Red supergiants are the most common final evolutionary stage of stars that have initial masses between 8 and 35 times that of the Sun¹. During this stage, which lasts roughly 100,000 years¹, red supergiants experience substantial mass loss. However, the mechanism for this mass loss is unknown². Mass loss may affect the evolutionary path, collapse and future supernova light curve³ of a red supergiant, and its ultimate fate as either a neutron star or a black hole⁴. From November 2019 to March 2020, Betelgeuse—the second-closest red supergiant to Earth (roughly 220 parsecs, or 724 light years, away)^5,6—experienced a historic dimming of its visible brightness. Usually having an apparent magnitude between 0.1 and 1.0, its visual brightness decreased to 1.614 ± 0.008 magnitudes around 7–13 February 2020⁷—an event referred to as Betelgeuse’s Great Dimming. Here we report high-angular-resolution observations showing that the southern hemisphere of Betelgeuse was ten times darker than usual in the visible spectrum during its Great Dimming. Observations and modelling support a scenario in which a dust clump formed recently in the vicinity of the star, owing to a local temperature decrease in a cool patch that appeared on the photosphere. The directly imaged brightness variations of Betelgeuse evolved on a timescale of weeks. Our findings suggest that a component of mass loss from red supergiants⁸ is inhomogeneous, linked to a very contrasted and rapidly changing photosphere.

红超巨星是最常见的恒星最终演化阶段，其初始质量是太阳的 8 到 35 倍¹。在这个持续大约 100,000 年¹的阶段，红超巨星经历了大量的质量损失。然而，这种质量损失的机制是未知的²。质量损失可能会影响红超巨星的演化路径、坍缩和未来的超新星光变曲线³，以及它作为中子星或黑洞⁴的最终命运。从 2019 年 11 月到 2020 年 3 月，参宿四——距离地球第二近的红超巨星（大约 220 秒差距，即 724 光年）^5,6-经历了其可见亮度的历史性暗淡。^{它的视星等通常在 0.1 到 1.0 之间，其视觉亮度在 2020 年 2 月7}日至 13 日左右下降到 1.614 ± 0.008 星等 7——这一事件被称为参宿四大变暗。在这里，我们报告了高分辨率的观测结果，表明参宿四的南半球在大变暗期间在可见光谱中比平时暗十倍。观测和建模支持这样一种情况，即最近在恒星附近形成了尘埃团，这是由于光球上出现的凉爽斑块的局部温度下降所致。直接成像的参宿四亮度变化在数周的时间尺度上演变。我们的研究结果表明，红超巨星^{8质量损失的一个组成部分}是不均匀的，与对比鲜明且变化迅速的光球层有关。