Betelgeuse

Betelgeuse (α Orionis, α Ori, Alpha Orionis) is a semiregular variable star located 427 light-years away. It is the second brightest star in the constellation Orion [and the ninth brightest star in the night sky. Although it has the Bayer designation "alpha", it is not as bright as Rigel (Beta Orionis). It is a vertex of the Winter Triangle asterism
Betelgeuse is a red supergiant, one of the physically largest stars known. If it were placed at the center of our solar system, its outer surface would possibly extend to the orbit of Jupiter. As it is, Betelgeuse is so large and so close that it is the star with the third largest angular diameter as viewed from Earth [1], smaller only than the Sun and R Doradus, and one of only a dozen or so stars that telescopes have imaged as a visible disk (See photo, at right and a picture of hotspots on Betelgeuse.) The angular diameter of Betelgeuse was first measured in 1920–1921 by Michelson and Pease using an astronomical interferometer on the Mount Wilson 100 inch telescope.
== Fading ==
A pulsating semiregular variable star, Betelgeuse is subject to multiple cycles of increasing and decreasing brightness due to changes in its size and temperature. The astronomers who first noted the dimming of Betelgeuse, Villanova University astronomers Richard Wasatonic and Edward Guinan, and amateur Thomas Calderwood, theorize that a coincidence of a normal 5.9 year light-cycle minimum and a deeper-than-normal 425 day period are the driving factors. Other possible causes hypothesized by late-2019 were an eruption of gas or dust, or fluctuations in the star's surface brightness.
By August 2020, long-term and extensive studies of Betelgeuse, primarily using ultraviolet observations by the Hubble Space Telescope, suggest that the unexpected dimming was probably caused by an immense amount of superhot material ejected into space. The material cooled and formed a dust cloud that blocked the starlight coming from about a quarter of Betelgeuse's surface. Hubble captured signs of dense, heated material moving through the star's atmosphere in September, October, and November before multiple telescopes observing the more marked dimming in December and the first several months of 2020.
By January 2020, Betelgeuse had dimmed by a factor of approximately 2.5 from magnitude 0.5 to 1.5, and reported still fainter in February in The Astronomer's Telegram at a record minimum of +1.614, noting that the star is currently the "least luminous and coolest" in the 25 years of their studies and also calculating a decrease in radius. Astronomy magazine described it as a "bizarre dimming", and popular speculation inferred that this might indicate an imminent supernova. Astronomers note that the supernova is expected to occur within approximately the next 100,000 years and is thus unlikely to be imminent.
Category:Orion constellation
Category:Stars
Category:Variable stars