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Giant star Betelgeuse
Photo: Alan Dyer / StockTrek Images / imago images
The unusual astronomical phenomenon could even be seen with the naked eyes: In the winter of 2019, the brightness of Betelgeuse in the constellation of Orion decreased by up to two thirds for several weeks.
That excites astronomers as well as hobby stargazers.
Because some saw it as an indication of an imminent supernova.
To witness the consequences of such an explosion of a star would be a sensation.
Because when Galileo Galilei presented his telescope developed for celestial science in 1609, the last chance to document the death of the stars in our Milky Way was already gone.
Johannes Kepler described the last supernova in our galaxy as early as 1604.
Since then, astronomers have had to look at other Milky Way galaxies.
But the hope of seeing such a brief flash of light in the sky even without a telescope was not fulfilled for the time being.
The explanation: The reduced radiance that usually precedes the phenomenon was due to other causes at Betelgeuse.
The darkening of the super star was triggered by a cloud of dust that had pushed itself in front of the red giant. Researchers around Miguel Montargés from the Paris Observatory have now announced new details. Accordingly, a large, cool spot triggered the dust veil, as an international team of researchers reports in the journal Nature after evaluating data from the Very Large Telescope of the European Southern Observatory in Chile.
The observations show what exactly happened when this spot appeared: The temperature of the star's surface, which is normally around 4000 degrees Celsius, decreased by up to 500 degrees. The dark spot took up up to four fifths of the star disc visible in the telescope. "For the first time we see how the appearance of a star changes over a period of weeks," says Montargés.
The research team suspects that the cool, dark spot was merely the side effect of a large ejection of matter from the star.
And by cooling the surface, some of the ejected matter could condense into dust.
So it is the combination of a large, cool spot on the star's surface and a resulting haze of dust that was responsible for the great darkening of Betelgeuse, according to Montargés and his colleagues.
Researchers at the Leibniz Institute for Astrophysics in Potsdam had already come to similar findings based on the evaluation of data from the Hubble space telescope.
The current study suggests that the scientists were right.
In April 2020 Betelgeuse shone again in the usual brightness.
But Montargés does not give the all-clear with a view to an imminent explosion of the star.
Too little is known about this last phase of stellar evolution: "Although Betelgeuse's current behavior is not a harbinger of its doom, it remains possible that the star may explode without warning."
Bloated giant
Betelgeuse is a red giant star and is 724 light years away from us.
With good telescopes, astronomers can even see details on the surface.
Like all stars and our sun, it draws its energy from the fusion of atomic nuclei for millions of years.
But when the gigantic fusion machine runs out of fuel and the end of its life approaches, the massive star inflates.
Betelgeuse is about 900 times larger than our sun.
During this phase, stars release a lot of gas into their surroundings and eventually explode as a supernova - leaving behind a neutron star or a black hole.
joe / dpa