💥 First: astronomers capture a star's explosion as it deforms its surface

Observing the Universe often means contemplating phenomena that occurred millions of years ago. But sometimes, the opportunity arises to capture a cosmic event almost in real-time, as if we were witnessing a star's transformation live. This rare possibility recently materialized for astronomers who were able to observe a crucial moment in the life of a massive star.

Thanks to the Very Large Telescope of the European Southern Observatory, an international team managed to capture the explosion of a star at the precise moment it broke through its surface. This observation, made just 26 hours after the initial detection of the supernova, represents a first in the history of astronomy. The speed of intervention was crucial, as this transient phase lasts only a few hours before becoming undetectable. The star in question, located about 22 million light-years away in the galaxy NGC 3621, offered an exceptional opportunity to study the first moments of a stellar explosion.



The discovery of this supernova triggered a race against time for astronomers. Yi Yang, a professor at Tsinghua University, submitted an observation proposal less than twelve hours after being informed of the event. The rapid approval from ESO allowed the telescope to be pointed at the nascent supernova, thus capturing valuable data about its initial geometry. This exceptional responsiveness was made possible by international collaboration.

The technique used for this observation, spectropolarimetry, revealed details that would otherwise be invisible. Although the exploding star appears as a mere point of light from Earth, analyzing the polarization of its light unveiled its three-dimensional shape. This method exploits the fact that light emitted by non-spherical objects exhibits specific polarization characteristics that betray their geometry.

The collected data showed that the initial explosion had a surprising olive-like shape, revealing a well-defined axial symmetry. This particular geometric configuration suggests the existence of common physical mechanisms governing the explosion of massive stars. As the explosion progresses and collides with surrounding matter, its shape flattens but retains its original axis of symmetry, thus providing valuable clues about internal processes.

This discovery allows astronomers to refine their theoretical models about the end of life of massive stars. The star at the origin of SN 2024ggi was a red supergiant, approximately twelve to fifteen times more massive than our Sun and five hundred times larger. Understanding these explosions helps to better grasp the stellar life cycle and how heavy elements disperse into space to form new generations of stars and planets.

Spectropolarimetry: seeing the invisible

Spectropolarimetry combines two light analysis techniques to reveal information that neither spectroscopy nor polarimetry alone can provide. By studying how light is polarized at different wavelengths, astronomers can determine the shape and orientation of cosmic objects too small to be resolved directly. This approach is particularly useful for supernovae, where geometric details are essential for understanding explosion mechanisms.

Light polarization occurs when light waves vibrate preferentially in a particular direction. In the case of spherical stars, this polarization is generally zero because the vibrations cancel each other out in all directions. However, when the emitting object is not perfectly symmetrical, such as during an asymmetric explosion, the light shows clear polarization that betrays this asymmetry.

The FORS2 instrument on the Very Large Telescope is specially designed for this type of measurement. It can detect tiny variations in light polarization, thus allowing the three-dimensional shape of objects located millions of light-years away to be reconstructed. This unique capability was crucial in revealing the olive-like shape of the SN 2024ggi explosion, demonstrating the power of this observation technique.

The applications of spectropolarimetry extend far beyond the study of supernovae. It is used to analyze accretion disks around black holes, study exoplanet atmospheres, and characterize interstellar dust. Each technical advance opens new windows on the Universe, allowing astronomers to answer fundamental questions about the nature of cosmic objects.

The life and death of massive stars

Massive stars, those with at least eight times the mass of our Sun, have a brief but spectacular existence. Their large mass generates such high pressure and temperature in their core that they burn their nuclear fuel at an accelerated rate. While our Sun will live for about ten billion years, a star with twenty solar masses can exhaust its reserves in just a few million years, leading to a violent end.


The final phase begins when the star's core exhausts its hydrogen, then its helium, fusing increasingly heavier elements up to iron. Iron represents a point of no return because its fusion consumes energy instead of producing it. Deprived of its internal energy source, the core collapses under its own weight in about a second, creating a shock wave that propels the star's outer layers into space.

This collapse and explosion release colossal energy, temporarily surpassing the brightness of an entire galaxy. The heavy elements synthesized during the star's life are scattered into the interstellar medium, enriching the gas from which new stars and planets will form. Without these explosions, the Universe would lack elements like oxygen, carbon, or iron that are essential for life.

The remnant of the explosion depends on the initial mass of the star. For stars with eight to twenty solar masses, a neutron star usually remains, while more massive stars can form black holes. Each supernova thus represents not only an end, but also the beginning of new cosmic cycles, participating in the permanent recycling of matter in the Universe.