'Sloshing' Supernova Sheds Light on Star's Death

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By tracing radioactive material in the remains of a nearby exploded star, scientists have a new understanding of what happened in the star’s final moments and how similar explosions create the calcium, gold, iron and other elements spread throughout the cosmos.

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The discovery comes from NASA’s Nuclear Spectroscopic Telescope Array, or NuSTAR, which was launched in 2012 to home in on the highest energy X-ray radiation emanating from celestial objects.

Astronomers took a look at a popular target, Cassiopeia A, which is the remnant of star that exploded some 11,000 years ago. In visible light, Cas A is an expanding spherical cloud of debris stretching 10 light years, or some 60 trillion miles,  across the sky.

NuSTAR’s X-ray eye shows a different scene. The explosion that marked the star’s death was not symmetrical. Rather than jets — generated by the spinning star’s collapse — as some computer models predict, the detonation more likely was triggered by its core sloshing around, similar to the disrupted surface of a pot of boiling water.

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“Stars are spherical balls of gas, and so you might think that when they end their lives and explode, that explosion would look like a uniform ball expanding out with great power,” NuSTAR’s lead scientist Fiona Harrison, with the California Institute of Technology, said in a statement.

“Our new results show how the explosion’s heart, or engine, is distorted, possibly because the inner regions literally slosh around before detonating,” she said.

When a massive star runs out of hydrogen for nuclear fusion, gravity eventually takes the upper hand and begins crushing it, building up pressure inside and fusing together even heavier elements.

When there is nothing left to fuse, at the very center of the star tiny particles called neutrinos form and start heating material just behind the shock wave.

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Astrophysicist Brian Grefenstette, also with Caltech, likens the process to boiling water.

“You’re heating up the water. That makes bubbles that rise up and the top of your boiling water sloshes around a little bit,” Grefenstette told reporters.

Neutrinos cause a similar phenomenon in the heart of a collapsing star.

“That’s where you get your big bubbles,” Grefenstette said. “They come up and they make ripples in the shock wave.”

“It sort of pushes the material out of the way, just like the bubbles in your pot (of boiling water.) In this case, they’re letting the shock wave out and the shock wave tears apart the rest of the star,” he said.

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The extreme heat and pressure of supernova explosions fuse lighter materials together to create heavier elements, such as gold, calcium and iron — the very substances that are in our bodies, our cars and everywhere in the material universe.

“If you bought an American car, it wasn’t made in Detroit two years ago,” said astronomer Robert Kirshner, with the Harvard-Smithsonian Center for Astrophysics. “The iron atoms in that steel were manufactured in an ancient supernova explosion that took place 5 billion years ago,” he said.

The research appears in this week’s Nature.

Image: NuSTAR’s high-energy X-ray data, in blue, from radioactive material in the supernova remnant Cassiopeia A.