For the first time, scientists have without a doubt observed not one but two collisions between black holes and neutron stars. These two separate mergers occurred 10 days apart in January 2020 and were seen by the Laser Interferometer Gravitational-Wave Observatory (LIGO) and Virgo facilities, which detect invisible gravitational waves.

The achievement marks the long-awaited completion of a trifecta of events observed by gravitational-wave interferometers: black hole–black hole collisions, neutron star–neutron star collisions and now, at last, black hole–neutron star collisions. Although the LIGO-Virgo collaboration had previously identified two candidates for this type of merger in 2019, lingering uncertainties about those events precluded any definitive discovery claim. This time, however, the telltale signatures of black holes feasting on neutron stars were unmistakable.

“It wasn’t that surprising, but it was just like, ‘Finally, it’s there,’” says Zsuzsa Márka, a Columbia University astrophysicist, LIGO collaborator, who was a co-author of the study announcing the discovery. The paper was published on June 29 in the Astrophysical Journal Letters.

The 2020 collisions each occurred independently in distinct, widely separated regions of the sky and at astronomically vast distances from Earth. One, on January 5, involved a black hole with a mass nearly nine times greater than that of the sun and a neutron star that was almost twice as massive as our star. The other, on January 15, involved a black hole of 5.7 solar masses and a neutron star packing one and a half times our sun’s heft. Based on the short period in which both collisions took place, physicists now estimate that a merger between a black hole and neutron star occurs approximately once a month somewhere within a billion light-years of the solar system.

Albert Einstein’s 1916 prediction of gravitational waves, or ripples in spacetime that can be caused by the motions of extremely massive objects, has borne fruit for physicists since 2015. In September of that year, LIGO detected gravitational waves from the collision of two black holes. Subsequently, LIGO’s capabilities were upgraded, and Italy’s Virgo and Japan’s Kamioka Gravitational-Wave Detector (KAGRA) joined in the detection of gravitational waves—leading to more observations of binary black hole mergers and the first detection of a binary neutron star collision in 2017. In a way, the observation of a neutron star coalescing with a black hole “completes our collection,” says Chase Kimball, an astrophysics graduate student at Northwestern University and a co-author of the research.

The interferometers at LIGO, Virgo and KAGRA each consist of two arms that “wiggle” slightly because of perturbations from passing gravitational waves. During the two 2020 events, the signals produced by these wiggles—charmingly known as chirps—were striking, Márka says, especially in the case of the first merger on January 5. “It was definitely a beautiful chirping event,” she adds.

Earlier interferometer observations in April and August 2019 caught scientific and media attention as potential black hole–neutron star mergers, says Alessandra Buonanno, director of the Max Planck Institute for Gravitational Physics in Germany, a LIGO collaborator and a co-author of the June 29 study. The particulars for both of those events eroded confidence in their designation, however, whereas the most recent signals were more definitive. Specifically, the April 2019 signal was not clear and could have instead been the result of detector noise, whereas one of the objects involved in the August 2019 collision fell into the “mass gap,” a theoretical range in which neither black holes nor neutron stars are thought to exist. If such an object was a neutron star, it would be the heaviest on record. If it was a black hole, it would be the lightest ever found. Befuddled, researchers are still debating exactly what they saw. Yet, because each merger is a one-time affair, no further information is likely to materialize from that faraway event to deliver a definitive answer.

Often, astrophysicists studying these mergers hope to also see accompanying electromagnetic emissions from an event—some sparks of celestial light produced in addition to gravitational waves by the cosmic cataclysm. This time, however, there was no such luck: the two 2020 observations were characterized as neutron star–black hole mergers based on gravitational waves alone rather than any electromagnetic signal, says François Foucart, a physics professor at the University of New Hampshire, who was not involved in the research.

Prior to the 2020 observations, physicists were not sure what would happen in this type of merger—if the much more massive black hole would swallow the neutron star in a single bite, Pac-Man-style, or if instead its tidal forces would shred the star before engulfing it like a tornado ripping apart a house. In the latter case, one would expect there to be a pileup of hot, glowing debris around the black hole, which a high-powered telescope could detect. Buonanno confirms that no glows or other electromagnetic signals were observed in either collision. Still, she adds, that does not mean such light-based counterparts will not be observed in future collisions because their creation depends on factors such as the masses, velocities, orientations and cosmic environs of the black hole and neutron star.

The discovery also brings scientists one step closer to learning about how these types of binaries form, Kimball says. Perhaps each of the two progenitor pairs were born and lived out their lives as stars together. Or they could have begun to circle each other later in their relative life spans as members of a globular cluster—such clusters contain dense swarms of stars at their center. These two mergers alone do not give us the answers, he adds, but the hope is that eventual demographic studies of a larger population of black hole–neutron star collisions will reveal which pathway is more common.

Future observations of these mergers may also reveal clues about another mystery: how our universe came to be filled with gold, platinum and other heavier elements, Foucart says. He adds that about half of the elements heavier than iron are forged in massive cosmic collisions or explosions, and a better sense of the frequency of black hole–neutron star mergers will tell us what proportion of the universe’s allotment of heavier elements they produce.

Currently, the LIGO and Virgo detectors are being upgraded in preparation for an observing run scheduled to begin after June 2022. KAGRA, the detector in Japan, will go online for that run. These updates will increase the detectors’ combined ability to pinpoint the precise points on the sky where an event occurs and, in turn, aid astronomers in scanning the right regions of the heavens with traditional telescopes to try and capture electromagnetic counterparts, Foucart says.

“Seeing these neutron star–black holes for the first time is just the tip of the iceberg of this population,” Buonanno says.