, these dancers retain a memory of their turbulent youth: of being kicked out of star clusters or made to whirl by close neighbors.

In the heart of a dense star cluster, a true forge reigns: here stars, like glowing blanks, collide and merge under gravity, and these mergers are not a catastrophe but an opportunity. Each strike of the stellar hammer transfers spin to the future black hole.