Its name may not ring a bell, but you must have already noticed it, when you rolled your eyes one evening: sporting a beautiful orange hue, it is the sixth brightest star visible in mainland France.
“When I started to take an interest in the stars, around the age of 10, Betelgeuse was the first one I managed to identify in the sky.
Our house in Collioure was oriented to the south and in winter, when I returned from college, I could see the three stars aligned above the garden of Orion's belt and her, a little higher up.
Miguel Montargès, the Paris Observatory astrophysicist who remembers these first appearances, has now unraveled one of the secrets of the red giant: in an article published Thursday in the journal Nature, his team finally explains why this star is so sparkling abruptly faded just over a year and a half ago, setting the scientific community in turmoil.
Live training of "stardust"
At the end of November 2019, the researcher, who at the time worked for the Institute of Astronomy at the University of Louvain, in Belgium, received an e-mail warning him of a curious decrease in luminosity of Betelgeuse. He doesn't pay attention: after all, the star of the constellation Orion is a variable star. Its radiance evolves in two cycles, of approximately 400 and 3000 days. In mid-December, however, the rapid drop in magnitude convinced him to point the Very Large Telescope (VLT) of the European Southern Observatory (ESO), located in Chile, towards the giant. The data confirms that something is happening.
In the media, a little music begins to be heard: what if Betelgeuse was at the end of its life, ready to explode into a supernova? Attractive trail. Scientists could have learned a lot, the last phenomenon of its kind having been observed in ... 1604, in the time of Johannes Kepler! But Miguel Montargès does not believe it for a single second: “It is a hypothesis that I did not consider at all and I did not even understand why it came out. I don't understand how you can relate a loss of luminosity to an impending supernova. Especially since Betelgeuse is much too young to be able to be at the end of his life. Among researchers who are really working on the subject, we have not even discussed it. "
So what happened? According to Miguel Montargès' team, which relies on several images captured until March 2020, Betelgeuse expelled a large amount of gas before part of its surface began to cool and darken, due to of its variability. This cooling would have caused the gas to condense into a cloud of dust which would have veiled the star for a few weeks. “Red supergiants are massive stars surrounded by a large shell of gas and dust. We know that they are constantly losing material, but we don't know how that happens. We may have seen the unfolding of the event. And even the live formation of "stardust", similar to that which participated in the appearance of life on Earth,because the stars are real factories of elements such as carbon and oxygen.
And what has become of this dust around Betelgeuse? Because of Covid-19, we'll never know. It is a regret of Miguel Montargès: “We should have continued to observe it in June 2020. But from the end of March all the observatories in the world have closed because of the pandemic. The last image published in March 2020 was obtained three days before the closure of the VLT, when Betelgeuse was turning very bright again. According to the researcher, it is possible that the warming of the star caused the destruction of the cloud. He pleads for more observations of the red giants, in order to find other examples of the phenomenon.