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Could the giant star Betelgeuse really have already exploded?

14.6.2023

Translation: machine translated

It has been rumoured for some time that Betelgeuse will soon go supernova. Now there are new indications that it may have already collapsed.

More and more speculation has recently centred on whether the red giant star Betelgeuse in the constellation Orion could be about to explode in a supernova. One indication of this was that it flickered strangely - sometimes appearing brighter and sometimes dimmer than usual. Around three years ago, the all-clear was given.

However, a research team led by Hideyuki Saio from the University of Tokyo has now presented a new study, which has been published in advance on the ArXiv preprint server. It is to be published in the journal "The Monthly Notices of the Royal Astronomical Society", but still has to go through the peer review process. In it, the astronomers come to the conclusion that the end of Betelgeuse could actually be closer than previously thought. The dead star would then shine for a few weeks as a striking beacon that would be brighter than our neighbouring planet Venus - the third brightest object in the sky after the sun and moon! Even now, Beteigeuze can be seen in the daytime sky, but when it explodes, it will be even more spectacular.

Two to three massive stars per galaxy and century should explode in a supernova. In such a gravitational collapse, the inner core of the giant collapses because the fusion of the chemical elements there comes to a standstill. In the past 400 years, nothing has happened in this respect in our home galaxy: a supernova is therefore long overdue in purely mathematical terms.

Massive stars go through all the fusion cycles of nuclear fusion. Due to the high temperature and density inside stars, the atomic nuclei of chemical elements fuse in a fixed order. In the case of our lightweight sun, this is essentially hydrogen, the lightest chemical element. In stellar giants with a few tens of solar masses, even iron can be produced towards the end of their lifetime. Fusion cannot go any further because no more energy is released beyond this point. The consequence: the star's central heat source dries up, the gas and radiation pressure inside drops and the giant collapses under its own gravity.

In 1987, this happened in the Large Magellanic Cloud, our nearest neighbouring galaxy 170,000 light years away. Despite the enormous distance, the collapsed star called Sanduleak -69° 202a, a blue supergiant with 17 solar masses, flared up brightly and was easy to see with small telescopes.
Betelgeuse is the main star in the constellation Orion. It normally shines with an average brightness of 0.5 magnitudes and weighs in at almost 20 solar masses. In 2019 and 2020, it caused quite a stir because its brightness decreased in a mysterious way ("great dimming"). Was a supernova imminent? The all-clear was quickly given, because only a dust cloud passing directly in front of Betelgeuse had dimmed the star.

In fact, however, it cannot be ruled out that Betelgeuse could light up as a supernova in a few tens of years. Saio and his team are now presenting new arguments in favour of this. They looked at the light curves of the giant star. They found evidence of radial pulsations of the star. Four observed pulsation periods could be substantiated with models. The team linked one of them to carbon burning in the interior of Betelgeuse. The researchers therefore came to the conclusion that the carbon burning phase is in principle complete. Betelgeuse is now entering an unstable phase and could soon collapse.

When this happens, everything happens very quickly: the collapse takes place on a timescale of milliseconds. After the collapse, the rebound from the dense stellar core, together with myriads of neutrinos that are created, drives the bright supernova. It is even possible that this has already happened, as we will only be able to see its stellar explosion on Earth around 650 years after the collapse due to Betelgeuse's distance.

Gravitational wave researchers are also very interested in the explosion because the supernova should shake space-time so violently that Einstein space-time waves would be detected for the first time by a core-collapse supernova. It remains exciting.

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Cover image: © ESO/M. Montargès et al / The view of Betelgeuse from SPHERE in December 2019 / CC BY 4.0 CC BY (detail) Betelgeuse is close enough to us that it appears extended rather than point-like. The red supergiant can already be easily recognised in the sky. When it shines as a supernova, it will be even more spectacular.

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Could the giant star Betelgeuse really have already exploded?

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