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Betelgeuse (from Arabic:إبط الجوزاء Ibṭ al-Jauzā’, meaning "the armpit of Orion") is a large red supergiant star in the Orion constellation. It is usually the ninth-brightest star in the night sky, and the second-brightest in Orion.
It is also one of the largest and most luminous observable stars. It has a radius between 950 to 1,000 times wider than the Sun. This means it is 1.322 to 1.392 billion kilometers in diameter. Betelgeuse was the first star other than the Sun whose size was known. The star's distance from Earth is about 640 light-years. Its average absolute magnitude is about −6.02.
Betelgeuse is less than 10 million years old. It evolves rapidly because of its high mass. It is moving through the interstellar medium at a speed of 30 km/s, creating a shock wave over 4 light-years wide.
An observation by the Herschel Space Observatory in January 2013 showed that the star's winds are crashing against the surrounding interstellar medium. Betelgeuse is one of the red supergiant stars to have a bow shock: others are Mu Cephei and IRC-10414. Betelgeuse can be seen at the upper left in Orion.
Betelgeuse is also called Alpha Orionis (α Orionis, abbreviated Alpha Ori, α Ori).
- Davis, Kate (AAVSO Technical Assistant, Web) 2000. Variable star of the month: Alpha Orionis. American Association of Variable Star Observers (AAVSO). 
- Harper, Graham M. et al. 2008. A new VLA-Hipparcos distance to Betelgeuse and its implications. The Astronomical Journal 135 (4): 1430–40. 
- Levesque E.M. 2010. The physical properties of red supergiants. Astronomical Society of the Pacific. 425 Hot and Cool: Bridging Gaps in Massive Star Evolution ASP Conference Series: 103. 
- "Betelgeuse braces for a collision". ESA. 2013-01-22. http://www.esa.int/Our_Activities/Space_Science/Betelgeuse_braces_for_a_collision. Retrieved 2013-01-23.