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Peer into the heart of an exploding star

For the first time, we have telescopes strong enough to see the radioactivity at the heart of the supernova known as Cas A. What scientists have seen there helps unravel the mystery of how a star dies.

The first map of radioactivity in a supernova remnant. The blue shows radioactive material mapped in high-energy X-rays using NuStar.

Every second, another star explodes. When it does so, it gives birth to the very metals and minerals out of which the universe is made. This we know. But until this week, the exact process by which the giant balls of fire meet their dramatic death was something of a mystery.

Thanks to Caltech and NASA's Nuclear Spectroscopic Telescope Array (NuStar), that mystery is beginning to unravel.

By using NuStar's orbiting telescopes, scientists determined that the insides of stars actually slosh around a bit before stars burst into the beautiful cosmic mess that we call a supernova. They figured this out by looking at Cassiopeia A (Cas A), a supernova produced when a star with more than eight times the mass of our own sun exploded about 11,000 years ago. Since that time, the remnants from the big blast have fanned out dramatically -- Cas A takes up 10 light years of space in the cosmos, making it visible from Earth. It is believed to have appeared in our skies in roughly 1671.

"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," Fiona Harrison, the principal investigator of NuStar at Caltech, 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."

Although Cas A has been observed by astronomers for years through optical, infrared, and X-ray telescopes, NuStar's high-energy X-ray detectors allowed them to see radioactive elements in the supernova for the first time -- and solve a perplexing problem.

Previous computer simulations of supernova blasts often show the explosion stalling after the main shock wave peters out without enough force to rip the star apart. The NuStar telescopes, however, were able to see for the first time titanium-44, a radioactive element found at the heart of the exploding star. The way the element was distributed led the scientists to believe that the gasses in the star literally sloshed around, causing the shock wave to re-energize and giving the system enough energy to blow apart.

"With NuStar we have a new forensic tool to investigate the explosion," said Brian Grefenstette of Caltech, the lead author of a paper published Thursday in the journal Nature. "Previously, it was hard to interpret what was going on in Cas A because the material that we could see only glows in X-rays when it's heated up. Now that we can see the radioactive material, which glows in X-rays no matter what, we are getting a more complete picture of what was going on at the core of the explosion."

Although not quite as dramatic as actually seeing a gigantic star go supernova, the Caltech video below shows a computer simulation of how one is formed, based on the new research.