Unusual stellar explosion will expand our understanding of stars

A study led by the University of Turku discovered a stellar explosion rich in oxygen and magnesium. This discovery will help to inform our understanding of the later life stages of massive stars.

These types of supernova explosions are produced during the deaths of massive stars, with the elements seen in a stellar explosion reflecting the composition of the dying star at the time of the explosion.

“Stars are glowing balls of gas of mostly hydrogen, the lightest element in nature. They shine by fusing atomic nuclei together to create heavier elements and energy,” explained Hanindyo Kuncarayakti, a research fellow at the Academy of Finland and the Department of Physics and Astronomy at the University of Turku, Finland.

How are massive stars structured?

Massive stars – which result in stellar explosions at their time of death – have around eight times the mass of the Sun or more. Their structures are similar to that of an onion, with layers of different elements inside them. Their outer layers mostly consist of hydrogen; however, within the deeper layers of the star, heavier elements like helium, carbon, and oxygen exist.

Kuncarayakti said: “During its lifetime, a star may lose some, or even most, of its mass. The most common way is through ejecting streams of particles, a process known as stellar winds, which also occur in the Sun.

“Some stars lose their mass very vigorously, and may completely strip all of their hydrogen envelopes. As a result, the inner layers may become exposed. The mass lost by the star may remain in the vicinity of the star, creating circumstellar matter.”

Previously, astronomers have identified supernovae with circumstellar matter that is rich in hydrogen, as well as those rich in helium. Furthermore, in 2021, researchers discovered supernovae with carbon-oxygen circumstellar matter. These different types of objects represent a sequence of stellar envelope stripping and the accumulation of stripped matter around the star, starting from hydrogen, which is the lightest and outermost element.

Understanding how stars lose their mass

As a result of the stellar explosion discovery, Kuncarayakti’s team could possibly extend our understanding of a massive star’s structure and where they lose its mass. Supernova (SN) 2021ocs was observed in a survey using the 8.2m European Southern Observatory (ESO) Very Large Telescope (VLT) in Chile.

“The spectrum looked like nothing we have seen before. It had strong features of oxygen and magnesium, and the object was unusually long-lasting and blue,” Kuncarayakti said.

These observations suggest that the oxygen-magnesium-rich expanding gas from the stellar explosion of SN 2021ocs could potentially be crashing into circumstellar matter. Matter such as this could have been formed by the precursory star, via mass loss only around 1,000 days prior to the supernova explosion.

As such, the observations act like a time machine, probing the dying star’s activities shortly before the final explosion.

Seppo Mattila, Professor of Astronomy at the University of Turku, who also participated in the study, concluded: “By observing new types of supernovae, we gain valuable information about the later stages of life of massive stars.

“This, on the other hand, creates new challenges for our theories on stars’ evolution.”

In addition to Kuncarayakti and Mattila, researchers Takashi Nagao, Claudia Gutierrez, and Rubina Kotak from the University of Turku contributed to the study.

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