Skip to Content

A supermassive black hole lit up a collision of two smaller black holes

black hole merger with supermassive black hole
An artist's illustration of the S190521g event, in which two black holes merged within the accretion disk of a supermassive black hole.Caltech/R. Hurt (IPAC)

Astronomers from Caltech have reported that they’ve observed a collision between two black holes. Normally such an event is invisible, but this time a more massive black hole sitting nearby helped illuminate the other two as they collided. If confirmed, the findings, published in Physical Review Letters, would be the first optical observations ever made of a black hole merger.

What happened: First detected in May 2019 and dubbed S190521g, the merger happened about 4 billion light-years away, within the vicinity of a supermassive black hole called J1249+3449. This object is 100 million times more massive than the sun, with a diameter roughly the size of Earth’s orbit around the sun. 

The two smaller black holes, each one about 150 times more massive than the sun, stumbled into the supermassive’s accretion disk: a swirl of stars, gas, and dust that is slowly being sucked toward its event horizon, beyond which not even light can escape. 

When the two smaller black holes collided, the force sent the merged object hurtling out of the accretion disk at about 700,000 kilometers per hour, the researchers calculate. As it sped into space, the merged black hole lit up the surrounding gas in the disk, producing light that was a trillion times brighter than the sun. 

How astronomers linked the merger with the supermassive black hole: S190521g, like many other black hole mergers, produced ripples in space-time that were detected on Earth by LIGO, a gravitational wave observatory. When those observations came in, an automated alert was sent out to the world’s telescopes to see if they could optically observe any events in the night sky that might coincide with the merger. About 34 days later, the Zwicky Transient Facility in California spotted the light produced by the merger event, and scientists followed a fiery trail back to J124942.3+344929. 

The implications: Black holes are not supposed to be visible, but these new findings suggest we can actually visualize these objects by observing the surrounding matter they light up. It’s not all that different from how the Event Horizon Telescope snapped the now-famous image of a supermassive black hole, which was released last year. That image is not exactly the black hole itself, but rather the glowing gas and dust bordering its event horizon.

Establishing a way to closely observe black hole mergers in accretion disks might allow us to answer questions about how matter interacts with these objects, and whether a merger can be predicted before it happens. The Caltech team believes it’s possible these black holes were the result of a long chain of previous mergers—and S190521g was simply the latest.

What’s next: The findings still have to be confirmed. Astronomers will be doing a more detailed analysis of S190521g to see if the merger and the flare associated with J124942.3+344929 really are connected.  

Deep Dive


How to safely watch and photograph the total solar eclipse

The solar eclipse this Monday, April 8, will be visible to millions. Here’s how to make the most of your experience.

The great commercial takeover of low Earth orbit

Axiom Space and other companies are betting they can build private structures to replace the International Space Station.

The race to fix space-weather forecasting before next big solar storm hits

Solar activity can knock satellites off track, raising the risk of collisions. Scientists are hoping improved atmospheric models will help.

Inside the quest to map the universe with mysterious bursts of radio energy

Astronomers still don’t know what causes fast radio bursts, but they’re starting to use them to illuminate the space between galaxies.

Stay connected

Illustration by Rose Wong

Get the latest updates from
MIT Technology Review

Discover special offers, top stories, upcoming events, and more.

Thank you for submitting your email!

Explore more newsletters

It looks like something went wrong.

We’re having trouble saving your preferences. Try refreshing this page and updating them one more time. If you continue to get this message, reach out to us at with a list of newsletters you’d like to receive.