Betelgeuse's Great Dimming

Summary

In late 2019 and early 2020 Betelgeuse, the red supergiant in Orion, faded to about a third of its usual brightness, and talk of a supernova spread. It recovered by April 2020; the cause was an eruption from its surface, a cooler patch and dust, not the end of the star.

The explanation is agreed in outline. How much of the fading was dust and how much was cooling is still argued, and the star turned out to have something else to reveal: a probable companion, reported in 2025.

What is documented

The star. Betelgeuse is a red supergiant, a massive star late in its life, and a variable one: its brightness normally rises and falls. NASA describes a pulsation of about 400 days that astronomers had followed for nearly two centuries. It also varies on a longer cycle of about six years.

The fading. Betelgeuse began to dim noticeably in late 2019, and the change was visible to the naked eye. It reached its faintest around 7 to 13 February 2020, at visual magnitude 1.614 ± 0.008, a fall of about 1.2 magnitudes. NASA described the star as having lost more than two-thirds of its brilliance by mid-February. By April 2020 it was back to its normal brightness.

What came first. Andrea Dupree and colleagues had been taking ultraviolet spectra of Betelgeuse with the Hubble Space Telescope. In September, October and November 2019 the spectra showed dense, heated material moving outward through the star's southern hemisphere at about 200,000 miles (320,000 kilometres) per hour. According to the team, the material travelled millions of miles from the surface, cooled enough to form dust, and blocked light from about a quarter of the star's surface.

The images. A team led by Miguel Montargès imaged the surface of Betelgeuse with the SPHERE instrument on the European Southern Observatory's Very Large Telescope in December 2019, January 2020 and March 2020. During the dimming the star's southern hemisphere was ten times darker than usual in visible light. "For once, we were seeing the appearance of a star changing in real time on a scale of weeks," Montargès said. In Nature in June 2021 the team concluded that a clump of dust had formed close to the star, over a patch of its surface that had cooled, after a bubble of gas had been ejected.

The eruption. In 2022 Dupree's team described the event as a surface mass ejection: Betelgeuse lost part of its visible surface in 2019, material weighing several times as much as the Moon, about 400 billion times the mass of a typical coronal mass ejection from the Sun. The star's 400-day pulsation was disrupted, and the star was still settling afterwards.

The measurements that disagree. The studies do not agree about the dust. In March 2020 Emily Levesque and Philip Massey argued that a drop in temperature alone could not explain the dimming. In December 2020 Graham Harper and colleagues argued, from the star's photospheric temperatures, that no new dust was required. In 2021 Sofya Alexeeva and colleagues reported a drop of at least 170 K in the star's effective temperature on 31 January 2020, which they attributed to a large dark spot, and concluded that dust obscuration was unlikely to be the cause. In 2022 Daisuke Taniguchi and colleagues used images from the Japanese weather satellite Himawari-8, whose pictures of the Earth include Betelgeuse in the background, and found that a cooling of about 140 K and extra dust had each contributed about 0.6 magnitudes, almost equal shares.

The companion. In 2024 two independent studies, by Jared Goldberg and colleagues and by Morgan MacLeod and colleagues, proposed that Betelgeuse's roughly six-year cycle is caused by a companion star in orbit around it. In July 2025 Steve Howell and colleagues reported a probable detection with speckle imaging on the 8.1-metre Gemini North telescope. Their 2020 images, taken during the Great Dimming when the companion was predicted to lie directly in line with the star, showed no companion, as expected; their 2024 images showed a faint source 52 milliarcseconds from Betelgeuse, about 6 magnitudes fainter at a wavelength of 466 nanometres, at a significance of 1.5 standard deviations. The discoverers proposed the name Siwarha, Arabic for "her bracelet". In November 2025 a Hubble search by Goldberg's team found no ultraviolet signature of the companion and, combining this with X-ray data suggesting that the companion is a young star still forming, excluded masses of 1.5 solar masses or more. In February 2026 Dupree's team reported absorption and outflows in Betelgeuse's atmosphere that increase after the companion passes in front of the star, consistent with a trailing wake. In July 2026 Montargès and colleagues reported that the SPHERE instrument had detected the candidate at high significance in December 2024, 52.32 ± 0.18 milliarcseconds from the star; if the two stars are the same age, they estimate it to be a young main-sequence star of 2.6 to 3.1 solar masses. They note that formal confirmation awaits a second observation. In September 2026 Lynn Matthews and colleagues reported radio observations from 2021 to 2026 in which the elongation of Betelgeuse's image varied with the companion's orbital phase, which they found consistent with periodic disturbance of the star's atmosphere by the companion.

Leading explanations

A surface eruption, a cooler patch and dust (the leading explanation, peer-reviewed). Hubble saw material leaving the star's southern hemisphere from September to November 2019, as the fading began, and the Very Large Telescope saw that hemisphere darken. Montargès and colleagues modelled a dust clump forming over a cool patch; Dupree's team traced the dust to a surface mass ejection. Nothing in this is exotic: red supergiants shed mass, and this was an unusually large, well-observed episode.

Cooling alone, without new dust (peer-reviewed, contested). Harper and colleagues and Alexeeva and colleagues found that a cooler surface could account for the fading. Levesque and Massey, and the Himawari-8 study, found that it could not account for all of it. The balance between cooling and dust is the part of the case that remains open.

A tidal encounter (proposed, not sufficient). Hailey Aronson, Thomas Baumgarte and Stuart Shapiro calculated in 2022 how much a close fly-by of another body could darken a star by distorting it, and concluded that the effect was likely not large enough to explain the Great Dimming by itself.

An imminent supernova (rejected). ESO stated in 2021 that the Great Dimming was not an early sign that the star was heading for its end.

Current status

Later explained. The Great Dimming was a surface event: material ejected in 2019, a cooler region of the surface and, on most analyses, dust that formed from the ejected gas. The competing analyses differ on proportions, not on the kind of cause, and none of them involves the star's core or an approaching supernova. A resolution of the cooling-versus-dust question would refine the record without changing its status.

The companion is live science. It is proposed to explain Betelgeuse's six-year cycle, and two studies link it to disturbances in the star's atmosphere, but its detection awaits formal confirmation and the estimates of its mass depend on assumptions about its age. Whether the companion had any part in the 2019 eruption has not been established.

Last reviewed: September 2026. Records are provisional. Where the evidence changes, the entry changes. Found an error? Tell us.