Some 870 million years ago two dead stars became one. Their merger shook the fabric of space with a gravitational wave that swept through Earth on August 14, 2019, rippling through three pairs of carefully calibrated lasers designed to detect their passage. An automated system sent out a preliminary alert 21 seconds later, vibrating smartphones and pinging laptops around the world.

A few years after the Nobel Prize–winning first gravitational-wave detection, which stemmed from a pair of colliding black holes, such alerts had become commonplace. This time, however, astrophysicists instantly knew that the observed event was special. “My jaw dropped when I saw the data,” says Geoffrey Lovelace of California State University, Fullerton, a member of the Laser Interferometer Gravitational-wave Observatory (LIGO) Scientific Collaboration.

The wave was detected by LIGO in the U.S. and the Virgo Observatory in Italy at 21:11:18 UTC on August 14, 2019. An automatic first pass pegged it as resulting from an unprecedented merger between a pair of bodies too light to classify, sending astronomers scrambling to look for additional electromagnetic emissions from the event. Subsequent analysis recategorized the signal as a collision between a black hole and a neutron star, a stellar remnant in which gravity squeezes an entire sun’s mass into a ball the size of a city. This may be the first such event detected with confidence and, after black hole–black hole mash-ups and mergers between two neutron stars, the third variety of collision detected by gravitational waves. While the classification remains uncertain, this event, now known as GW190814, marks the beginning of a new era of astrophysical studies, with implications for how researchers understand Einstein’s general theory of relativity, the deaths of stars and the behavior of extreme matter.

An “Off-The-Charts” Signal

Chad Hanna, a LIGO collaborator and astrophysicist at Pennsylvania State University, was celebrating his wedding anniversary with his wife when his phone went off. His group specializes in rapid classification of LIGO events, so he immediately logged in to check the wave’s details. “The first thing I knew was that it was extremely significant,” Hanna says, “kind of off-the-charts loud.”

The LIGO-Virgo collaboration’s algorithmic pipeline spits out a basic classification based on the shape of a wave, its duration and other factors almost instantly—Hanna’s team aims for under 20 seconds—so astronomers can immediately slew their telescopes in the celestial direction the wave came from.

On that August day, the automatic system confidently declared that at least one of the objects that produced GW190814 fell into the “mass gap,” a wasteland, spanning three to five solar masses, seemingly bereft of black holes and neutron stars. All known black holes weigh more than five suns, while all known neutron stars—born from lighter stars that stopped short of becoming black holes—weigh less than three suns. A mass gap detection would have been a first for LIGO-Virgo—one that would have sharpened the theoretical line separating the heaviest neutron stars from the lightest black holes—but the preliminary label would not last. “There was a handoff around the globe,” says Jocelyn Read, an astrophysicist at C.S.U. Fullerton and a LIGO member, beginning with researchers in the U.S. that afternoon and with calculations continuing in Europe well into the following morning.

American scientists woke up on August 14, 2019, to a new classification. Human analysis had pegged the event as a neutron star–black hole merger with greater than 99 percent confidence. LIGO-Virgo has heard the collisions of more than a dozen black hole pairs, as well as two pairs of neutron stars, but it has never conclusively heard the rumbles from a black hole swallowing a neutron star.

“This is something I’ve waited for for a long time,” says James Lattimer, an astronomy professor at Stony Brook University and a pioneering nuclear astrophysicist, who showed that neutron star–black hole mergers can spray heavy elements such as gold and uranium into space in his 1976 thesis.

Researchers detected a similar wave in April 2019, but they were not able to confirm that it came from deep space—the signal associated with that potential event, models suggest, has a 60 percent chance of originating from a terrestrial rumble, and such a spurious detection might be expected about once every 20 months. The August 2019 signal, however, initially appeared so clear that a false alarm would have been a once-in-trillions-of-years event. “When it’s more than the age of the universe,” Lovelace says, “you know it’s the real deal.” More comprehensive analyses later pegged a false alarm as a once-in-tens-of-thousands-of-years occurrence.

GW190814’s deafening signal, however, does not guarantee that astrophysicists have definitely bagged their first neutron star–black hole collision. While the current label clearly puts the heavier object in black hole territory (more than five suns), it leaves the lighter partner in the murky zone below three solar masses. Further analysis refined this partner’s weight to 2.6 solar masses, where its identity could still break either way—toward the universe’s heaviest-known neutron star or its lightest-known black hole. Theoretical arguments have led physicists to suspect that a black hole is more likely, but without more data they cannot be sure.

Looking For Light

Virgo’s detector in Italy—along with only one of LIGO’s two detectors—recognized the wave initially, but the collaboration was able to manually incorporate data from the second LIGO detector overnight. Triangulating from that third detection allowed researchers to pinpoint the source’s location in the sky more precisely than any previous wave so soon after detection. “I opened up [the new] sky map, and I was like, ‘Oh, they accidentally updated a blank sky map,’” Read recalls thinking before she noticed the tiny dot marking the wave’s origin.

The narrowed location, which amounted to 0.06 percent of the sky’s total area, came as a boon to astronomical teams hunting for a flash of gamma rays or visible light that could accompany the death of a neutron star, if that is what the lighter partner was. “In principle, it’s a manner of minutes to cover that area,” says Marcelle Soares-Santos, a cosmologist at Brandeis University, who coordinated follow-up observations using the Dark Energy Camera on a four-meter telescope in Chile.

The black hole may have shredded the potential neutron star, leaving behind a ring of glittering wreckage that faded as it fell into the hole’s waiting maw. Alternatively the black hole could have swallowed the neutron star in one clean gulp, with little left to see. LIGO-Virgo simulations for GW190814 predict the latter scenario, but no one knows for sure what actually transpired. Soares-Santos and other teams ultimately did not spot an accompanying flash, which she says may help refine theories of how this first-of-its-kind event could have played out.

Probing “Neutronium”

Theorists dream of catching the demise of a neutron star because competing theories describing the innards of the enigmatic objects abound. Nuclear physicists seek a glimpse inside the objects, where matter exists at densities that challenge the present best models. If the pressure dissolves neutrons into a plasma of fundamental particles, for instance, neutron stars of a certain mass should appear smaller than they otherwise would be. Fine features of the detected gravitational wave produced as the star spiraled into the black hole may reveal the star’s size and, accordingly, the consistency of the matter that fills it. Similarly, whether astronomers see a flash or not will also set limits on the star’s size. Such precise measurements of a neutron star’s dimensions are “sort of the holy grail of nuclear physics,” says Ben Margalit, a postdoctoral researcher at the University of California, Berkeley, who is not part of the collaboration that observed the event.

A black hole obliterating a neutron star also represents a new arena for testing general relativity. Applying Einstein’s theory of gravity to the smooth fabric of spacetime around black holes is tough enough, Lovelace says. Adding in hot, turbulent magnetized neutron star matter—an exotic substance sometimes called neutronium—elevates the challenge to a messy new level.

Even if this ripple in spacetime doesn’t divulge any of nature’s secrets, researchers feel confident it is just the first of many to come. “I hope it tells us something about black hole–neutron star [mergers],” Lovelace says. “But if not, it still makes me really optimistic that the gravitational sky is bright.”