Red giant stars like Betelgeuse frequently undergo changes in brightness, but the drop to 40% of its normal value between October 2019 and April 2020 surprised astronomers.
Red giants exhibit a behaviour known as pulsation, caused by changes in the area and temperature of the star's surface layers.
Instead, the astronomers say, temperature variations in the photosphere - the luminous surface of the star - most likely caused the brightness to drop.
The Betelgeuse star spot would be a hundred times larger than the Sun.
It is a supergiant star growing a super-sized star spot." said co-author Prof Albert Zijlstra from The University of Manchester, UK.