If a dying star is spinning very fast, its core can split during collapse to form two black holes that quickly merge into one.

When a giant, rapidly spinning star collapses, it does not always stay perfectly round. Supercomputer simulations show that the spinning motion can warp the star's core into a ring shape. This ring can break apart into two dense clumps. Each clump collapses to form its own black hole. These two baby black holes then spiral around each other and quickly merge into a single, larger black hole, sending out massive gravitational waves.