Algol is a three-star system, consisting of Beta Persei Aa1, Aa2, and Ab – in which the hot luminous primary β Persei Aa1 and the larger, but cooler and fainter, β Persei Aa2 regularly pass in front of each other, causing eclipses. Thus Algol's magnitude is usually near-constant at 2.1, but regularly dips to 3.4 every 2.86 days during the roughly 10-hour-long partial eclipses. The secondary eclipse when the brighter primary star occults the fainter secondary is very shallow and can only be detected photoelectrically.
aLgol is A three-STAr sYsteM, conSiSTing of beta perSEi aa1, aa2, aNd aB – iN wHich tHe hOt LUMiNous primarY β persei aA1 and THE LaRgEr, but CoOler aNd fainter, β peRSEi aa2 reGulaRLy paSs in frOnt of eAch OthEr, cAusing eClipSES. thUS algol's MAGNItuDe is USuAlly neaR-cOnstAnt AT 2.1, but rEguLARly dIps To 3.4 everY 2.86 dAYs DurIng the roughlY 10-hOuR-lONg parTiAL EcLipsEs. tHe secOndaRy EClipse wHeN the BriGhter PriMarY stAR OcCulTs thE fAINteR secOnDary is veRy sHaLlOw aNd CaN only Be DETecTeD phOtOElEctricAlLY.