Betelgeuse ^{Wikipedia permalink/938810722}

Betelgeuse is usually the eleventh-brightest star in the night sky and, after Rigel, the second-brightest in the constellation of Orion. It is a distinctly reddish semiregular variable star whose apparent magnitude, varying between +0.0 and +1.3, has the widest range displayed by any first-magnitude star. At near-infrared wavelengths, Betelgeuse is the brightest star in the night sky. Its Bayer designation is α Orionis, Latinised to Alpha Orionis and abbreviated Alpha Ori or α Ori.

Classified as a red supergiant of spectral type M1-2, Betelgeuse is one of the largest stars visible to the naked eye; imagined as being at the center of the Solar System, its surface would lie beyond the asteroid belt, it would engulf the orbits of Mercury, Venus, Earth, Mars, and possibly Jupiter. Nevertheless, there are several larger red supergiants in the Milky Way, including Mu Cephei and VY Canis Majoris. Calculations of Betelgeuse’s mass range from slightly under ten to a little over twenty times that of the Sun. It is calculated to be about 700 light-years from the Sun, indicating an absolute magnitude of about −6. Less than 10 million years old, Betelgeuse has evolved rapidly because of its large mass and is expected to end its evolution with a supernova explosion, most likely within 100,000 years. Having been ejected from its birthplace in the Orion OB1 Association—which includes the stars in Orion's Belt—this runaway star has been observed moving through the interstellar medium at a speed of 30 km/s, creating a bow shock over four light-years wide.

In 1920, Betelgeuse became the first extrasolar star whose photosphere’s angular size has been measured. Subsequent studies have reported an angular diameter (i.e., apparent size) ranging from 0.042 to 0.056 arcseconds; that range of determinations is ascribed to the following causes:

• non-sphericity
• limb darkening
• pulsations
• varying appearance at different wavelengths.

It is also surrounded by a complex, asymmetric envelope, roughly 250 times the size of the star, caused by mass loss from the star itself. The Earth-observed angular diameter of Betelgeuse is exceeded only by those of R Doradus and the Sun.

Starting in October 2019, Betelgeuse began to dim noticeably, and by January 2020 its brightness had dropped by a factor of approximately 2.5, from magnitude 0.5 to 1.5. On 1 February 2020, astronomers reported support for earlier dimmings, suggested a minimum brightness of the star on 21 February 2020 and noted that Betelgeuse is currently the "least luminous and coolest" in the 25 years of their studies.^[17] Despite speculation that the drop might indicate that Betelgeuse could soon explode as a supernova, astrophysicists consider this unlikely.

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