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Algol (Beta Per, β Persei, β Per), known colloquially as the Demon Star, is a bright star in the constellation Perseus. It is one of the best known eclipsing binaries, the first such star to be discovered, and also one of the first (non-nova) variable stars to be discovered. Algol is actually a three-star system (Beta Persei Aa1, Aa2, and Ab) in which the large and bright primary β Persei Aa1 is regularly eclipsed by the dimmer β Persei Aa2. Thus, Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every 2 days, 20 hours and 49 minutes during the roughly 10-hour-long partial eclipses. There is also a secondary eclipse (the "second minimum") when the brighter star occults the fainter secondary. This secondary eclipse can only be detected photoelectrically.〔 〕 Algol gives its name to its class of eclipsing variable, known as Algol variables. ==Observation history== The association of Algol with a demon-like creature (Gorgon in the Greek tradition, ghoul in the Arabic tradition) suggests that its variability was known long before the 17th century, but there is still no indisputable evidence for this.〔G.A. Davis, "Why did the Arabs Call Beta Persei "al-Ghul"?", ''Sky and Telescope'', 16 (1957), 177 (ADS ).〕 Investigating a calendar for lucky and unlucky days composed in Egypt some 3200 years ago, scholars have noted a significant periodicity of 2.85 days and several empirical tests indicate that this periodicity may be connected to Algol. The variability of Algol was first unambiguously recorded in 1667 by Italian astronomer Geminiano Montanari,〔G. Montanari, "Sopra la sparizione d'alcune stelle et altre novità celesti", in: ''Prose de Signori Accademici Gelati di Bologna'' (Bologna: Manolessi, 1671), pp. 369-92 ((Google books )).〕 but the periodic nature of its variations in brightness was not recognized until more than a century later, when the British amateur astronomer John Goodricke also proposed a mechanism for the star's variability.〔(ADS O.J. Eggen,"An Eighteenth Century Discussion of Algol", ''The Observatory'', 77 (1957), 191-197 ).〕 In May 1783 he presented his findings to the Royal Society, suggesting that the periodic variability was caused by a dark body passing in front of the star (or else that the star itself has a darker region that is periodically turned toward the Earth.) For his report he was awarded the Copley Medal. In 1881, the Harvard astronomer Edward Charles Pickering presented evidence that Algol was actually an eclipsing binary. This was confirmed a few years later, in 1889, when the Potsdam astronomer Hermann Carl Vogel found periodic doppler shifts in the spectrum of Algol, inferring variations in the radial velocity of this binary system. Thus Algol became one of the first known spectroscopic binaries. Dr. Joel Stebbins at the University of Illinois Observatory used an early selenium cell photometer to produce the first-ever photoelectric study of a variable star. The light curve revealed the second minimum and the reflection effect between the two stars. Some difficulties in explaining the observed spectroscopic features led to the conjecture that a third star may be present in the system; four decades later this conjecture was found to be correct.〔Meltzer, Alan S., A "Spectroscopic Investigation of Algol". ''Astrophysical Journal'', vol. 125, (1957), p.359, (BibCode:1957ApJ...125..359M )〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Algol」の詳細全文を読む スポンサード リンク
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