Supernovae: a first-of-its-kind star explosion raises new questions about these momentous events

Stars often end their lives with a dazzling explosion, creating and releasing material into the universe. This will then seed new life, leading to a cosmic cycle of birth, death and rebirth.

Astronomers around the world have been studying these explosions, called supernovae (derived from the Latin “an extremely bright new star”), and have discovered tens of different types.

In 2021, astronomers observed a bright supernova, dubbed SN2021yfj, two billion light years away. In a recent paper, published in Nature, astronomers observed it for more than a month and discovered that it exhibits the visible signatures of heavier elements – such as argon, silicon and sulphur – since the onset of the explosion. This was previously unobserved in any stellar explosion.

Supernovas violently eject stellar material into the cosmos, roughly keeping the same onion structure the star had before its death. This means that lighter materials – such as hydrogen and helium – will be in the outer layers and heavier ones – such as iron, silicon and sulphur – in the inner layers.

However, massive stars can lose part of their layers during their evolution via winds (like the Sun), great eruptions (like the star Eta Carinae), or a gravitational and energetic “tug of war” with a companion star in a binary system. When this happens, circumstellar material will form around the star and will eventually be hit by the ejected material in the explosion.

In a galaxy, there are an enormous number of stars. If you think that there are at least two trillion observed galaxies, you can picture what a vast playground of discoveries scientists play with every day. Although not all stars end with an explosion, the proportion is large enough to allow scientists to confirm and study their shell structure and chemical composition.

The luminosity (brightness) of the new discovery in terms of timeframe and behaviour was similar to other known and well-studied stellar explosions. The chemical signatures discovered in their electromagnetic spectra (colours) and their appearance over time pointed to a thick inner stellar layer expelled by the star.

This was then struck by material left in the star and expelled during the explosion. However, some traces of light elements were also present, in direct clash with the heavy elements as they should be found in stellar layers far apart from each other.

The astronomers measured the layer velocity to be around 1,000 km/s, consistent with that of massive stars called Wolf-Rayet, previously identified as progenitor stars of similar stellar explosions. They modelled both the luminosity behaviour and electromagnetic spectra composition and found the thick layer, rich in silicon and sulphur, to be more massive than that of our Sun but still less than the material ejected in the final explosion.

Heavy elements

The new discovery, the first of its kind, revealed the formation site of the heavy elements and confirmed with direct observations the complete sequence of concentric shells in massive stars. Some stars develop internal “onion-like” layers of heavier elements produced by nuclear fusion, which are called shells. The latest findings have left the astronomy community with new questions: what process can strip stars down to their inner shells? Why do we see lighter elements if the star has been stripped to the inner shells?

This new supernova type is clearly another curveball thrown by the Universe to the scientists. The energy and the layers composition cannot be explained with the current massive star evolution theory. In the framework of mass loss driven by wind (a continuous stream of particles from the star), a star stripped down to the region where heavy elements form is difficult to explain.

A possible explanation would require invoking an unusual scenario where SN2021yfi actually consists of two stars – a binary system. In this case, the stripping down of the principal star would be carried out by a strong stellar wind produced by the companion star.

An even more exotic explanation is that SN2021yfi is an extremely massive star, up to 140 times that our Sun. Instabilities in the star would release very massive shells at different stages of its evolution. These shells would eventually collide with each other while the star collapsed into a black hole, leading to no further material released into the cosmos during the explosion.

To improve our understanding of stellar evolution, we would need to observe more such objects. But our comprehension could be limited by their intrinsic rarity – because the possibility of finding another explosion like SN2021yfi is less than 0.00001%.

This article is republished from The Conversation, a nonprofit, independent news organization bringing you facts and trustworthy analysis to help you make sense of our complex world. It was written by: Cosimo Inserra, Cardiff University

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Cosimo Inserra receives funding from Foundation MERAC (Mobilising European Research in Astrophysics and Cosmology) and the Engineering and Physical Sciences Research Council (EPSRC).