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Crowded Star Cluster Explains The Strangest Gravitational Wave Yet Found

A gravitational wave detected in 2019 looks different from all the others detected, and the environment around the merging black holes may explain why.


Stephen Luntz

Stephen has a science degree with a major in physics, an arts degree with majors in English Literature and History and Philosophy of Science and a Graduate Diploma in Science Communication.

Freelance Writer

NGC 346 in the Small Magellanic Cloud. A cluster like this full of massive hot stars could be rich in black holes, which might explain the strangest gravitational wave we have found. Image Credit: NASA, ESA and A. Nota (STScI/ESA)
NGC 346 in the Small Magellanic Cloud. A cluster like this full of massive hot stars could be rich in black holes, which might explain the strangest gravitational wave we have found. Image Credit: NASA, ESA and A. Nota (STScI/ESA)

When the gravitational wave GW190521 was detected it stood out from the 90 or so other ripples in space-time we have seen, both for its immense distance and the enormous masses involved. There were also some other distinctive features that were harder to explain. Astronomers are now suggesting it is because the black holes involved were part of a tightly packed cluster. If so, this could have very interesting implications for how we see the early universe.

The black holes involved in GW190521 are the largest we have detected merging; estimated by various studies at 80-102 and 52-100 solar masses respectively. Even though several solar masses were converted to energy and dispersed in the waves we picked up 17 billion light-years away, this still left around 150 solar masses to make up the merged black hole. This marks the first confirmed example of a long-sought intermediate black hole, one with a mass between 100 and 100,000 solar masses


However, as a new paper points out, that is far from the end of what makes GW190521 stand out. Besides its intensity, the wave was distinctively brief. 

Most gravitational waves occur when two large masses, be they black holes or neutron stars, spiral into each other, getting closer and closer as they shed energy into the universe. It is thought these start out as ordinary binary stars, with first one and then the other undergoing supernova explosions to leave extraordinarily dense remnants behind.

“The shape and brevity – less than a tenth of a second – of the signal associated with [GW190521] lead us to hypothesize an instantaneous merger between two black holes, which occurred in the absence of a spiraling phase,” Professor Alessandro Nagar of the Instituo Nazionale di Disica Nucleare said in a statement.

In theory, two black holes could run into each other without spiraling, like two people not looking where they are going. However, black holes are tiny compared to the distances between star systems. On a random walk in a remote desert, it usually takes more than obliviousness to trip over someone.


Nagar and co-authors think the explanation could be that both the black holes were part of a densely packed star cluster. After all, collisions are far more likely for the heedless in a crowded room than wide open spaces. Instead of the two black holes being the remnants of stars born together, they may have originated in different parts of the cluster and eventually been thrown together. The collision was probably a two-stage process, with a previous close encounter placing the pair in a highly eccentric (elongated orbit) around each other. As with any dance between two such massive objects, the orbit would have decayed bringing them closer together, turning a flyby into a collision.

If the hypothesis is correct, it would make GW190521 the first example we have seen of a dynamical encounter between black holes, something theory predicts should occur, but only very rarely

A dense cluster could also have been the site for multiple previous mergers, which would explain how both black holes were so much larger than usual products of core-collapse supernovas. The larger hole at least is more massive than is thought to be possible from a stellar collapse.

Most stars are born in dense clusters that gradually disperse. More massive stars have shorter lives, making it plausible they could use up all their hydrogen and collapse into black holes before spreading occurs. Given how far back in time this event was, confirmation of the idea would indicate dense star clusters have been a feature of the universe almost from the beginning.


A flash of light detected at Palomar Observatory has tentatively been associated with GW190521, leading to the proposal both black holes were orbiting an even larger one, whose disk may have been disturbed by the product.

The study is published in Nature Astronomy.


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  • tag
  • Astronomy,

  • intermediate mass black holes,

  • black hole merger,

  • gravitaitonal waves