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WATCH: What Happens When 2 Neutron Stars Collide

An international team of astronomers has concluded that when it comes to theories about colliding neutron stars, Einstein got it right. Everybody else, not so much.

A neutron star is what's left when a star burns out and collapses in on itself, leaving a small, incredibly dense ball.

Einstein's theory of general relativity predicted that when two neutron stars collide, they would generate a gravitational wave, a ripple in space time.

That's exactly what physicists saw for the first time last summer with LIGO, the new gravitational wave observatory.

There were also plenty of theories about what else they'd see. For example, there were predictions about energetic emissions known as gamma rays.

"The old picture suggested that when the two neutron stars merged you launch this very narrow, very, very bright, very fast jet of gamma rays, says Mansi Kasliwal, Assistant Professor of Astronomy at Caltech in Pasadena and principal investigator for GROWTH, the Global Relay of Observatories Watching Transients Happen.

She says that old picture was wrong. It's true astronomers did see a burst of gamma rays, "But the brightness of this burst was rather wimpy."

Indeed, the gamma ray burst was 10,000 times weaker than what they were expecting.

Other measurements proved the theorists wrong as well. The ultraviolet light from the merger was bluer than theories said it should be, and the radio waves generated by the collision were predicted to fade over time. Instead they kept getting stronger.

Kasliwal and her colleagues now think they know where the theorists went wrong. The explanation appears in the journal, Nature.

Before the neutron stars collide, they rotate around each other. "So you have these neutron stars doing this dance around each other, coming closer and closer and closer together before they merge, says Kasliwal.

During the dance, 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 it doesn't get very far.

"The jet sort-of gets stuck," says Kasliwal. "Because there's so much stuff around, that this poor jet cannot just barrel through that and escape out into the interstellar medium."

Kasliwal says the jet transfers some of its energy into the cloud of stuff surrounding the merged neutron stars. This pushes the cloud outward, forming a kind of glowing cocoon.

It's the glowing cocoon that causes the blue ultraviolet light, and the persistent radio waves.

"We are now a hundred days after the merger ad most beautifully it keeps getting brighter and brighter and brighter, exactly as we predicted the cocoon model would do," says Kasliwal.

It's probably not surprising that modern theorists got things wrong. Predicting what kind of gamma rays, radio waves and X-rays colliding neutron stars would produce is "a complicated and messy problem," wrote Daniel Kasen, Associate Professor of Physics, Astronomy at the University of California, Berkeley, in an email to The Two Way. "You have to consider the structure of the neutron stars, the hydrodynamics of how material gets spit out in a merger, the nuclear physics of how heavy elements are synthesized, [and] the atomic physics and electromagnetism of how the ejected material radiates light." Understanding all this relies on lots of different kinds of physics.

Predicting the gravitational wave signal is "a much cleaner problem," wrote Kasen.

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Joe Palca is a science correspondent for NPR. Since joining NPR in 1992, Palca has covered a range of science topics — everything from biomedical research to astronomy. He is currently focused on the eponymous series, "Joe's Big Idea." Stories in the series explore the minds and motivations of scientists and inventors. Palca is also the founder of NPR Scicommers – A science communication collective.