One of the last mosaics unearthed in Pompeii beautifully stages an episode of Greek mythology: the catasterization of the hunter Orion, that is to say his transformation into a cluster of stars. Those of us who have not lost our connection with the night sky are bound to know this constellation framed by four very bright stars. The one representing Orion’s left shoulder is called Betelgeuse. However, at the end of 2019, this star suffered a disturbing and significant drop in luminosity, to the point that many media saw it as the harbingers of its imminent death.
A year and a half later, the star is still there, it has regained its usual brilliance and an international team led by French researchers publishes, Wednesday, June 16 in Nature, a study solving the mystery of the “great darkening” of Betelgeuse. To understand this investigation, we must first explain what this celestial object is. It is a red supergiant, a name that astronomers give to a massive star (here about 20 times the mass of the Sun) at the end of its life. Intuition could suggest that the larger a star, the more “fuel” it has and the longer its thermonuclear fusion reactions will last. But it is exactly the opposite that occurs: Betelgeuse and her giant sisters are burning the candle at both ends and their longevity is much less than a modest star like the Sun. When a massive star enters the red supergiant phase, it is only about 100,000 years old.
Veil of dust
First author of the article by Nature, Miguel Montargès, postdoctoral fellow at the Laboratory for Space Studies and Instrumentation in Astrophysics (Paris Observatory / PSL University), knows his Betelgeuse like the back of his hand: “I’ve been studying it for ten years and I did my thesis largely on it. ” When, at the end of 2019, the first messages arrive saying that the luminosity of the star is decreasing, he is frankly skeptical: “I thought it was just a low of her 400 day cycle, a little steeper than normal. Suddenly, I was quite a wait-and-see. But when, on December 19, I saw that the curve was going down sharply, I requested, through the emergency procedure, the observation time from the VLT ”, the Very Large Telescope of the European Southern Observatory (ESO), installed in Chile.
Thanks to its relative proximity to the cosmic scale (724 light years) and its gigantic diameter (900 times that of the Sun), Betelgeuse is indeed part of the handful of stars whose surface details can be observed. “Thanks to the VLT Sphere instrument”, explains Eric Lagadec, astrophysicist at the Côte d’Azur Observatory and co-author of the study. ” When we observed it in 2015 with Sphere, it was the first time that we made a direct image of a star ” other than the Sun.
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