Algol is Beta Persei, at about 94 light years, and it is the prototype of an entire class of variable star.
Its brightness drops from magnitude 2.1 to 3.4 and recovers, on a period of 2.867 days, about two days, twenty hours and forty-nine minutes. The dimming lasts a few hours and is obvious to an attentive naked eye: at minimum the star is visibly less bright than usual, and the change can be followed by comparison with neighbouring stars without any instrument.
The cause is eclipse. Algol is a multiple system with three confirmed components: a hot blue main-sequence star of type B8V, an orange subgiant of type K0IV, and a more distant third star of type F1V. The deep minimum occurs when the dimmer subgiant passes in front of the brighter blue star. This is what an eclipsing binary is, and Algol gives its name to the type: Algol variables.
The system also produced a genuine astrophysical puzzle, the Algol paradox. The less massive component is the more evolved one, which should be impossible if both formed together, since more massive stars evolve faster. The resolution is mass transfer: the subgiant was originally the more massive star, evolved first, expanded, and transferred much of its mass to its companion. The paradox and its solution were an important step in understanding binary evolution.
The Arabic ra's al-ghul means the head of the ghoul or demon; the star marks the severed head of Medusa held by Perseus.
One further detail bears on the tradition: the eclipse is partial, not total. The subgiant covers most but not all of the blue star's disc, which is why the minimum is a drop of about 1.3 magnitudes rather than a disappearance. The star never vanishes; it dims to roughly a third of its usual brightness and returns within about ten hours.
Sources AstronomyAndrew Fraknoi, David Morrison, and Sidney C. Wolff (OpenStax) with The Arabs and the StarsPaul Kunitzsch