# Betelgeuse (Alpha Orionis)

Betelgeuse (Alpha Orionis) is a star in the constellation Orion, magnitude 0.5.

**A red supergiant so vast it would swallow the orbit of Mars — and one day it will explode.**

If Betelgeuse took the Sun’s place, its surface would reach beyond the asteroid belt, engulfing the orbits of Mercury, Venus, Earth and Mars. It is a red supergiant of between ten and twenty solar masses, less than 10 million years old — a star that has lived fast. Within about 100,000 years it should explode as a supernova, shining as brightly as a half Moon for more than three months, with no harm to life on Earth.

Late in 2019 it began to fade, and by February 2020 it had lost about two-thirds of its light. Hubble observations suggest why: the star threw out a vast burst of gas, which cooled into dust and veiled part of its face. It is a restless [variable star](https://astropilot.space/glossary/variable-star) anyway, pulsing on a main period of about 400 days.

Its name is a medieval slip. The Arabic yad al-jawzā’, ‘the hand of al-Jawzā’’ (Orion), was misread in the 13th century — one letter for another — and Europe ended up with Betelgeuse.

Betelgeuse (Alpha Orionis) is 500 light-years away: the light we see from it set off around 1525, around the time Copernicus put the Sun at the centre of the Solar System.

## Did you know?

- **The first star measured.** In 1920 Albert Michelson and Francis Pease measured Betelgeuse’s apparent width with an interferometer on the Mount Wilson telescope — the first star beyond the Sun whose disc was measured.
- **Her bracelet.** Betelgeuse probably has a small companion orbiting close in. Its discoverers named it Siwarha, Arabic for ‘her bracelet’; the name became official in September 2025.
- **Outshining Rigel.** John Herschel saw Betelgeuse outshine Rigel in 1837 and 1839. Such swings may explain why Johann Bayer labelled it alpha, ahead of the usually brighter Rigel.
- **A runaway.** Born among the stars of Orion’s Belt in the Orion OB1 association, Betelgeuse was flung out. It now ploughs through space at 30 km/s, piling up a bow shock of gas ahead of it.

The orange-red shoulder of Orion, a vivid contrast with blue-white Rigel at the opposite corner. Its brightness wanders: compare it with Rigel, Procyon and Aldebaran every few weeks to follow its slow changes.

## When and where to see it

It is best placed in the evening sky around January, when it crosses the meridian at about 10 pm local time (around midnight in December). It can be seen from everywhere north of 83°S and never sets north of 83°N.

At its highest it stands 47° up from 50°N, 62° up from 35°N, 83° up from the equator, 48° up from 35°S.

## Facts

- Type: Star
- Constellation: Orion
- Designations: Alpha Orionis, α Ori, 58 Ori, HR 2061, HD 39801, HIP 27989
- Magnitude: 0.5
- Position: RA 5h 55.2m, Dec +7° 24′ (J2000)
- Spectral type: M1-M2Ia-Iab
- Colour: orange-red (B−V 1.85)
- Brightness: varies from magnitude 0 to 1.6
- Distance: 500 light-years
- Discovered: Known since antiquity; John Herschel noted its changes in brightness from 1836

Open AstroPilot to see when Betelgeuse (Alpha Orionis) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view. https://astropilot.space/object/betelgeuse