Algol (The Demon Star; Beta Persei; b Per)

Perhaps the best-known variable star, its magnitude varying between 2.2 and 3.5 in a period of 2.87 days. The variation occurs because Algol is an eclipsing binary system in which the two stars regularly cross in front of each other as viewed from Earth. Algol is regarded as the prototype eclipsing binary. The brighter of the components of Algol is a B star and the fainter a G star. The decline in brightness as the G star cuts off the light from its brighter companion takes four hours, and minimum lasts only twenty minutes. The secondary eclipse, when the fainter star causes a dip in brightness of only 0.06 magnitude, is not detectable by eye. Variations in the spectrum of Algol with a period of 1.862 years reveal the presence of a third star in the system. There is also evidence in the spectrum for mass transfer between the two close companions, supported by observations that Algol is a radio star, erratically flaring up to twenty times its normal radio brightness. The radio emission is attributed to gas streaming on to the primary star.