Conventional wisdom has it that stars keep their spherical shape because of the careful balance between gravitational pressure and the internal pressure caused by the nuclear fusion happening in their cores. When they run out of nuclear fuel, they undergo gravitational collapse at their core while the outer shell falls inward and rebounds. For particularly massive stars, this triggers a massive explosion (a supernova) that blows off the outer layers of the star, dispersing material into space and filling the interstellar medium (ISM).
During a short-lived phase in this process, the initial "breakout" shape of a supernova can be observed before the shock wave begins interacting with the surrounding material. Thanks to a team of astronomers using the ESO's Very Large Telescope (VLT) and a technique known as "spectropolarimetry," this has now been achieved for the very first time. While observing the supernova SN 2024ggi, located in the galaxy NGC 3621 22 million light-years away in the constellation Hydra, they obtained data on the geometry of the star's explosion that was never before possible.
The research was led by Yi Yang, an assistant professor at Tsinghua University in Beijing. He was joined by researchers from the European Southern Observatory (ESO), the Cynthia Woods Mitchell Institute for Fundamental Physics and Astronomy, the Hagler Institute for Advanced Study, the Weizmann Institute of Science, the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), the National Institute for Astrophysics (INAF), the International Gemini Observatory, the Institute for Frontier in Astronomy and Astrophysics (IFAA), and multiple universities. Their findings appeared in a study recently published in Science Advances.
As the name suggests, spectropolarimetry combines spectroscopy and polarimetry to measure the polarization of light across a range of wavelengths. This technique can reveal information about a supernova explosion that is impossible to obtain using other techniques due to the extremely small angular scales involved. Even though supernovae appear as a single point in telescope images, the polarization of the light can reveal things about the explosion itself. When it comes to most stars, the polarization of individual photons cancels out so that the net polarization is zero.
When astronomers measure a non-zero net polarization, they can use that measurement to infer the shape of an object, including supernovae. The only instrument capable of doing this is the FOcal Reducer and low dispersion Spectrograph 2 (FORS2) that was recently installed on the VLT. The supernova SN 2024ggi was first detected on the evening of April 10th, 2024, and was observed by the VLT telescope on the following day. Thanks to the rapid response by Yang and his colleagues, the ESO, and the sophistication of the instruments involved, the international team was able to unravel the shape of the explosion shortly after it happened.
What they observed is helping unravel the mystery behind the mechanisms that power massive-star supernovae, a fundamental question in stellar evolution that remains subject to debate. SN 2024ggi is a prime example, as its progenitor was a red supergiant star approximately 500 times the radius of the Sun and 12 to 15 times more massive. Based on the FORS2 data, the astronomers found that the initial blast was olive-shaped, which flattened as the explosion spread outwards. However, the axis of symmetry of the ejected material remained the same throughout. As Yang explained in an ESO press release:
The geometry of a supernova explosion provides fundamental information on stellar evolution and the physical processes leading to these cosmic fireworks. These findings suggest a common physical mechanism that drives the explosion of many massive stars, which manifests a well-defined axial symmetry and acts on large scales.
“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 co-author and ESO astronomer Dietrich Baade. "For a few hours, the geometry of the star and its explosion could be, and were, observed together." This discovery is reshaping our understanding of stellar evolution and the death of massive stars, and also demonstrates the effectiveness of international collaborations. Thanks to the data, astronomers are already able to rule out some of the current supernova models while improving on others.
Further Reading: ESO, Science Advances