Observations with the Hubble Space Telescope indicate that the unexpected ‘eclipse’ of the bright star Betelgeuse last winter was likely caused by the star’s release of a massive amount of hot material into space. A cloud of dark dust has formed on it, which has dimmed the light of Betelgeuse.
Betelgeuse is a red supergiant: a star that has become very swollen in its core due to changes in the fusion reactions. In October 2019, this star began to dim, and in mid-February of this year, the giant star was more than three times fainter than normal. Then its brightness returned to normal values.
Various explanations have been provided for this unexpected event. For example, Betelgeuse would have ejected clouds of dust or colossal ‘sunspots’ would have formed on its surface.
The Hubble observations now lead astronomers to conclude that there was indeed dark dust around the star. This obscuring material is said to have been created by a colossal convection cell rising from the interior of the star and reaching the cooler outer layers of its atmosphere.
The spectra recorded by the space telescope in the last quarter of 2019 indeed show that part of Betelgeuse initially became noticeably hotter and that the material in question moved outwards. The (initially) hot material was two to four times brighter than the star’s normal brightness and cooled to dark dust millions of miles from the star.
Astronomers are still in the dark about the cause of the eruption. Possibly the ascension of the convection cell was helped by the pulsations that Betelgeuse already shows. At the time of the eruption, the star was just swelling. It is not inconceivable that the phenomenon is a precursor to the supernova explosion that will eventually await the star. But it may take many thousands of years before it actually happens.
Currently, Betelgeuse cannot be observed with the Hubble Space Telescope because it is too close to the sun. However, the star can still be viewed with STEREO-A, a NASA satellite that normally observes the sun. Measurements from this satellite show that the brightness of Betelgeuse is declining again. This is surprising, because the star’s ‘brightness dips’ usually occur at intervals of well over a year. Astronomers are therefore looking forward with more than average interest to next autumn, when Betelgeuse will be visible again. (EE)