Scientists believe they have solved the mystery behind why one of the brightest stars in the sky suddenly became visibly darker.

Betelgeuse - a red supergiant located in the constellation of Orion - lost more than two-thirds of its brilliance, sparking fears the star was coming to the end of its life and could be about to explode.

Astronomers were left puzzled by the discovery, dubbed the "great dimming" in late 2019.

But an international team of researchers now believes a cloud of stardust was responsible.

The team reached its conclusion after analysing images of Betelgeuse over the years using the European Southern Observatory's (ESO) Very Large Telescope (VLT) in Chile.

They revealed in scientific journal Nature how the event was triggered by the formation of stardust obscuring half of Betelgeuse.

Miguel Montarges, from the Observatoire de Paris, France, and KU Leuven, Belgium, who is lead author on the study, added: "For once, we were seeing the appearance of a star changing in real time on a scale of weeks.

"We have directly witnessed the formation of so-called stardust."

But the dimming of Betelgeuse - which is about about 500 light-years from Earth - only lasted for a few months before the star returned to its original level of illumination in April 2020.

The surface of the star is subject to regular changes as giant gas bubbles move, shrink and swell - in a phenomenon known as pulsation.

Scientists believe that Betelgeuse ejected a large gas bubble during the dimming.

Shortly afterwards, as the surface of the star partially cooled, heavier elements in the gas, such as silicon, condensed into solid dust.

Professor Stefan Kraus from the University of Exeter, one of the study authors, added: "Ageing stars such as Betelgeuse have long been suspected to churn out flecks of dust, either through a constant wind or more localised surface ejections.

"Here, we see that Betelgeuse has ejected a massive dust cloud that has obscured half of the star's surface while drifting away into space."

The scientists will continue examining the star, which is around 1,000 times the size of the sun, in a bid to witness another gas bubble being ejected.

Study author and astronomer at the Harvard-Smithsonian Center for Astrophysics in the US, Andrea Dupree, said the research "affects our understanding of the evolution of all stars".