We know stars explode. We’ve seen them do it, ever since Chinese and Islamic astronomers at the turn of the last millennium caught sight of a “guest star” that turned out to be supernova SN 1006.

We also know why they explode. There are two main mechanisms — the first is when a white dwarf sucks matter from or crashes into a companion star and triggers nuclear fusion. The second is when the core of a massive star suddenly collapses under its own gravity.

But what we’re rather less clear on is how they explode — the exact process the star goes through to rip itself apart. We have computer models that try to simulate what might happen, but they don’t closely match what we see in reality. It’s clear that we’re missing something.

So to try and clear up some of the confusion, NASA has used its Nuclear Spectroscopic Telescope Array (NuSTAR) to map the X-rays being emitted from a supernova remnant. Specifically, researchers trained their scopes on Cassiopeia A, which we first saw explode about 300 years ago. It’s the strongest radio source outside of our solar system, though it’s very faint in the visible spectrum.

NuSTAR is complementing previous observations of the Cassiopeia A supernova remnant (red and green) by providing the first maps of radioactive material forged in the fiery explosion (blue) // NASA/JPL-Caltech/CXC/SAO

The team monitored the presence of titanium-44 — an unstable radioactive element produced in the heat of an exploding star. They found clumps of the element concentrated in the centre of the remnant.

That, say the researchers, points to a shock wave that ricocheted around the centre before blasting off the outer layers of the star. The team seeded their results into a computer model and found that it yielded a much better match with reality than previous attempts.

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

The results may also help to disprove a rival theory — that narrow jets of gas are launched around rapidly-rotating stars, which drive the explosion. While jets have been seen before around Cassiopeia A, NuSTAR saw no titanium-44 in these regions, suggesting they were not the explosive trigger.

“With NuSTAR we have a new forensic tool to investigate the explosion,” said the paper’s lead author, Brian Grefenstette of Caltech. “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 case isn’t yet settled, however — more work will be needed before we have a complete picture of how Cassiopeia A exploded.

“This is why we built NuSTAR,” said Paul Hertz, director of NASA’s astrophysics division in Washington. “To discover things we never knew — and did not expect — about the high-energy universe.”