The variability of Algol was first recorded in 1670 by Montanari, but it is probable that this property was noticed long before this time; the name Algol means "demon star," (from Arabic al ghûl, "the ghoul") which was probably given due to its peculiar behavior. Astrologically, Algol was considered the most unfortunate star in the sky.
The Algol paradox was a problem of stellar evolution:
Components of a binary star form at the same time, and massive stars evolve much faster than the less massive ones. The more massive component of Algol is still in the main sequence. The less massive is a subgiant, showing a later evolutionary stage. The paradox can be solved by mass transfer. When the more massive star became a subgiant, it filled its Roche lobe. Most of the mass was transferred to the other star, which is still in the main sequence. (In some binaries similar to Algol, a gas flow can actually be seen.)