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On April 10, 2024, astronomers noticed a supernova, labeled SN 2024ggi, in the spiral galaxy NGC 3621, 22 million light years away, located in the constellation Hydra. Fortunately, the supernova was detected soon enough after the explosion that astronomers worldwide were able to spring into action to study what happens during the early stages of a supernova after the explosion. One goal of this effort is to try to detect the shape of the blast wave that emerges from the dying star, as different theories of how the supernova evolves predict different outcomes in this case. The measurements must be made soon after the blast because the blast wave will become distorted when it starts to run into matter distributed around the star, such as dust and gas.
The result of this study has recently been published, and its conclusion is that the blast wave of this supernova was oblate, that is shaped something like an olive. (Note that this link is to the scholarly article in Science Advances; a less challenging account can be found at this Science News link.)
Stars produce their light by fusing the atomic nuclei they are made up of into heavier nuclei. Hydrogen is fused into helium; when hydrogen runs out, helium nuclei are fused into still heavier nuclei, carbon and oxygen principal among them. This process of lighter atomic nuclei being fused into heavier ones continues until the star produces iron; the fusion of an iron nucleus to produce a heavier nucleus produces no excess energy, so the fusion process shuts down. The energy produced by fusion creates an outward pressure that balances the force of gravity and keeps the star from collapsing. Once the nuclear fuel runs out, the outward pressure disappears, and the star collapses. If the star in question has amass equal to or greater than 8 times the mass of the Sun, that collapse will cause a supernova. When the outer layers of the star collide with the iron core of the star, the substance of the those outer layers rebound results in the shock wave as well as the huge surge in radiation. However, there is debate about what physical process within the stellar core may be giving the rebound an extra kick. Some of the proposed processes predict a perfectly spherical shock wave, while others predict asymmetric ones.
“The very first [particles of light] and matter do not shoot out spherically from the star’s surface,” says study coauthor Yi Yang, an astronomer at Tsinghua University in Beijing. “Scientifically this is very important, because the intrinsic shape of the shock breakout tells us a lot of how it was triggered at the heart of the star in the first place.”
While the findings can’t fully explain how this type of supernova is triggered, they do narrow the possibilities.
So how was it possible to determine the shape of the shock wave for a supernova that occurred 22 million light years away? It turns out that analysis of the polarization of the light from the supernova in its early stages will reflect the symmetry of the blast wave. A perfectly spherical blast wave will be perfectly unpolarized. However, the early radiation observed from SN 2024ggi displayed a polarization preference, which indicated that the blast wave had the shape of an oblate spheroid, longer along one particular axis, like an olive. This is consistent with a theory that involves the rebound shock wave driven by the prodigious number of neutrinos produced by the star’s collapse. However, SN 2024ggi is, at this point, the only supernova for which such early polarization data exist. This result must be reproduced many times before there can be assurance that one or another theory of supernovas supports the majority of the data. However, if these measurements can be done once, they can be repeated. It will take time and a certain amount of luck (by catching a supernova early in its explosion), but in time, a clear picture of the process will emerge.
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This analysis of Trump’s meeting with Mamdani is one of the most insightful I’ve read. Think it is spot on.
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