ALGOL

also called Beta Persei prototype of a class of variable stars called eclipsing binaries, the second brightest star in the northern constellation Perseus. Its apparent visual magnitude changes periodically over the range of (approximately) 2.2 to 3.5; even at its dimmest it remains readily visible to the unaided eye. The name probably derives from an Arabic phrase meaning demon, or mischief-maker, and the Arabs may have been aware of the star's variability even before the invention of the telescope. The first European astronomer to note the light variation was the Italian Geminiano Montanari in 1670; the English astronomer John Goodricke measured the cycle (69 hours) in 1782 and suggested partial eclipses of the star by another body as a cause, a hypothesis proved correct in 1889. The comparatively long duration of the eclipse shows that the dimensions of the two stars are not negligible in comparison with the distance between them. Small fluctuations in the period of the light variation can be explained by assuming that the system contains two other stars, which take no part in the eclipses. The presence of one companion with a period of 1.87 years has been confirmed by spectroscopic observations.