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ING Press Release
19 January, 2022

First Detection of a Supernova Explosion Originating From a Wolf-Rayet Star

A first-of-its-kind exploding star – thought to have existed only in theory – originating from a Wolf-Rayet star was recently discovered by an international team of collaborators led by Avishay Gal-Yam from the Weizmann Institute of Science in Israel. Observations were made with several telescopes around the world, including the William Herschel Telescope (WHT) and through the Isaac Newton Group (ING) Override Programme of targets of opportunity, using the Auxiliary-port CAMera (ACAM) imager and spectrograph.

In the not-so-distant past, the discovery of a supernova – an exploding star – was considered a rare occasion. Today, advanced measuring instruments and analysis methods make it possible to detect an average of 50 explosions every day. On the one hand, the improved techniques used by present-day astrophysicists to spot supernovae may have made these celestial events less of an attraction over the years. On the other, the greater number of observations has also increased the probability that researchers spot rarer types of explosions that have so far existed only as theoretical constructs.

The core of every massive star is fuelled by nuclear fusion, wherein the nuclei of lighter elements fuse together to form heavier elements. The fusion of four hydrogen nuclei results in the formation of a helium atom, while several helium nuclei combined result in the formation of carbon, oxygen, and so on. The last element that will naturally form through nuclear fusion is iron, which is the most stable atomic nucleus.

In normal circumstances, the energy produced at the star's core maintains extremely high temperatures that cause its gaseous matter to expand, thus preserving the fine balance with the force of gravity, drawing the star's mass toward its center. Once the star runs out of elements to fuse and stops producing energy, this balance is disrupted, causing the star to collapse in on itself, which in turn leads to a black hole at the heart of the star and to the star's explosion. This explosion releases into the universe the heavy elements created during the life of the star.

The entire process is naturally very lengthy. The life spans of massive stars are considered relatively short, a few million years at most. A low-mass star, like the Sun, in comparison, has a life expectancy of about 10 billion years. The succesive processes of nuclear fusion at the core of massive stars lead to their stratification, in which the heavy elements are concentrated at the core, with lighter elements in the outer layers.

Wolf-Rayet stars are particularly massive stars that are missing one or more of the external layers that are made up of lighter elements. In this way, instead of hydrogen – the lightest element – the star's surface is characterised by the presence of helium, or even carbon and heavier elements. One possible explanation for this phenomenon is that strong winds blowing due to high pressure at the star's envelope, disperse its outermost layer, thus causing the star to lose one layer after another over several hundred thousand years.

Observations of these stars are snapshots, of a moment during a long evolutionary process. Nonetheless, despite their relatively short life spans and their state of progressive disintegration, the supernova explosion of a Wolf-Rayet star has yet to have been definitely observed.

Analysis of the ever-growing number of supernova discoveries has led to the hypothesis that Wolf-Rayet stars simply don't explode – they just quietly collapse into black holes – otherwise, we would have been able to observe one by now. This hypothesis, however, has just been shattered owing to the discovery, made by the Weizmann group and their international team of collaborators, of a supernova originating from this type of star.

Spectroscopic analysis of the light emitted from the explosion showed spectral signatures that are associated with specific elements. In this way, the researchers were able to show that the explosion contained carbon, oxygen and neon atoms, the latter an element that has not yet been observed spectroscopically in any supernova to date.

Moreover, the researchers identified that the matter emitting light did not in itself participate in the blast but rather originated from the space surrounding the volatile star. This, in turn, supports their hypothesis in favor of strong winds that took part in stripping the star of its outer envelope.

On 12 June, 2019 the team led by Gal-Yam used their target-of-opportunity allocation as part of the Optical Infrared Coordination Network for Astronomy (OPTICON) programme and they requested observations with ACAM. The override programme on the WHT allowed ING to quickly schedule the observations within hours and to obtain a spectrum of the supernova within 5.8 days of maximum. This spectrum subsequently confirmed the weakening and disappearance of the CIII and OIII absoprtion features seen earlier, and made observations consistent with an explosion of a massive Wolf–Rayet star.

The images which led to the discovery of supernova SN2019hgp, and a spectrum showing the detection of key elements, making this a first-of-its-kind supernova discovery. Credit: Weizmann Institute of Science, Israel. Large format: JPG.

Since this observation is the first of its kind, the leader of the discovery team, Avishay Gal-Yam, says that it's not yet certain that all such stars share the same fate. "We can't say at this stage whether all Wolf-Rayet stars end their lives with a bang or not. It might be that some of them do collapse quietly into a black hole. We estimate that the mass that was dispersed during the explosion is equal to that of the Sun or a slightly smaller star; the star that exploded was significantly heavier – having a mass at least ten times greater than that of the Sun. So where did the majority of mass end up?".

Gal-Yam suggests a midway scenario, in which both possible fates are met at the same time: once nuclear fusion is exhausted at the star's core, an explosion takes place that blasts some of the mass into space, while the remaining mass collapses in on itself, forming a black hole. "One thing’s certain, this is not the 'silent' collapse often referred to in the past."

Since this discovery was made, another similar explosion of a Wolf-Rayet star has been observed, implying that this phenomenon is indeed not a unique occurrence. "It is possible that the better our detection and measurement instruments become, the more this type of explosion – today considered rare and exotic – will become a common sight."

About the William Herschel Telescope

Based on observations made with the William Herschel Telescope (WHT) operated on the island of La Palma by the Isaac Newton Group of Telescopes (ING) in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias (IAC). The ING is funded by the Science and Technology Facilities Council (STFC-UKRI) of the United Kingdom, the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO) of the Netherlands, and the IAC in Spain. IAC's contribution to ING is funded by the Spanish Ministry of Science, Innovation and Universities.

Journal article

A. Gal-Yam, R. Bruch, S. Schulze, Y. Yang, D. A. Perley, I. Irani, J. Sollerman, E. C. Kool, M. T. Soumagnac, O. Yaron, N. L. Strotjohann, E. Zimmerman, C. Barbarino, S. R. Kulkarni, M. M. Kasliwal, K. De, Y. Yao, C. Fremling, L. Yan, E. O. Ofek, C. Fransson, A. V. Filippenko, W. Zheng, T. G. Brink, C. M. Copperwheat, R. J. Foley, J. Brown, M. Siebert, G. Leloudas, A. L. Cabrera-Lavers, D. Garcia-Alvarez, A. Marante-Barreto, S. Frederick, T. Hung, J. C. Wheeler, J. Vinkó, B. P. Thomas, M. J. Graham, D. A. Duev, A. J. Drake, R. Dekany, E. C. Bellm, B. Rusholme, D. L. Shupe, I. Andreoni, Y. Sharma, R. Riddle, J. van Roestel & N. Knezevic, 2022, "A WC/WO star exploding within an expanding carbon–oxygen–neon nebula", Nature, 601, 201. Paper: DOI | Nature.

More information

"Astronomers discover first supernova explosion of a Wolf-Rayet star", IAC press release, 12th January 2022.

Avishay Gal-Yam (Weizmann Institute of Science, Rehovot, Israel)

Javier Méndez (ING PR Officer)
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Last modified: 19 January 2022