A black hole swallowing a neutron star — a star more massive than our sun but only about the size of a city — has been observed for the first time ever.

Each of these space monsters is among the most extreme and mysterious phenomena in the universe. The new find, described in The Astrophysical Journal Letters Tuesday, shows how the very fabric of the universe gets roiled when the two come together.

Researchers actually found not just one, but two black holes making snacks of neutron stars. Their noshing happened about a billion years ago, but was so intense that it shook space-time and sent out ripples that only recently hit the Earth, triggering giant detectors built to sense these waves.

What happens when strange meets stranger in the cosmos
While scientists have previously used these detectors to spot a couple of black holes merging, and a pair of neutron stars colliding, never before have they had a chance to see what happens when these two different kinds of beasts encounter each other.

"It was just a matter of time," says Gabriela González, a physicist at Louisiana State University, who explains that eventually seeing something like these events was expected. But it was surprising the detections came back-to-back, just 10 days apart, in January of 2020.

An analysis shows that one of the black holes had a mass nine times bigger than our sun, and gulped a neutron star with about two times our sun's mass. The other black hole had about six times the mass of the sun, and ate up a neutron star with 1.5 times the sun's mass.

While black holes are famous for having such a strong gravitational pull that nothing — not even light — can escape, neutron stars are plenty weird too.

"I think people often hear them described as the less extreme cousins of black holes, which I think is really a disservice to how cool they are," says Maya Fishbach, a gravitational-wave astronomer at Northwestern University, who explains that neutron stars seem to be protons and neutrons all smushed together into a very, very dense sphere.

When a neutron star meets a black hole that's much more massive, like the recently observed events, says Susan Scott, an astrophysicist with the Australian National University, "we expect that the two bodies circle each other in a spiral. Eventually the black hole would just swallow the neutron star like Pac-Man."

"We are pretty sure, based on what we do know, that for these particular systems, the neutron star would have just plunged into the black hole without emitting any light, without being ripped to pieces," agrees Fishbach.

An alert was sent out to astronomers as soon as the detectors found these events, but no one saw any flash of light with telescopes.

Monster collisions trigger monster waves
Being able to detect this kind of previously-unobservable event is just the latest step forward in the study of gravitational waves, which are waves sent through space by powerful collisions. Albert Einstein predicted the existence of gravitational waves more than a century ago, although even he had some doubts. The first time they were actually detected, in 2015, was such a big deal that work leading to the feat was almost immediately awarded a Nobel Prize. So far, scientists have detected more than 50 events.

"Throughout all of human history, since we've been looking up at the stars, we've been gathering information just based off of the light that we see. And then, you know, with gravitational wave astronomy, which only really started five or six years ago, we suddenly have this brand new channel of information," says Chase Kimball, a graduate student at Northwestern University. "It's like flipping the sound on, on a silent movie, or something like that."

He calculates that a black hole eats a neutron star roughly every 30 seconds somewhere in the whole observable universe, though scientists would have to be looking in the right place with the right kind of equipment to detect it. Within one billion light-years of Earth, it happens roughly once per month.

Upgrade of wave detector may detect more weirdness

The chance to understand this kind of cosmic activity is why researchers spent decades designing and building gravitational wave detectors. These L-shaped giants send lasers down pipes that are more than two miles long. When a gravitational wave passes through the devices, it warps space and changes the length of the lasers' paths. The detectors are able to catch changes in length that are so subtle, they're a fraction of the width of a subatomic particle.

The U. S. has two of these detectors, funded by the National Science Foundation, that together are known as LIGO. There's another in Italy, called Virgo, and one in Japan called KAGRA, although it was not online during these detections. Over 2,000 scientists around the world work together to detect and analyze gravitational wave events.

Over time, LIGO has been improving its detectors to make them more and more sensitive. In the beginning, the detectors only registered two events in a period of about four months. During the last observing run, which ended in March of 2020, the detectors were able to detect an event around every six days.

After an upgrade now underway that should be finished by next summer, "we hope to be on the mark for about one per day or so," says Michael Landry, head of the LIGO Hanford Observatory, who called the current work "the most complicated and impactful upgrade" since 2015.

He says collecting information on more black holes eating neutron stars should ultimately help reveal what these oddball stars are made of. "It's like a nuclear physics laboratory, an extraordinary nuclear physics laboratory," says Landry.

In the future, if a neutron star is able to get close enough to a black hole before plunging in, it could get ripped to pieces and put on a visible fireworks display that astronomers can watch — much like the dramatic collision of two neutron stars seen in 2017.

"I think it would be really exciting to have more events where we see gravitational waves and light from the same source," says Fishbach.

There are also whole other classes of objects out there, beyond two massive objects crashing together, that astronomers still hope to see. "A star that goes supernova," says Landry, "should give off gravitational waves as a pulse — a sort of cymbal clash of gravitational waves."

Scientists also hope to sense objects that make waves continually. A neutron star that is slightly asymmetrical, for example, could perturb space as it rotates, sending out a constant whirring stream of waves.

"We believe that we will, in time, detect these kind of continuous streams of waves," says Scott. "And this will give us a lot of extra information about the composition of neutron stars, which is a really big holy grail of astronomy and astrophysics at the moment."

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