< When A City-Size Star Becomes A Black Hole's Lunch, The Universe Roils

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Today astronomers say they have spotted one outer space monster eating another. For the first time ever, they've seen a black hole gobbling a neutron star. NPR's Nell Greenfieldboyce reports on how scientists were able to spy on this cosmic snack.

NELL GREENFIELDBOYCE, BYLINE: Black holes are famous for their gravitational pull, which nothing, not even light, can escape. And then there's neutron stars.

MAYA FISHBACH: Neutron stars are very weird.

GREENFIELDBOYCE: Maya Fishbach is an astronomer at Northwestern University. She says neutron stars are made of protons and neutrons, the stuff you find inside atoms. But they're crushed together into a shockingly dense sphere.

FISHBACH: That's, you know, heavier than our sun and can comfortably fit within the city of Chicago.

GREENFIELDBOYCE: Now scientists say they've caught a black hole eating a neutron star in one giant gulp. And then 10 days later, they saw another black hole do the same thing.

FISHBACH: For these particular systems, the neutron star would have just plunged into the black hole without emitting any light.

GREENFIELDBOYCE: If all this noshing didn't put out detectable light, then how did researchers spot it? By sensing gravitational waves. Those are the ripples in space-time created by powerful, violent events out in the universe. Gravitational waves were predicted to exist by Albert Einstein over a century ago but not detected until 2015. Chase Kimball is a graduate student at Northwestern. He says the ability to register gravitational waves has been a game-changer for astronomy.

CHASE KIMBALL: So it's like, you know, flipping the sound on on a silent movie or something like that where we'd previously just been, you know, watching the universe. And now we can listen to it through these gravitational waves.

GREENFIELDBOYCE: In this case, the black holes gobbling neutron stars generated gravitational waves that took about a billion years to reach Earth. In January of 2020, the waves triggered three giant detectors. One of them was the LIGO Hanford Observatory in Washington State. Mike Landry is in charge here.

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MIKE LANDRY: And so these ones are kind of nice, these temporary ones.

GREENFIELDBOYCE: He cleans his shoes on a scrubber, then puts on white booties and a cap to enter a warehouse-sized clean room.

LANDRY: We're just transitioning into the main experimental hall.

GREENFIELDBOYCE: He walks beside a huge metal tube that stretches off into the distance. It's more than two miles long. There's another tube here just like it. They're joined together. From above, they look like the capital letter L. When a gravitational wave rolls through here, it literally warps and stretches space just a tiny, tiny bit - like, a fraction of the width of a subatomic particle. This detector can catch that change thanks to laser beams traveling inside each arm of the L.

LANDRY: You have a real sensitive measure of the change in the relative length of the arms.

GREENFIELDBOYCE: Landry shows off big pieces of new equipment waiting to be installed. He says over the last six years, gravitational wave detectors have logged more than 50 extreme events, like pairs of black holes merging or a couple of neutron stars colliding and now black holes eating neutron stars.

LANDRY: I found that to be an astonishing speed to mature the field. I think that's been a surprise and a good surprise.

GREENFIELDBOYCE: The upgrades currently being made should be done by next summer. And who knows what kind of exotic thing they'll find then? Landry thinks the improvements should let the detector capture a previously undetectable celestial event just about every day.

Nell Greenfieldboyce, NPR News.

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