An Event So Wild It Could Make Anyone Feel Cosmically Small

Gravitational waves have brought us long-sought proof of a cataclysmic meeting between a black hole and a neutron star.

An artistic rendering of a collision between a black hole and a neutron star
Carl Knox / OzGrav / Swinburne University

Are you in the mood to feel small? Like cosmically small? And not because of the usual dreamy, slightly cheesy stuff that space can offer—the idea that we exist on a tiny speck of rock clinging to our beautiful sun in the darkness. I’m talking about some truly wild action, so intense that it warps space-time, the invisible scaffolding that holds up everything we know, and reverberates for hundreds of millions of light-years.

Then astronomers have got something for you.

An international team of researchers announced today that it has detected evidence of one of the most extreme objects in the cosmos, a black hole, colliding with another of the most extreme objects in the cosmos, a neutron star, forming an even bigger black hole. And the team caught it happening not once, but twice.

Until now, these kinds of mergers, as astronomers call them, were purely hypothetical. Theoretical models predicted that they could, and should, happen. Astronomers had found evidence of other mash-ups between extreme astrophysical objects; in the past several years, they have detected mergers between two black holes and mergers between two neutron stars. But they couldn’t be certain that a meeting between one of each was possible, that nature could really bring them together, until one day, in a flash, the evidence arrived on Earth.

“This crazy concept of a black hole–neutron star merger—not just two black holes, not just two neutron stars, but this odd combination—actually exists,” Mansi Kasliwal, an astronomy professor at the California Institute of Technology who studies such mergers but was not involved in the new discoveries, told me.

Astronomers know the mergers happened because they detected the impacts of the collisions. Such cataclysmic events send gravitational waves fanning out in all directions. They ripple across space, bending and squishing it as they go, and pass through everything they encounter—whole galaxies, stars, and planets. And since 2015, we’ve had the capacity, via supersensitive instruments, to detect these gravitational waves as they wash over us.

The gravitational waves from both mergers were detected in January 2020, just 10 days apart. The events unfolded far beyond our own galaxy: One occurred about 900 million light-years from Earth; the other happened even farther away. “Merger” is a polite description of what astronomers believe actually happened. In both cases, the black holes were significantly bigger than their neutron-star counterparts. Rather than slowly shredding the neutron stars apart, the black holes likely enveloped them whole. “We think the black hole kind of cleanly ate the neutron star in one bite,” Phil Landry, an astrophysicist at California State University at Fullerton and a co-author of the new findings, told me.

The colossal nature of these events might give the impression that when the gravitational waves finally reach Earth, they announce themselves in some momentous manner. But no, these cosmic collisions show up as tiny blips—almost imperceptible distortions in the beams of light suspended in a sophisticated piece of technology. If you think too hard about this, the scale of it can be dizzying: These collisions, so large at their origin, make only the smallest impression when they reach us, and we in turn are just that small, compared with the unfathomably large scale of the phenomena we’re trying to measure.

And these massive events are happening on a cosmic level all the time. Based on the new findings, the researchers estimate that a mash-up between a black hole and a neutron star occurs about once a month within 1 billion light-years of Earth. Such events probably take place in every galaxy in the universe, including our own—we just haven’t detected one here yet.

The mergers of black holes and neutron stars provide astronomers with a window into the lives of stars. These objects, after all, were once stars themselves—the classic kind, whose only job is to keep churning and stay luminous. When some of these massive stars run out of fuel, they can implode in a brilliant explosion called a supernova, and their leftovers can become an invisible, bottomless void (a black hole) or a dense, luminous jewel (a neutron star).

How do a black hole and a neutron star find themselves together, close enough to experience another nature-altering event? They could start out together, as two stars that happen to experience supernovas one after the other. Stars stay stars because they produce enough energy to stave off the always present pressure of gravity that’s trying to crumple them. “If you are in a binary stellar system and one of the stars goes supernova and blows a bunch of material on the other star, suddenly that balance is thrown completely out of whack,” Landry explained. The force of this cosmic sneeze could push the other star to explode too. One becomes a black hole and the other a neutron star, and one day they meet.

Another scenario: Picture the center of a galaxy, where the conditions are so extreme that even objects as gargantuan as black holes and neutron stars are slung around as if they’re dangly toys on a baby mobile. Here, the conditions make cosmic crashes more likely.

The mergers of extreme objects, in all their possible combinations, could help solve a mystery that scientists are still trying to understand: how the universe got some of its heaviest metals, including gold, silver, and platinum. Supernovas contributed, but not enough to account for them all. Astronomers found evidence in 2017 that a collision between two neutron stars produced some heavy elements, so such events also spew such metals. Perhaps convergences of black holes and neutron stars do too? Not all black holes would take their time chomping on neutron stars, and the process of shredding them could produce some spectacular chemical reactions. “We just might need neutron stars and black holes to make the heaviest of the heavy elements,” Kasliwal said.

Having established that neutron stars and black holes can merge, researchers now hope to detect another mixed merger (and another, and another). And they want to catch a glimpse of light from it too. Astronomers managed to capture the afterglow of those two colliding neutron stars back in 2017, but they found nothing this time. Maybe the black hole was indeed as efficient at gobbling up its companion as the researchers think. Or maybe the events were simply too far away, and the light too dim for our telescopes to spot.

When I asked Kasliwal what she might see, if she could hover over these extraordinary astrophysical objects in the moments before they meet, she laughed. Her eyes wouldn’t do it justice; such collisions produce a dazzling light show in nearly every wavelength, and our little human eyeballs can’t see them all. And anyway, she’d want to keep a safe distance. “I wouldn’t want to get too close,” she said, laughing. “I would be shredded to pieces.”