Once again, scientists have detected gravitational waves — ripples in the fabric of space and time created by objects moving throughout the Universe. And this time, the celestial signal stems from a never-before-seen event: the merger of two neutron stars. In contrast to past observations of gravitational waves, the event was also detectable by regular light telescopes, giving scientists unprecedented insight into this cosmic collision.

a never-before-seen event

This is because all four previous wave detections have come from the mergers of black holes, which are events that don't emit light. But these waves were created from the violent collision of two distant neutron stars, the superdense leftovers of stars after they’ve collapsed. When these two objects combined, they spiraled around each other rapidly before smashing into one another, creating a gigantic fireball of light visible to telescopes on Earth.

Three different gravitational wave observatories across the globe picked up the signal in August: the dual US-based observatories operated by LIGO (which made the first wave detection in history last year) as well as a third observatory, Virgo, which is located in Italy. Thanks to this trio of inputs, astronomers located the area where the merger occurred, narrowing it down to a very small patch in the southern sky.

Once the general area was known, LIGO mobilized the rest of the astronomy community. Within just a few hours of the detection, thousands of astronomers operating up to 70 ground-based and space-based telescopes were searching the sky — eventually spotting the explosive leftovers of the merger. They continued to observe the event for weeks after the collision, learning more about how the chaotic object evolved over time.

the beginning of a new era known as “multi-messenger astronomy”

Until now, light was really the only tool astronomers had to study objects in space. Scientists can learn more about distant objects by observing them in various wavelengths of light — from visible light to light we can’t see, like X-rays and infrared. But now, both light and gravitational waves can be used to study celestial events together, marking the beginning of a new era known as “multi-messenger astronomy.”

“This is a revolution in astronomy, of having thousands of astronomers focus on one source for weeks and having this collaboration unravel in seconds, in hours, then days, and weeks,” Vicky Kalogera, an astrophysicist at Northwestern University and one of the LIGO collaborators, tells The Verge. “For us, that’s the Holy Grail.”

A long time coming

This astronomical revolution comes less than two years after the first gravitational wave was detected. Astronomers have been trying to figure out how to detect these ripples for the last century, ever since they were first predicted by Albert Einstein in his theory of general relativity. Einstein argued that objects in the Universe actually warp the space and time around them. And when they move, they create waves in this space-time, a bit like a boat leaving ripples in a pond.

detecting these waves is an incredibly difficult process

But detecting these waves is an incredibly difficult process. The ripples from nearby planets and stars, for instance, are much too small to pick up from Earth. That’s why scientists look for the biggest waves they can find — ones coming from the most massive objects in the Universe moving at rapid speeds. Merging black holes and neutron stars offered the perfect targets.

When these superdense objects combine, they actually spiral around each other, growing closer and closer together over time. The spinning increases in frequency until the two objects are revolving around each other up to several times per second before combining in one forceful impact. This sort of activity creates gargantuan gravitational waves, which travel through the Universe at the speed of light. They fade away during their journey, but still make it to Earth in a diminished form. The LIGO and Virgo observatories have to use incredibly sensitive laser technology to pick up the signals from these waves. (Learn more about how those observatories work here.)

Until now, all four of the detections made by LIGO have been from mergers of black holes

Until now, though, all four of the detections made by LIGO have been from mergers of black holes. These discoveries told scientists a great deal about the types of black holes found in our Universe, but they don’t offer much opportunity for follow-up observations. Black holes have incredibly strong gravitational pulls, so nothing — not even light — can escape from them. Even if astronomers could pinpoint where a black hole merger occurred, telescopes that observe light wouldn’t be able to see anything. That’s why astronomers have been eager to find merging neutron stars.

Scientists have been getting a lot better at locating where these mergers come from, too, thanks to the recent addition of Virgo. The first three detections were made by LIGO’s observatories alone, but the fourth signal was also seen by Virgo. Having three detectors pick up waves makes it much easier to find the sources of these signals in the sky. By timing when the waves reach each detector, astronomers can triangulate the location of the wave source in space, similar to how three GPS satellites are used to pinpoint the location of something on Earth.

Astronomers just needed a source other than black holes that they could actually see.

Finding the signal

On August 17th at 8:41AM ET, just before LIGO and Virgo were scheduled to stop observations after a months-long run, both of LIGO’s observatories in Washington and Louisiana picked up what looked to be a gravitational wave signal. Immediately, astronomers suspected that it was from two neutron stars colliding, since the wave perturbed LIGO’s instruments for over a minute and a half (much longer than previous signals from black holes, which lasted just fractions of a second). It was a sign that the merging objects were much smaller than black holes. “Neutron stars are so much smaller than black holes, so they get much closer together before they merge,” Laura Cadonati, a LIGO collaborator and professor of physics at Georgia Institute of Technology, tells The Verge. “So you can observe the waves for a long time, and get a nice, long, beautiful signal.”

