What caused the dramatic brightness drop observed in late 2019 and early 2020?

Answer

A surface mass ejection forming a dust cloud

The 'Great Dimming' was caused by the star ejecting a massive chunk of surface material; this material cooled and formed a large dust cloud that temporarily blocked about a quarter of the star's visible light.

What caused the dramatic brightness drop observed in late 2019 and early 2020?
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