Algol

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 over the range of 2.1 to 3.4 with a period of 2.87 days. 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. A third star, which does not take part in the eclipses, revolves about the other two with a period of 1.862 years.

Eclipsing variable star

Eclipsing variable star, pair of stars revolving about their common centre of mass in an orbit whose plane passes through or very near the Earth. An observer on the Earth thus sees one member of the binary pass periodically over the face of the other and diminish its light through an eclipse. The star Algol was the first suggested as an eclipsing binary, by John Goodricke, in 1782. Thousands are now known. By combining measurements of the brightness variations with spectroscopic information for both stars of the pair, astronomers can determine the mass and size of each star. See also variable star; binary star.