Algol

The galactic position and direction of Algol relative to Earth's Sun. Note that, at this extreme scale, the two stars are effectively in the same place.

Algol, the 'Demon' star, and its three attendants in the constellation of Perseus.

The Algol eclipse: every 69 hours, Algol's yellow component crosses in front of its blue companion star. This event reduces the star's brightness by more than a magnitude, an effect that lasts for four hours.

The Arabs gave the name 'Algol' ('The Demon') to this blue dwarf star, almost certainly because it is one of the most variable stars in the sky. This variation in brightness is caused by a companion star, not directly visible from Earth.

This companion is a yellow, G-type star, much fainter than the blue primary. The two orbit each other very closely - so closely that matter is exchanged between them - in a period of about 2 days and 21 hours. Once in each orbit, from our viewpoint on Earth, the yellow star crosses in front of the blue, reducing its brightness considerably. This effect lasts for about four hours.

As Algol's yellow primary star is circled by its companion, the less massive blue star loses streamers of matter from its surface.

The Algol system contains at least one other star, quite distant from the other two, and orbiting them in a little under two years. There is also evidence for as many as three other companions, which would make Algol a sextuple star system.

Algol lies some 90 light years (28 parsecs) from the Earth. When not eclipsed by its companion, its magnitude of +2.1 makes it the second brightest of the stars in the constellation of Perseus.