Betelgeuse’s brightness dimmed, and we finally understand why
Astronomers have discovered the cause of the ‘Great Dimming of Betelgeuse’: a cloud of dust partially concealing it from us.
As one of the largest stars visible to the naked eye, the red supergiant star Betelgeuse is a familiar sight to professional and amateur astronomers alike. That’s perhaps why it was so surprising when the star’s brightness started to drop in October 2019.
By February 2020, the star, which marks the right shoulder in the constellation of Orion, had hit a record low of only 40 per cent of its usual brightness.
This dramatic drop sparked speculation that Betelgeuse was about to go supernova s that is, reach the end of its life as a red supergiant, collapse,
and then bounce into a Ƃery explosion so bright we’d even be able to see it in the daytime. It wasn’t immediately clear to astronomers whether or not this was the case, since a supernova hasn’t been observed in our Galaxy since astronomer Johannes Kepler saw one in 1604.
But it never happened, and by April 2020, Betelgeuse was back to its normal brightness. Now, images of the star, taken with the European Southern Observatory’s Very Large Telescope, along with data from the GRAVITY instrument, have revealed what happened to it.
“We have directly witnessed the formation of so-called stardust,” said Dr Miguel MontargÄs, from the Observatoire de Paris, France, and KU Leuven, Belgium. “For once, we were seeing the appearance of a star changing in real time on a scale of weeks.”
The surface of Betelgeuse is always changing. Giant bubbles of gas grow, shrink and move around within the star, and occasionally it burps one out. Before the Great Dimming began, Betelgeuse released one of these bubbles. Then a patch of the star’s surface cooled down, and this temperature drop allowed the gas to cool enough to condense into solid dust.
This cloud of dust partially concealed Betelgeuse from the Earth, particularly in the southern region.
“The dust expelled from cool evolved stars, such as the ejection we’ve just witnessed, could go on to become the building blocks of terrestrial planets and life,” said 'mily Cannon, a PhD student at KU Leuven. “Looking up at the stars at night, these tiny, twinkling dots of light seem perpetual. The dimming of Betelgeuse breaks this illusion.”