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.