Third-Generation

In the past decade, astronomers have “heard” almost 400 mergers between black holes. The signals were carried by gravitational waves – tiny ripples in spacetime. They’ve revealed that some of the black holes probably had undergone earlier mergers – making them third-generation black holes. Gravitational waves are produced by the motions of any object. But the waves are extremely weak. So far, the only ones that have been detected were produced by mergers involving black holes or neutron stars – dense, heavy objects that come together in a fraction of a second. The characteristics of the waves reveal the masses of the merging objects. They also reveal how the objects were spinning, and how they were orbiting before the merger. And those details provide hints to the existence of third-generation black holes. One example was discovered in late 2024. A black hole about 20 times the mass of the Sun merged with one about six times the Sun’s mass. Scientists determined that the heavier black hole probably formed from an earlier merger. They even calculated the details of those black holes: about 7 and 13 times the mass of the Sun. Third-generation black holes probably form in places where lots of black holes are jammed close together, such as the hearts of star clusters. That keeps a merged black hole from escaping – setting up the possibility of more mergers ahead. Script by Damond Benningfield