Supernova death throes
Three-dimensional turbulent mixing in a stratified burning oxygen shell: yellow ashes of sulphur are being dredged up from the underlying orange core Arnett, Meakin and Viallet/AIP Advances

A new view into the final, turbulent death throes of a supernova has given an insight into how their final explosions outshine entire galaxies.

Turbulent mixing inside stars causes them to expand, contract, eject and explode, according to a 3-D model. It is the first of its kind to represent the start of a supernova collapse in three dimensions, according to its developer W. David Arnett, a professor of astrophysics at the University of Arizona.

Arnett created the model with Casey Meakin and Nathan Smith at Arizona and Mazime Viallet of the Max-Planck Institut fur Astrophysik. He traced his fascination with turbulence to 1987A, the first supernova of 1987. Located in a nearby galaxy, it was bright enough to see with the naked eye. It was previously unknown why the material ejected by its explosion appeared to mix with material previously ejected from the star.

Existing models envisioned a star as a series of concentric circles, with heavier elements like iron and silicon in the center and lighter elements like carbon, helium and oxygen towards the surface. The heavier elements exert a powerful gravitational pull on the lighter elements, which compacts the star and increases pressure, driving temperatures high enough create neutrinos.

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As neutrinos shoot out from the star, they take energy with them. The lost energy reduces the ability of the lighter gases to fight the core's gravitational pull. Instead of cooling down, the star contracts further.

Arnett said: "It heats up and burns faster, making more neutrinos and speeding up the process until you have a runaway situation."

Scientists reached these conclusions by analysing light and radioactivity from supernovas, then creating models of physical processes that yield similar results.

The new model, however, shows something entirely different. It reveals a wild, turbulent interior that spits out star remnants prior to the final explosion, much the way rapidly heating a pot causes water to boil over the edge.

Arnett explained: "We still have the concentric circles, with the heaviest elements in the middle and the lightest elements on top, but it is if someone put a paddle in there and mixed it around. As we approach the explosion, we get flows that mix the materials together, causing the star to flop around and spit out material until we get an explosion."

"That's what see in supernova remnants," he added, referring to the ring of heavy and light elements that form nebulas around stars that went supernova.

"We see those ejections of star material, and how they mix with material expelled from the star during its final explosion. Other models cannot explain this."

Researchers also needed more data, as supernovas are rare and difficult to find. Over the past decade, however, smaller observatories like the Katzman Automatic Imaging Telescope (KAIT) and Palomar Supernova Factory, have used sophisticated electronics to look for miniscule changes in brightness that might indicate a supernova.

When they are discovered, researchers turn larger telescopes on them to gather more detailed information. The data has produced a new understanding of how some stars die.

Arnett said: "Instead of going gently into that dark night, it is fighting. It is sputtering and spitting off material. This can take a year or two. There are small precursor events, several peaks, and then the big explosion.

"Perhaps we need is a more sophisticated notion of what an explosion is to explain what we are seeing."