Betelgeuse, also known by its Bayer designation Alpha Orionis (shortened to α Orionis or α Ori), is the ninth-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 0.0 and 1.3, the widest range of any first-magnitude star. Betelgeuse is one of three stars that make up the Winter Triangle, and it marks the center of the Winter Hexagon. The star's name is derived from the Arabic إبط الجوزاء Ibt al-Jauzā' or يد الجوزاء Yad al-Jauzā', meaning "the hand of Orion".
The star is classified as a red supergiant of spectral type M1-2 and is one of the largest and most luminous observable stars. If Betelgeuse 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 range from 5 to 30 times that of the Sun. Its distance from Earth was estimated in 2008 at 640 light-years, yielding a mean absolute magnitude of about −6. Less than 10 million years old, Betelgeuse has evolved rapidly because of 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/s, 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. An observation by the Herschel Space Observatory in January 2013 revealed that the star's winds are crashing against the surrounding interstellar medium.
In 1920, Betelgeuse became the second star (after the Sun) to have the angular size of its photosphere measured. 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 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 these 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.
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