26 hours after its discovery in the galaxy NGC 3621 (towards the Hydra cluster, about 22 million light-years away), spectropolarimetry observations on ESO's Very Large Telescope revealed for the first time the early geometry of the explosion — with the shock erupting through the front of the red supergiant (12–15 solar masses, 500 times the radius of the Sun).
A rapid observation using the European Southern Observatory's (ESO) Very Large Telescope (VLT) has revealed the explosive death of a star just as the explosion ripped through the front of the star. For the first time, astronomers have revealed the shape of the explosion in its earliest, transient phase. This brief initial phase would not have been visible until a day later, and it helps answer a whole series of questions about how massive stars become supernovae.
When the supernova explosion SN 2024ggi was first detected on the night of April 10 local time, Yi Yang, a senior lecturer at Tsinghua University in Beijing, China, and the lead author of the study, had just landed in San Francisco after a long flight. He knew he had to act quickly. Twelve hours later, he sent a proposal for an observation to ESO, and after a very rapid approval process, they turned the VLT in Chile on the supernova on April 11, just 26 hours after the first detection.
SN 2024ggi is located in the galaxy NGC 3621 in the direction of the Hydra cluster at a distance of “only” 22 million light-years, close in astronomical terms. With a large telescope and the right instrument, the international team knew they had a rare opportunity to discover the shape of the explosion shortly after it happened. “The first VLT observations captured the phase where material accelerated by the explosion close to the center of the star was shot through the star’s surface. Over a few hours, the geometry of the star and its explosion could be observed, and were observed, together,” said Dietrich Bade, an astronomer at ESO in Germany and co-author of the study published today in Science Advances.
"The geometry of a supernova explosion provides fundamental information about stellar evolution and the physical processes that lead to these cosmic fireworks," Yang explains. The exact mechanisms behind supernova explosions of massive stars, with masses greater than eight solar masses, are still under debate and are one of the fundamental questions scientists want to answer. The source of the supernova was a red supergiant star with a mass of 12-15 solar masses and a radius 500 times larger than the Sun, making SN 2024ggi a classic example of a massive star explosion.
We know that throughout its life, a typical star maintains its spherical shape because of a very precise balance between the forces of gravity that want to squeeze it in and the pressure of its nuclear fusion that wants to expand it. When its last fuel source runs out, nuclear fusion begins to blow out. In massive stars, this is the beginning of a supernova: the core of the dying star collapses, the layers of mass around it fall on top of it and are thrown back. The shock wave created by this blow propagates outward and breaks the star apart.
As the shock wave breaks through the star's surface, it releases enormous amounts of energy - the supernova then becomes very bright and observable. For a very short phase, it is possible to study the shape of the supernova's initial "burst" before the interaction between the explosion and the material surrounding the dying star.
This is what astronomers have now achieved for the first time using the VLT and the technique of "spectropolarimetry", which provides information about the geometry of the explosion that other types of observations cannot provide because the angular sizes are too small. Although the exploding star looks like a single point, the polarization of its light contains hidden clues about its geometry, which the team was able to decipher.
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