In the spiral galaxy NGC 3621, located 22 million light-years away, a red supergiant roughly 500 times the Sun’s radius is where the supernova, SN 2024ggi, originated. Observing this stage of a supernova is extremely rare, as most explosions are either too distant or detected too late to capture the initial shockwave as it emerges.
When SN 2024ggi erupted on April 10, 2024, an international team of astronomers, under the direction of Yi Yang from Tsinghua University, promptly obtained viewing time on the European Southern Observatory’s Very Large Telescope (VLT) in Chile. The VLT began collecting data on the event just 26 hours after the supernova was first spotted by the ATLAS survey, capturing the moment when material debris driven by the explosion near the star's core ultimately burst through its surface.
The progenitor star, with a mass between 12 and 15 times that of the Sun, ended its life after exhausting its nuclear fuel. Its core collapsed into a neutron star, and the surrounding layers bounced outward in a catastrophic explosion. The shockwave took roughly a day to reach the visible surface due to the star’s enormous magnitude.
By measuring the polarisation of the supernova's light using spectropolarimetry, the team was able to obtain precise details of the explosion's geometry. According to the data, the shockwave was slightly flattened (resembling an olive) but expanded symmetrically while interacting with circumstellar material. These findings cast doubt on models where the shockwave is driven by neutrinos, which would result in extremely asymmetric explosions. They also suggest that magnetic fields may influence shaping asymmetries seen in later phases.
This discovery, which was published in Science Advances, provides a unique view into the physics of stellar death and advances knowledge of how supernovae affect the universe's composition and development.