Betelgeuse: Why Did the Red Supergiant Suddenly Dim?

Betelgeuse, the red giant shining in Orion, surprised astronomers with an unexpected event: between 2019 and 2020 its brightness dropped by 40%, a phenomenon dubbed the "Great Dimming". Observations from space and ground telescopes revealed that the star expelled a massive amount of surface material, forming a dust cloud that temporarily blocked visible light. After the dimming peak, Betelgeuse began a slow recovery, yet its brilliance hasn't fully returned to pre‑dimming levels.

Computer simulations suggest these episodic mass ejections stem from internal convulsions, where huge gas bubbles rise and explode. Another mystery is the roughly 2100‑day periodicity seen in brightness and radial velocity, possibly hinting at a hidden stellar companion. Astronomers are trying to directly detect this potential satellite using high‑resolution interferometry.

Betelgeuse remains a natural laboratory for studying red supergiants, stars that live only a few million years before exploding as supernovae. Each new observation brings us closer to understanding how these colossal stars die and their impact on the cosmos.

Betelgeuse's dimming has been scrutinized through spectroscopic studies that revealed significant changes in silicon (Si I) and titanium (Ti I) absorption lines, indicative of photospheric cooling. Interferometric measurements with the Very Large Telescope Interferometer (VLTI) showed a reduction of the apparent angular diameter by about 5%, consistent with the formation of a silicate cloud at temperatures of 800–1000 K.

Three‑dimensional hydrodynamic simulations based on supercritical convection models indicate that large convective cells can generate shock waves that, upon reaching the surface, trigger the ejection of masses on the order of 10⁻⁴ M☉. This process is amplified by differential rotation, which may favor asymmetric dust‑cloud structures.

Regarding the 2100‑day periodicity, long‑term light‑curve analysis (AAVSO) combined with radial‑velocity data (Gaia DR3) suggests a sinusoidal modulation possibly arising from a binary orbit with a period of roughly 5.8 years. A companion of 1–3 M☉, likely a B‑type star or a compact object, could explain the observed radial‑velocity variations. However, direct observations with the CHARA Array have yet to confirm a secondary luminous point, probably due to the strong infrared emission of the surrounding cloud.

Future observations with the James Webb Space Telescope (JWST) and the Laser Interferometer Space Antenna (LISA) could detect weaker infrared or gravitational‑wave signals, providing decisive evidence on the nature of the possible companion and the mass‑ejection mechanism. These data will be crucial to refine evolutionary models of red supergiants and to predict more accurately the timing of their final supernova explosion.