How Betelgeuse changed in brightness in 2019–2020
This animation combines four real images of the red supergiant star Betelgeuse, the first taken in January 2019 and the others taken in December 2019, January 2020 and March 2020, during the star’s unprecedented dimming. All images, which allow us to resolve the surface of the star, were taken with the SPHERE instrument on ESO’s Very Large Telescope.
Astronomers were puzzled to see Betelgeuse’s brightness decrease, but later found that it was caused by a “dusty veil” that partially concealed the southern part of the star.
Credit : ESO/M. Montargès et al./L. Calçada
Betelgeuse is the second brightest star in the constellation Orion (the hunter). It is a red supergiant and one of the largest stars known, about 1,000 times larger than the Sun. It is also one of the brightest stars in our sky, shining like more than 100,000 suns combined. Such properties imply an imminent end for this stellar mastodon. With an age of only a few million years, Betelgeuse is already nearing the end of its life and is destined to explode into a supernova.
During the winter of 2019-2020, Betelgeuse captured the attention of the astronomical community, becoming much less bright than it had been for at least 150 years. To understand the reason, an international team of astronomers led by a researcher from Paris Observatory - PSL at LESIA (Observatoire de Paris - PSL / CNRS / Sorbonne University / University of Paris), using the SPHERE instrument at the ESO Very Large Telescope (Chile), and GRAVITY on its interferometer, the VLTI, conducted a campaign of observations.
It was by comparing two images of Betelgeuse, one taken in December 2019, the other older one obtained in January 2019, that the team had noticed a darkening on the southern part of Betelgeuse, without being able to determine its origin.
The team continued its observations relentlessly and highlighted a minimal activity of the star at the end of January 2020 followed by a return to its nominal level of brightness in March 2020 : "In the space of a few weeks, it was an unprecedented opportunity to witness changes of a magnitude never before observed on the surface of the star," comments Miguel Montargès, a post-doctoral fellow at the Paris Observatory - PSL and first author of the study.
By publishing an article on June 17, 2021 in the journal Nature, the team reveals, through its unpublished observations of the star, the origin of its decrease in brightness. Two successive phenomena have contributed to it : a local cooling has occurred on its surface and has caused a formation of dust ; this has taken place in front of the star, an area in which was also a cloud of gas previously ejected by the star. This new dust constitutes the famous stardust. "The exceptional dimming of Betelgeuse allowed us to see this immense star expelling its matter into space. This is an essential phenomenon for understanding the evolution of matter on a cosmic scale. "underlines Pierre Kervella, astronomer of the Observatoire de Paris - PSL at LESIA and co-author of the study.
Observed in early 2020, the decrease in brightness of Betelgeuse could have foreshadowed its imminent explosion into a supernova. It would then have become visible in daylight, and would have been as bright as the full moon at night. Such an event has not been observed in our Galaxy since the XVIIᵉ century. Needless to say, the means of observation were not as accurate then as they are now.
While the researchers still do not know exactly what the warning signs of a supernova are, their publication definitively confirms that Betelgeuse’s diminishing luminosity is not the harbinger of its imminent agony.
For Emily Cannon of the KU Leuven in Belgium, second author of the publication, "it was fascinating to follow day after day the evolution of the light curve of this star and to receive its images. This star, so familiar, appeared different for a few months, breaking with the constancy of the firmament. According to Miguel Montargès, "this breakthrough will allow us to better understand the fate of massive stars - progenitors of supernovae, neutron stars and black holes - but also how they enrich the Galaxy with heavy elements that can be at the origin of the formation of new planets. "
These results are the result of the unparalleled precision delivered by the SPHERE and GRAVITY instruments, to the construction of which the teams of the Observatoire de Paris - PSL and the CNRS at LESIA, have been closely associated. LESIA is now developing new instruments that will allow to continue to follow Betelgeuse with more details, such as the adaptive optics imager MICADO that will equip the 39 m diameter Extremely Large Telescope (ELT), under construction by ESO in Chile.
Bibliography
This research work was the subject of a paper entitled "A dusty veil shading Betelgeuse during its Great Dimming" by Miguel Montargès et.al. to be published on June 17, 2021 in the journal Nature.
