Unique shape of star’s explosion revealed just a day after detection

Swift observations with the European Southern Observatory’s Very Large Telescope (ESO’s VLT) have revealed the explosive death of a star just as the blast was breaking through the star’s surface. For the first time, astronomers unveiled the shape of the explosion at its earliest, fleeting stage.

This brief initial phase wouldn’t have been observable a day later and helps address a whole set of questions about how massive stars go supernova.

When the supernova explosion SN 2024ggi was first detected on the night of 10 April 2024 local time, Yi Yang, an assistant professor at Tsinghua University in Beijing, China, and the lead author of the new study, had just landed in San Francisco after a long-haul flight. He knew he had to act quickly.

Twelve hours later, he had sent an observing proposal to ESO, which, after a very quick approval process, pointed its VLT telescope in Chile at the supernova on 11 April, just 26 hours after the initial detection.

Seizing a rare astronomical opportunity

SN 2024ggi is located in the galaxy NGC 3621 in the direction of the constellation Hydra “only” 22 million light-years away, close by in astronomical terms. With a large telescope and the right instrument, the international team knew they had a rare opportunity to unravel the shape of the explosion shortly after it happened.

“The first VLT observations captured the phase during which matter accelerated by the explosion near the center of the star shot through the star’s surface. For a few hours, the geometry of the star and its explosion could be, and were, observed together,” says Dietrich Baade, an ESO astronomer in Germany, and co-author of the study published in Science Advances.

“The geometry of a supernova explosion provides fundamental information on stellar evolution and the physical processes leading to these cosmic fireworks,” Yang explains.

Understanding the mechanics of supernovae

The exact mechanisms behind supernova explosions of massive stars, those with more than eight times the mass of the sun, are still debated and are one of the fundamental questions scientists want to address. This supernova’s progenitor was a red supergiant star, with a mass 12 to 15 times that of the sun and a radius 500 times larger, making SN 2024ggi a classical example of a massive star explosion.

We know that during its life a typical star keeps its spherical shape as a result of a very precise equilibrium of the gravitational force that wants to squeeze it and the pressure of its nuclear engine that wants to expand it. When it runs out of its last source of fuel, the nuclear engine starts sputtering.

For massive stars, this marks the beginning of a supernova: the core of the dying star collapses, the mass shells around fall onto it and bounce off. This rebound shock then propagates outward, disrupting the star.

Once the shock breaks through the surface, it unleashes immense amounts of energy—the supernova then brightens dramatically and becomes observable. During a short-lived phase, the supernova’s initial “breakout” shape can be studied before the explosion interacts with the material surrounding the dying star.

Revealing the explosion’s hidden geometry

This is what astronomers have now achieved for the very first time with ESO’s VLT, using a technique called ‘spectropolarimetry’. “Spectropolarimetry delivers information about the geometry of the explosion that other types of observation cannot provide because the angular scales are too tiny,” says Lifan Wang, co-author and professor at the Texas A&M University in the US, who was a student at ESO at the start of his astronomy career.

Even though the exploding star appears as a single point, the polarization of its light carries hidden clues about its geometry, which the team were able to unravel.

The only facility in the southern hemisphere capable of capturing the shape of a supernova through such a measurement is the FORS2 instrument installed on the VLT. With the FORS2 data, the astronomers found that the initial blast of material was shaped like an olive.

As the explosion spread outwards and collided with the matter around the star, the shape flattened but the axis of symmetry of the ejecta remained the same.

Implications for supernova models and science

“These findings suggest a common physical mechanism that drives the explosion of many massive stars, which manifests a well-defined axial symmetry and acts on large scales,” according to Yang.

With this knowledge, astronomers can already rule out some of the current supernova models and add new information to improve other ones, providing insights into the powerful deaths of massive stars.

“This discovery not only reshapes our understanding of stellar explosions, but also demonstrates what can be achieved when science transcends borders,” says co-author and ESO astronomer Ferdinando Patat.

“It’s a powerful reminder that curiosity, collaboration, and swift action can unlock profound insights into the physics shaping our universe.”