WATCH: What Happens When 2 Neutron Stars Collide : The Two-Way Turns out that Einstein was right about what happens when neutron stars collide. An international team of astronomers has confirmed his theory for the first time.

WATCH: What Happens When 2 Neutron Stars Collide

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RAY SUAREZ, HOST:

When it comes to predicting certain star collisions, an international team of astronomers say Einstein got it right. Everyone else, not so much. NPR science correspondent Joe Palca explains where more modern theorists got things wrong.

JOE PALCA, BYLINE: Neutron stars are what's left when a star burns out and collapses in on itself, becoming a small, incredibly dense ball. Einstein predicted that when two neutron stars collided, they would cause a gravitational wave - a ripple in space time. That's exactly what physicists saw for the first time last summer with their new gravitational wave observatory. There were also plenty of theories about what else they'd see; for example, energetic bursts of a kind of emission known as gamma rays.

MANSI KASLIWAL: The old pictures suggested that when the two neutron stars merge, you launch this very narrow, very, very bright, very fast jet of gamma rays.

PALCA: Mansi Kasliwal is an astronomer at Caltech. She says that old picture was wrong. It's true astronomers did see a burst of gamma rays.

KASLIWAL: But the brightness of this burst was rather wimpy.

PALCA: Wimpy?

KASLIWAL: Yeah, it was very wimpy.

PALCA: Ten thousand times weaker than what they were expecting. And there were other measurements that didn't fit the theories. Kasliwal and her colleagues now think they know why the theorists got it wrong. The explanation appears in the journal Nature. Before the neutron stars collided, they rotated around each other.

KASLIWAL: So you have these neutron stars doing this dance around each other, coming closer and closer and closer together before they merge.

PALCA: During the dance, Kasliwal says the stars start to break apart, forming a cloud of stuff. When they finally do merge, a jet of gamma rays does in fact form, but...

KASLIWAL: The jet sort of gets stuck. It hits a roadblock because there's so much stuff around that this poor jet cannot just barrel through that and escape out into the interstellar medium.

PALCA: Kasliwal says the jet energizes the stuff and pushes it outward, forming a kind of cocoon. That glowing cocoon also explains some of the other unexpected measurements that followed the merger, such as a blue ultraviolet glow. Now, this was only the first time astronomers were able to directly observe the collision of two neutron stars. It's possible that observations from another merger will mean the theories will have to be revised again - maybe even Einstein's. Joe Palca, NPR News.

(SOUNDBITE OF JOHN WILLIAMS' "CANTO BIGHT")

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