Algol, also known as « Persei, is a well-known variable star, as well as a binary star. Its light variations, explained
by John Goodricke in 1783, are caused by mutual eclipses of two stars moving in orbits around their common center of gravity, within a period of about 69 hours. When the dimmer star eclipses the brighter star, Algol fades to one-third of its original brightness, from magnitude 2.3 to 3.5, for about 10 hours. Only a slight dimming is apparent when the brighter star eclipses the dimmer star.
The larger star of the Algol system (spectral type B8) has a mass of about 5 solar masses and a radius 3.1 times the solar radius; the other star (spectral type gK0) has a mass about equal to that of the Sun and a radius 3.7 times the solar radius. The two stars are separated by 15 solar radii. A third, more distant star is also part of the Algol system, but it has no effect on the light variations.
Many eclipsing variables are broadly similar to Algol. In some the two stars appear to be essentially spherical. In others, however, named W Ursae Majoris stars after their prototype, the stars are ellipsoidal, being distorted by their mutual tidal attractions.