Betelgeuse (/ˈbiːtəldʒuːz/ or /ˈbɛtəldʒuːz/), also known by its Bayer designation Alpha Orionis (α Orionis, α Ori), is the eighth brightest star in the night sky and second brightest in the constellation of Orion. Distinctly reddish, it is a semiregular variable star whose apparent magnitude varies between 0.2 and 1.2, the widest range of any first-magnitude star. Betelgeuse is one of three stars which make up the Winter Triangle and marks the center of the Winter Hexagon. The star's name is thought to be derived from the Arabic يد الجوزاء Yad al-Jauzā' meaning "the Hand of al-Jauzā'", i.e. Orion, with mistransliteration into medieval Latin leading to the first character y being misread as a b.

The star is classified as a red supergiant of spectral type M2Iab and is one of the largest and most luminous known stars. If it were at the center of the Solar System its surface would extend past the asteroid belt, possibly to the orbit of Jupiter and beyond, wholly engulfing Mercury, Venus, Earth and Mars. Estimates of its mass are poorly constrained, but have recently ranged from 5 to 30 times that of the Sun. Its distance was estimated in 2008 at 640 light-years from Earth, yielding a mean absolute magnitude of about −6.02. Less than 10 million years old, Betelgeuse has evolved rapidly due to its high mass. Having been ejected from its birthplace in the Orion OB1 Association—which includes the stars in Orion's Belt—this crimson runaway has been observed moving through the interstellar medium at a supersonic speed of 30 km/sec, creating a bow shock over 4 light-years wide. Currently in a late stage of stellar evolution, the supergiant is expected to proceed through its life cycle before exploding as a type II supernova within the next million years.

Betelgeuse was the first star (after the Sun) to have its photosphere measured in 1920. Since then, researchers have used telescopes with different technical parameters to measure the stellar giant, often with conflicting results. Studies since 1990 have produced an angular diameter (apparent size) ranging from 0.043 to 0.056 arcseconds, an incongruity largely caused by the star's perceived tendency to periodically change shape. Due to limb darkening, variability, and angular diameters that vary with wavelength, many of the star's properties are not yet known with any certainty. Adding to the computational challenges, the surface of Betelgeuse is obscured by a complex, asymmetric envelope roughly 250 times the size of the star, caused by colossal mass loss. Possible stellar companions orbiting within this circumstellar nebula may also contribute to the star's enigmatic behavior.

Read more about BetelgeuseObservational History, Visibility, Properties, Star System

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