When two black holes find each other in the vastness of space, they can form a binary odd couple—and it’s more common than researchers expected. Scientists think most black holes are born when a star dies and its core collapses. This is also the case for binary black holes, pairs of black holes that orbit around each other and slowly move closer together. These pairs are often the remains of stars that lived together, died together and continued to coexist following their collapse. Eventually these two draw close enough together to merge into one faster-spinning black hole that sometimes shoots off into space. Now research is suggesting that these whirling loners can find love again. New work suggests many black hole pairs are so-called hierarchical mergers: one large, fast-spinning black hole born from a previous merger and the partner it somehow found in the cosmos. This picture of black holes’ origin stories could help clarify how supernova physics work and how galaxies evolve, scientists say. Knowing how black holes form and change is also crucial for understanding general relativity and the history of the universe. On supporting science journalism If you're enjoying this article, consider supporting our award-winning journalism by subscribing. By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today. In two studies released recently in Physical Review Letters, scientists describe how they used published data from the Laser Interferometer Gravitational-Wave Observatory, which tracks merging black holes through the spacetime ripples they release, and other observatories to identify this subpopulation. Each team used a different technique to track the black holes’ mass, spin and tilt, and both captured a distinct group of binary black holes that have “strong signatures” of hierarchical merging—one of the pair is much larger, is spinning faster and orbits out of alignment with...