Astronomers have peered into the core of an exploding star in its final moments of existence, for the first time in history.

Using NASA's Nuclear Spectroscopic Telescope Array, a high-energy X-ray observatory, the first map of radioactive material in a supernova remnant has been created.

The research is a major step in unravelling the mystery of how stars explode in a supernova. The remnant from Cassiopeia A, known as Cas A, reveals how shock waves likely rip apart enormous dying stars.

Fiona Harrison, the principal investigator of NuSTAR at the California Institute of Technology (Caltech) in Pasadena, said: "Stars are spherical balls of gas, and so you might think that when they end their lives and explode, that explosion would like a uniform ball expanding out with great power.

"Our new results show how the explosion's heart, or engine, is distorted, possibly because the inner regions literally slosh around before detonating."

The 3-D simulation below shows how the explosion might have looked.

Brian Grefenstette, the lead author of the paper, told Science Daily: "With NuSTAR we have a new forensic tool to investigate the explosion. 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."

The results were published in today's issue of Nature. The superimposed images of the Cas A supernova remnant were taken by NASA's Chandra and NuSTAR orbiting telescopes.

The blue shows NuSTAR's map of the distribution of titanium produced in the core of the explosion 340 years ago. The red and green show X-ray emissions of heated iron and silicon/magnesium, as picked up by Chandra.

NuSTAR is the first telescope capable of producing maps of radioactive elements in supernova remnants. In the case of Cas A, the element with the unstable nucleus produced at its core is titanium-44.

The map shows the element is concentrated towards the centre of the remnant. It offers a potential solution to the mystery of how the star died, as the research suggests the star "sloshed" around and re-energised the stalled shock wave. This finally allowed the star to blast off its outer layers.

The Cas A remnant was created when a star exploded as a supernova, in which the light ejected reached Earth a few hundred years ago. The map shows the remnant when it was young. Stars up to eight times the size as our sun die violent deaths.