At the same time LIGO got its signal, NASA’s Fermi space telescope (in orbit around Earth) detected an intense burst of high-energy light, known as a gamma ray burst, coming from deep space. Astronomers have suspected that neutron stars may create these beams of high-energy radiation when they collide because the explosions are so hot and powerful. Detecting a burst at the same time as a wave signal made the astronomers confident they were seeing two neutron stars merge.

Meanwhile, astronomers initially thought Virgo had missed the signal, since it wasn’t showing up in the observatory’s data. But after a further look, scientists realized Virgo had picked it up; the wave signal was just incredibly faint. It turned out the merger occurred in a part of the sky that is a bit of a blindspot for Virgo, which is a byproduct of the observatory’s location on Earth. “Virgo in a way missed it, because it happened to be in a narrow part of the sky where Virgo couldn’t quite catch it,” says Kalogera.

But the fact that Virgo missed it actually helped astronomers figure out where the signal was coming from: the scientists knew the exact spot in the southern sky that Virgo could not see. That knowledge, combined with the data from LIGO’s two observatories, helped the collaboration to pinpoint exactly where the waves were coming from, narrowing the signal’s home to a patch of sky of just 30 square degrees. That’s a small sample of the night sky, which is 40,000 square degrees.

A call to arms

As soon as the LIGO team suspected they had caught a new wave, text alerts were sent to astronomers around the world, telling them to get ready for a hunt. Five hours later, LIGO and Virgo shared a sky map with a general location. Seven hours after that, the aftermath of the collision had been pinpointed and the source of the gravitational waves was discovered.

text alerts were sent to astronomers around the world, telling them to get ready for a hunt

The ground-based Swope observatory in Chile saw it first, snapping images in visible light. Then other telescopes — both on the ground and in space — found it, too, measuring light from across the electromagnetic spectrum. Astronomers gathered as much data about the event as possible, measuring everything from X-rays and ultraviolet light to infrared and radio waves. “This is the reason we all become scientists,” Andy Howell, an astronomer at Las Cumbres Observatory Global Telescope Network, one of the first observatories to spot the event, tells The Verge. “There’s nothing like the feeling knowing you’re one of the first people in the world to see a new phenomenon.”

It’ll be some time before astronomers decode everything they’ve seen from these follow-up observations. But in the meantime, scientists have begun to paint a portrait of this collision using the data they’ve gathered. Based on the LIGO measurements, the two neutron stars combined 130 million light-years away, much closer than the black hole mergers which occurred billions of light-years beyond Earth. And each neutron star was between 1.1 and 1.6 times the mass of our Sun, though they were probably just about 10 miles across.

scientists have begun to paint a portrait of this collision using the data they’ve gathered

Their resulting impact is known as a kilonova, an incredibly explosive event. The merger creates a gargantuan fireball, and the superdense materials from the two stars shoot outward in all directions. Initial light measurements from the kilonova show just how fast that material was moving, too: the outer layers of the kilonova sped away from the event at speeds close to one-third the speed of light, according to astronomers’ estimates. These events aren’t just explosive either; they’re also thought to be factories for the production of the heaviest elements in the Universe. And the light emitted from the kilonova showed how those elements, such as gold, were produced in the wake of the merger.

Kilonovae have been mostly theoretical until now, and these observations confirm a lot of what astronomers expected to see during such an event — as well as opened up a few questions. “Theorists have been working on kilonova for the past decade, and now with this one singular event, all this decade of work is all kind of coming to a head,” Tony Piro, principal investigator of the Swope Supernova Survey, tells The Verge. “It’s really a momentous event.”

“It does feel good to find the source we’ve always expected to find.”

And this is just the beginning. It’s possible LIGO and Virgo saw more neutron star signals during their latest observational run, and that more could be seen in the future. The three observatories have been offline since August 25th, but engineers are working to make their instruments more sensitive to wave detection. The next observational run is slated to begin next summer, and once that starts, we could see an explosion of wave detections. For now, astronomers are basking in this latest discovery — something they’ve been hoping to find for decades.

“[Finding neutron stars] was super important as the motivation for building LIGO,” says Kalogera. “And now we’ve finally detected them. It should not take away from our first discovery, but it does feel good to find the source we’ve always expected to find.”