When a massive star explodes, it can be seen across millions of light years. But until now it was unclear what the earliest phases of such a supernova look like. Now, for the first time, astronomers have managed to observe the geometry of this initial stage of the explosion. Just 26 hours after a supernova lit up around 22 million light-years away, they watched the shock wave from the explosion break through the surface of the star. This revealed for the first time the actual shape of this eruption: the supernova’s shock wave emerged from the star’s surface in an olive-like structure. This elliptical, symmetrical shape of the explosion provides valuable information about the mechanism of such a supernova. At the same time, it also refutes some models that assume an amorphous, non-symmetrical shock wave.
The shape and size of a star are based on a finely tuned balance of two forces: the gravitational effect of its mass pulls it together, while the radiation pressure generated by nuclear fusion within its interior acts outwards. Over large parts of the stellar life cycle, these forces are balanced and the size of the star changes only slightly. However, this changes when the star’s hydrogen supply runs out. Then the radiation pressure predominates for a while and swells the star into a red giant or supergiant. However, when the last of the fusion fuel is used up, the outward force decreases. As a result, the star core begins to collapse under its own gravity. The surrounding layers of material first fall inwards, then a rebound occurs, through which material is thrown outwards again – a supernova explosion occurs, which destroys the star.
Unexplained initial stages
A supernova becomes visible when the shock wave from the explosion breaks through the surface of the star. This releases enormous energies in the form of radiation and fast particles. The star explosion is now visible from far away. “The geometry of a supernova explosion provides fundamental information about star evolution and the physical processes that lead to these cosmic fireworks,” explains lead author Yi Yang from Tsinghua University in China. Because exactly how the shock wave arises and races through the star is still not fully understood. This is also because the information required for this – for example the orientation and symmetry of the explosion – can only be seen in the early phase of the supernova. Later, interactions between the shock wave and the material around the dying star make observation more difficult.
Yang and his team had the opportunity to observe this crucial early phase on April 10, 2024: a telescope discovered a newly shining point of light in the spiral galaxy NGC 3621, around 22 million light-years away – relatively close by astronomical standards. Yang received the message via an automatic alarm system just as he had landed in San Francisco. The astronomer then immediately applied for observation time at the Very Large Telescope (VLT) of the European Southern Observatory ESO in Chile in order to observe the supernova named SN 2024ggi. The cause of the stellar explosion was a red supergiant with 12 to 15 solar masses and a radius that was 500 times larger than that of the Sun. This makes SN 2024ggi a classic example of the explosion of a massive star.
Burst in the shape of an olive
The astronomers managed to get observation time on the Very Large Telescope at short notice: just 26 hours after the supernova was discovered, they set their sights on the point of light with the FORS2 instrument installed on the VLT. This is the only one in the southern hemisphere that has the resolution and technology necessary to capture the shape of such a supernova more precisely. This is possible using spectropolarimetry, which analyzes the orientation and spectrum of the polarized light released when a star explodes. “Spectropolarimetry provides information about the geometry of the explosion that is not accessible with other observation methods because the angular scales are much too small,” explains co-author Lifan Wang from Texas A&M University. Although the exploding star appears as just a single point, the polarization of its light holds clues to its geometry. “Our observation campaign at SN 2024ggi has produced one of the two earliest spectropolarimetric data sets of such a short-lived event,” the astronomers report. “The other was measured 1.36 days after the shockwave breakthrough of supernova SN 2023ixf.”
(Video: ESO)
SN 2024ggi observation data showed the first photons of the stellar shock wave erupting from the dying star’s surface. “The remainder of the shock was still embedded in the star’s optically dense atmosphere or circumstellar medium,” report Yang and his colleagues. The material shooting out from the star’s surface was initially symmetrical and elliptical – its shape resembled that of an olive. As the explosion spread outward over the next few hours, it collided with the material of the stellar envelope. This flattened their shape but maintained their symmetry and alignment. “These results suggest a common physical mechanism that drives the explosion of many massive stars – it shows a well-defined axial symmetry and operates on large scales,” explains Yang. The mechanism at work shapes the explosion from the first shock wave outbreak through to the expansion phase of the explosion.
These observations allow astronomers to rule out some of the existing supernova models and refine others. The symmetrical shape of the shock wave from SN 2024ggi refutes some supernova models, according to which small-scale instabilities or the influence of neutrinos lead to an amorphous and non-symmetrical explosion. “This discovery not only changes our understanding of stellar explosions, but also shows what is possible when science pushes boundaries,” says co-author Ferdinando Patat from ESO. “It underscores that curiosity, collaboration and rapid action can provide deep insights into the physics that shape our universe.”
Source: Yi Yang (Tsinghua University, Beijing) et al., Science Advances, doi: 10.1126/sciadv.adx2925