Betelgeuse
Betelgeuse — the red supergiant marking Orion's shoulder, some 500–700 light-years away (the distance is genuinely uncertain). One of the largest stars visible to the naked eye — so vast that, in the Sun's place, its surface would engulf the orbits of Mercury, Venus, Earth and Mars, out to the asteroid belt — it is a semiregular variable famous for its 2019–2020 “Great Dimming” (a dust cloud, not an imminent explosion) and is destined to end as a supernova, astronomically soon. In 2024, speckle imaging with the Gemini North telescope revealed a long-suspected close companion star, nicknamed “Betelbuddy” — a hot young star orbiting only ~4 AU out, thought to drive Betelgeuse's ~6-year brightness cycle. The detection is tentative and awaits confirmation.
Key facts
Science
Physical
- Mass
- 3.58 × 10³¹ kg · 18 M☉
- Radius
- 3.55 AU · 764× wider than the Sun
- Density
- 0.0000569 kg/m³ · 5.7e-08× water
- Spectral type
- M1-2 Ia-ab (red supergiant)
Composition
- By mass
- A cool, colossal red supergiant — so large that, in the Sun's place, it would swallow the orbits of Mercury, Venus, Earth and Mars, out to the asteroid belt — with an extended, unsteady hydrogen/helium envelope over a core fusing heavier elements. One of the few stars whose disk has been directly imaged; its distance is genuinely uncertain (~500-700 ly). It will end as a supernova, astronomically soon. In July 2025, a team led by Steve Howell (NASA Ames) reported the first direct detection of a close companion — provisionally named Siwarha (“Betelbuddy”) — from Gemini North speckle imaging taken in December 2024: a hot, young star of roughly 1.5 solar masses on a ~6-year, nearly edge-on orbit about 4 AU out, likely the source of the star's long ~5.9-year secondary brightness variation. The companion detection is tentative (reported at low significance) and awaits confirmation (ApJ Letters 988, L47).
Temperature
- Surface
- 3,600 K (3327 °C)
Radiation & temperature
- Surface temperature
- 3,600 K (3327 °C)
- Peak wavelength
- 805 nm · near-infrared (mostly invisible to the eye)
Discovery & history
- Method
- Naked-eye star
- Name
- From Arabic (a corrupted 'Yad al-Jauza', the hand of Orion); Alpha Orionis.
Its variability was noted by John Herschel in 1836; in 1920 it became the first star (after the Sun) to have its disk directly measured. Its 2019-20 'Great Dimming' was traced to a dust cloud it ejected.
Images
Images from open archives · credit shown per image