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Betelgeuse, also called α Orionis, is a large red star in the constellation of Orion. It is the 12th brightest star in the sky, a red supergiant, and is a vertex of the Winter Triangle.

The name is a contraction of the Arabic Yad al-Jauza`, translated as the "armpit of the central one," indicating its position in the constellation of Orion. Because of its rich reddish color the star has frequently been referred to as the "martial one," and in Astrology portends military or civic honors. Because it is the first star of Orion to rise, the title of "roarer" or "announcer" has been assigned to it by ancient writers. Other names are:

Physical and other characteristics

Observation data
Semiregular Variable, type SR c  
Right ascension (RA, α) (J2000,00) 05 h 55 m 10.29 s
Declination (Dec, δ) (J2000,00) +07° 24' 25.3"
Distance from Earth 427 ± 92 LY (131 pc)
Visual brightness (V)  +0.45m (+0.3m to +0.6m (+1.3m))
Absolute magnitude -5.3m to -5.0m 
Physical characteristics
Mean Diameter (dS = 1,392,000 km) 385,000,000 km | ~ 276.6 dS
2.57 AU | (452 s.u.)
Surface area  
Mass ~ 40 × 1030 kg
Relative mass to the Sun
(mS = 1.9891 × 1030 kg)
~ 20 mS
Surface gravity  
Spectral type M1-2 Ia-Iab (Red Super Giant)
Surface temperature 3600 K
Color (B-V) +1.850 (Orange-red) | (1.500)  
V-I 2.32
Luminosity (LS = 3.827 × 1026 J s-1) ~ 10,000 LS
Orbital characteristics
Period of rotation 
Period ~ 2070 d to 2355 d

Betelgeuse is of great interest astronomically. It is also one of the first stars to have its diameter measured with a stellar interferometer. The diameter was found to be variable, ranging from 290,000,000 km to 480,000,000 km. At maximum diameter, the star would extend out beyond the orbit of Mars if put in the Sun's place.

Astronomers confidently predict that Belegeuse will ultimately undergo a type II supernova explosion. Opinions are divided as to the likely timescale for this event. Some regard the star's current variability as suggesting that it is already in the carbon burning phase of its life cycle, and will therefore go supernova at some time in the next thousand years or so. Skeptics dispute this contention and regard the star as being likely to survive much longer. There is consensus that such a supernova would be a spectacular astronomical event, but would not represent any significant threat to life on Earth given the star's current distance.

In science fiction, Ford Prefect, a character in the Hitchhiker's Guide to the Galaxy, hails from a world "in the vicinity of Betelgeuse."