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Algol, star β of the constellation Perseus, the most famous of eclipsing variables, whose visual magnitude, typically 2.2, suffers, every 69 hours or so, a decrease of splendor up to size 3.4. The variability was discovered by G. Montanari in 1669, but the frequency was only recognized in 1782 by J. Goodricke, who also gave the exact explanation. The distance from Earth is 100 light years.
Algol is one of the few stars visible to the naked eye to show a marked variability is the prototype of the variables and Algol is actually a triple star, where the main star, Algol A is regularly eclipsed by weaker companion, Algol B. its apparent magnitude, typically of 2.12, drops to 3.39 every 2 days, 20 hours and 49 minutes for a period of 10 hours, which is equivalent to the duration of the partial eclipse. The secondary eclipse, which occurs when the main pass in front of the secondary, can only be detected photoelectrically. A 2.69 UA wheel, around the common center of mass and in a period of 680 days, Algol C, a white star of the main sequence with a mass of 1.7 M⊙. The association of alcohol to a demon, probably due to his behavior, suggests that the variability of the star were also known before the seventeenth century, although there is no firm evidence about.The variability of Algol was recorded for the first time in 1670 by Geminiano Montanari, but the periodicity of its variations in brightness was recognized only after more than a century by amateur John Goodricke. In May 1783 Goodricke presented his findings to the Royal Society, suggesting that variability was caused by a dark body that passed in front of the star. For her report on Algol he was awarded the Copley Medal later.
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To open the video, click on the picture, good view from your Alessandro Brizzi.