What’s the True Shape of a Supernova? | European Southern Observatory
Astronomers have observed a supernova just a day after it was first detected. In the early stages of the blast, the explosion has not yet interacted with the material around the star, retaining its true shape. This initial shape has now been revealed for the first time. This video summarises the discovery.
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 would not 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 April 10, 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 sent an observing proposal to the European Southern Observatory (ESO). After a very quick approval process, the VLT telescope in Chile was pointed at the supernova on April 11, 2024, just 26 hours after the initial detection.
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 today 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. 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.
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. "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."
Credit: European Southern Observatory (ESO)
Directed by: Angelos Tsaousis, Martin Wallner
Editing: Angelos Tsaousis
Written by: Malika Nora Duffek, Kira-Marie Mikosch
Footage and photos: ESO, Luis Calçada, Angelos Tsaousis, Martin Kornmesser, Daniele Gasparri, Christoph Malin, Babak Tafreshi
Scientific consultant: Paola Amico, Mariya Lyubenova
Based on research by: Y. Yang et al., Science Advances, 2025
Duration: 1 minute, 24 seconds
Release Date: Nov. 12, 2025
#NASA #ESO #Astronomy #Space #Science #Stars #Supernovae #SN2024ggi #Galaxies #NGC3621 #Hydra #Constellations #Astrophysics #Heliophysics #Universe #VLT #FORS2 #Spectropolarimetry #ParanalObservatory #Chile #Europe #STEM #Education #HD #Video
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