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Astronomers have captured one of the most detailed early looks at a dying star after a type II supernova in the nearby galaxy NGC 3621 was caught only 1.2 days after it erupted. The blast, named SN 2024ggi, sits about 7 million parsecs from Earth. Early readings came from the FORS2 instrument on the Very Large Telescope in Chile, giving researchers a rare view of how the shock broke through the star’s surface and pushed outward in a fixed direction.
Polarization happens when light waves align instead of moving in random directions. Most supernovae show almost none of it because their blasts expand in a round shape. When the blast is slightly stretched, the scattered light no longer cancels out, and a small part of it becomes aligned. This lets researchers map the shape of the explosion without taking a direct picture.
The first FORS2 measurements produced a straight line on the Q-U plot, which meant the blast followed a clear axis from the start. The position angle was about 66 degrees. This showed that the shock did not expand evenly and instead pushed harder along one direction as it emerged from the star’s outer layers.
High-energy elements such as oxygen and carbon followed the same axis as the main light. Hydrogen features, which form farther from the center, appeared more mixed. This pattern showed that the earliest light escaped along the central axis while cooler material filled a wider region.
About ten days after the blast, the polarization changed sign but kept almost the same direction. This shift meant the visible shape changed from stretched to flattened, while the axis itself stayed fixed. The debris continued to point the same way even as the structure changed during expansion.
Between day two and day seven, the axis turned by nearly sixty degrees. During this period, the expanding material ran into gas the star had released earlier in its life. That gas formed a tilted disk around the star. The short rotation in the measured angle came from the combined effect of the disk and the explosion before the original axis took over again.
These findings provide clues to how the core collapsed. One model predicts a chaotic, uneven blast driven by neutrinos. Another point is rotation and magnetic fields, which can force matter to move along a stable direction. SN 2024ggi stayed aligned from the first day through almost a month, which supports the rotation-based model. The blast was moderately stretched, enough to create steady polarization without the extreme shapes seen in some other events.
Several other facilities also followed SN 2024ggi in the weeks after the event, including the 2.4-meter Lijiang Telescope in China, the 3.6-meter Telescopio Nazionale Galileo in Italy, and the 6.5-meter Magellan telescopes in Chile. But the earliest polarization data that revealed the axis came from the VLT’s FORS2.
Only a small number of type II supernovae have been measured this early. SN 2023ixf, another nearby event, showed uneven expansion too, but its first readings came after the shock had already crossed the outer layers. SN 2024ggi is the first case in which astronomers caught the breakout itself and later saw the same axis deeper in the hydrogen envelope.
The early and consistent data make SN 2024ggi one of the clearest recorded examples of a massive star showing its final structure in real time. The event suggests that some stars do not explode evenly and instead follow a stable direction shaped by their rotation and magnetic fields long before they collapse.