Fiery star deaths may look different than we thought

A new study of Cassiopeia A sheds light on the explosive process


Supernovae are the funeral pyres of very massive stars. They forge most of the basic elements we know, including the ones that make up our bodies. But we still don't know exactly how they happen. It's getting a little clearer though, and a new study published today in Science could address part of the mystery.

Researchers looked at supernova remnant Cassiopeia A in a unique way — one that allowed them to map its interior in three dimensions. The story told by that map reveals large-scale structures in the interior of the remnant, a previously undiscovered trait that suggests stars might explode differently than astronomers had imagined, says study author Dan Milisavljevic, a postdoctoral fellow with the Smithsonian Astrophysical Observatory.

These explosions are the most catastrophic events that occur in nature

These explosions are the most catastrophic events that occur in nature, and they happen all the time. We detect one every second in our sky; some are so large they can be seen with the naked eye, like the one Johannes Kepler observed in 1604. They’ve also been extensively researched. But a lot of the research focuses on extragalactic explosions — ones hundreds of millions of light-years away, Milisavljevic says. That’s too great a distance to inspect the fine details, details scientists need to understand what’s going on in the interior of the remnants.

"If you had a light bulb that’s really distant, you wouldn’t know what it's made of. If it’s nearby, you could see the filaments," he says.

Cassiopeia A is much closer and relatively young. It’s just 11,000 light-years from us, and the explosion was first sighted over 300 years ago. With Cassiopeia A, all the debris from the explosion is laid out in plain sight. "The bomb’s gone off," Milisavljevic says. "So did it explode equally in all directions, or was it preferential?"

Previously, scientists thought that supernova explosions were inherently random. Astronomers had painted a picture of dying stars as spherically symmetrical objects that, as they burn up their remaining fuel, collapse inward before exploding outward symmetrically — a notion Milisavljevic calls "cartoonish." Even if the stars were spherical and symmetrical, their heaviest elements would remain close to the center due to the laws of physics.

Milisavljevic and co-author Robert Fesen have found evidence that both of these ideas are false. The structure in this ejecta material that suggests that the explosion wasn’t random and that some asymmetry in the star or its explosion led to the formation of these structures. This could give astronomers reason to believe that stars behave differently than imagined before their massive explosions.

Cassiopeia A gif

You can inspect the new Cassiopeia A data yourself in this webapp made by the research team.

First, they identified structures six light-years wide in the remnant. As they looked more closely, the researchers noticed the formations were surrounded by "bubbles" of empty space. In fact, these bubbles may be what caused the structures. Milisavljevic and Fresen claim they were created by nickel-56, a heavy radioactive material that escaped further outward from the explosion than anyone had previously imagined. As the nickel moved away from the explosion, it decayed. The radioactive decay created pressure, pushing the lighter materials around it away, creating a cavity surrounded by these structures. Milisavljevic and Fesen believe they’ve found six of these large-scale formations.

The two are still looking for more proof to back up these initial findings. If these bubbles were created by nickel, there’s evidence that scientists could examine — the presence of iron, Milisavljevic says. That’s because when radioactive nickel decays, it forms cobalt and eventually iron. Right now they can see iron deposits in about half of the bubbles, but the evidence is still missing from the rest.

"Very smart people have looked."

"Very smart people have looked, and they haven’t seen it. But we say that we haven’t looked hard enough," Milisavljevic says. He argues modern observatories just aren’t sensitive enough to pick up the signal of the remaining iron — a problem that could have a remedy in the next few years when the James Webb Space Telescope launches. The Webb Telescope will scan the universe from space almost a million miles away from Earth, so it will deal with less interference and be able to see in higher resolution.

From there, astronomers and supernova modelers can use this new data as they try to learn what might cause this asymmetry in the moments before a supernova explosion. There are already a few possibilities, according to Milisavljevic. One is that the stars’ shape might get disrupted by the magnetic fields created by their spin. Another is that the increased density of a star about to go supernova could trap neutrinos, and those neutrinos would produce enough of an energy variation in different regions of the star to disrupt the symmetry.

A third option is that the explosive force could expand symmetrically, but the material of the star around it could be sloshing as the star destabilizes. There is evidence that points to this, according to Milisavljevic. A star underwent an eruption in 2009 that was initially thought to be a supernova. Closer inspection showed there wasn’t enough energy released to classify it as such, and the star basically burped these smaller explosions until one final massive eruption occurred in 2012. "That must be associated with some kind of turbulence inside that mixed the interior," Milisavljevic says.

"My hope is that, with our result, perhaps the modelers can try and determine which of these mechanisms is most consistent with the structure we see in Cas A," Milisavljevic says.

Cassiopeia A chart
Each slit on this chart represents one of the positions observed by the team in their survey of Cassiopeia A.

The three-dimensional map of Cassiopeia A wasn’t what Milisavljevic and Fesen originally set out to do when they arrived at Kitt Peak National Observatory a year ago. That night, with smooth jazz playing in observatory ("That’s Rob," Milisavljevic jokes), the duo used their reserved telescope time to study the outermost regions of Cassiopeia A. Good weather made for quick observations, and they found themselves with leftover time. Asymmetrical explosion had already been discussed, so they used the Mayall 4-meter telescope spectrograph to peer into Cassiopeia A’s interior. "It didn’t take very long, actually, I think that exposure was 20 minutes," says Milisavljevic. "That was enough to convince us that we needed to come back and do it again, and tear it apart, do as much of the remnant as we could." With evidence in hand, they applied for more time to map the interior of Cassiopeia A over the the course of the following year.

"We’re quite bewildered about how the explosion proceeds," Milisavljevic says. "You look at the debris field of an exploded star and we can see how the insides were shaped by the explosion process — we’re finally getting a real understanding of how stars explode."