Astronomers have, for the first time, captured the moment a massive star exploded, observing the blast just as it burst through the star's surface.

The discovery, made with the European Southern Observatory's Very Large Telescope (VLT) in Chile, offers unprecedented insight into the earliest stages of a supernova - a fleeting phase that would have vanished within a day.

The explosion, known as SN 2024ggi, was first detected on the night of 10 April 2024 local time. Yi Yang, an assistant professor at Tsinghua University in Beijing and the study's lead author, had just landed in San Francisco when he received word of the event.

"He knew he had to act quickly," ESO said. Within 12 hours, Yang had submitted an observing proposal to ESO, which rapidly approved it. The VLT was pointed at the supernova on 11 April - just 26 hours after its discovery.

SN 2024ggi lies in the galaxy NGC 3621, in the constellation Hydra, about 22 million light-years away - relatively nearby in astronomical terms. With the VLT's powerful instruments, an international team of researchers captured the shape of the explosion within hours of its onset.

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

Yang added: "The geometry of a supernova explosion provides fundamental information on stellar evolution and the physical processes leading to these cosmic fireworks."

Massive stars - those more than eight times the mass of the Sun - end their lives in spectacular supernovae, though the exact mechanisms behind these explosions remain debated. The progenitor of SN 2024ggi was a red supergiant star with a mass 12 to 15 times that of the Sun and a radius roughly 500 times larger, making it a textbook example of a massive-star explosion.

A star maintains its spherical shape through a balance between gravity, which pulls inward, and the pressure from nuclear fusion, which pushes outward. When that fusion ceases, the star's core collapses under its own weight. The outer layers then fall inwards, rebound off the dense core, and send a powerful shockwave outward - tearing the star apart.

Once this shockwave bursts through the star's surface, it releases a huge amount of energy, making the supernova suddenly visible. This brief "shock breakout" phase provides a unique opportunity to study the explosion's original geometry before it interacts with surrounding material.

Using a technique called spectropolarimetry, the team was able to infer the explosion's shape despite it appearing as a single point of light.

"Spectropolarimetry delivers information about the geometry of the explosion that other types of observation cannot provide because the angular scales are too tiny," said Lifan Wang, co-author and professor at Texas A&M University.

The observations were made with the FORS2 instrument, the only facility in the southern hemisphere capable of capturing such data. Analysis revealed that the initial blast of material was shaped like an olive. As the explosion expanded and collided with surrounding gas, the shape flattened, though the symmetry of the ejecta remained consistent.