Black holes can merge inside stars without destroying them.

Giant stars that live in the swirling gas disks around huge galaxy cores can weigh hundreds of times as much as our Sun. These same disks also contain many smaller black holes. Scientists wanted to see if these giant stars can trap passing black holes to build black hole pairs that crash together and emit gravitational waves, which are ripples in space. To test this idea, researchers ran three-dimensional computer models of a black hole hitting a star. The star survives the collision with very little damage. It warms up slightly and loses less than one percent of its mass. Meanwhile, drag from the stellar gas quickly slows down the captured black hole. It sinks straight to the center of the star in less than ten thousand seconds. If another black hole is already waiting there, they pair up and can merge rapidly.

Why it matters

Gravitational-wave detectors like LIGO have seen merging black holes, but their origins are unclear. This work reveals a new way: black holes can pair up inside a giant star in an active galaxy's disk. The star survives the process, and the binary merges in less than ten thousand years, fast enough to produce detectable waves.

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