When one supernova commenced, it looked like an olive — at least before it got shaken and stirred.
This insight, reported in the Nov. 12 Science Advances, comes from new observations taken in the wake of a massive star’s death. As some of the most comprehensive views ever captured of a supernova’s first moments, the findings give astronomers important clues about how these explosions begin.
On April 10, 2024, a supernova was detected in a nearby galaxy. Over the next 26 hours, an international collaboration of astronomers sprang into action to gather additional observations of the explosion before it progressed too far. Their efforts produced the earliest look at the shape of any supernova — the explosive death of a massive star — and revealed its blast wave breaking through the stellar surface.
“It’s a very important set of observations,” says astrophysicist Adam Burrows of Princeton University, who was not involved with the study. “The modern theory of supernova explosions seems to be validated in broad outlines by these data.”
For most of their lives, stars at least eight times the sun’s mass generate an outward pressure through the fusion of hydrogen and helium atoms in their core, counteracting gravity. But once these stars run out of fuel, that pressure disappears and the core collapses. The upper layers of the star follow suit, and as they hit the core, they create a rebounding shock wave that splits the star’s surface and releases an immense amount of energy and light, which we see as a supernova. How exactly the shock wave starts is a long-standing question.
Fortuitously, the shock wave’s shape can reveal what initiated it. But this fleeting view must be captured before the shock wave is disrupted by the material surrounding the star, which can take just hours.
To capture the snapshot of the April 2024 supernova, astronomers used the European Southern Observatory’s Very Large Telescope in Chile, which was able to look at the polarization, or orientation, of the supernova’s light. Using a technique called spectropolarimetry, the researchers used the light’s polarization to re-create the explosion’s shape in its first moments. Their results showed that the light emanated not uniformly, like the light from a typical star, but elongated, shaped like an olive.
“The very first [particles of light] and matter do not shoot out spherically from the star’s surface,” says study coauthor Yi Yang, an astronomer at Tsinghua University in Beijing. “Scientifically this is very important, because the intrinsic shape of the shock breakout tells us a lot of how it was triggered at the heart of the star in the first place.”
While the findings can’t fully explain how this type of supernova is triggered, they do narrow the possibilities.
The observations support the theory that the shock wave is initiated by ghostly subatomic particles called neutrinos being energized deep in the stellar interior, which heats the infalling upper layers like water boiling in a pot. Just as boiling water bubbles erratically, the star’s material bubbles up in an irregular pattern, which eventually averages out into an asymmetric shock wave. This general theory seems to be confirmed by the data, Burrows says, but specific details still need to be worked out. And that will require more observations.
“This is a unique set of data which may presage much better stuff for the future as we start to see, with [upcoming surveys], many, many more of these supernovas,” Burrows says. “If a fraction of them can be followed up with this type of precision, I think we will see a new era of dialogue between theoretical study of these explosions and their observational validation.”