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Algol is the common name of the star Beta Persei. This star has the unusual property of having regular variations in brightness: its magnitude changes regularly between 2.3 and 3.5 over a period of 2 days, 20 hours and 49 minutes. Algol is a so-called eclipsing binary: it is actually two stars in close orbit about one another, and when (as seen from Earth's line of sight), the second, dimmer star passes in front of its companion once per orbit, the amount of light reaching earth is decreased, followed by an increase as the companion moves out of the way.

The variability of Algol was first recorded in 1670 by Geminiano 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. In the constellation Perseus, it represents the eye of the Gorgon Medusa. Astrologically, Algol was considered the most unfortunate star in the sky.

The Algol paradox was a problem of stellar evolutionIn astronomy, stellar evolution is the sequence of changes that a star undergoes during its " lifetime", the millions or billions of years during which it emits light and heat. Over the course of that time, the star will change radically. Stellar evolutio:

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 sequenceThe main sequence of the Hertzsprung-Russell diagram is the curve where the majority of stars are located in this diagram. Stars located on this band are known as main-sequence stars or dwarf stars . The coolest dwarfs are the red dwarfs. This line is so. The less massive is a subgiant star , showing a later evolutionary stage. The paradox can be solved by mass transferMass transfer is the phrase commonly used in engineering for physical processes which involve molecular and convective transport of atoms and molecules within physical systems. Mass transfer includes both mixing and separation processes. Some common examp. When the more massive star became a subgiant, it filled its Roche lobeThe Roche lobe is the region of space around a star in a binary system within which orbiting material is gravitationally bound to that star. If the star expands past its Roche lobe, then the material outside of the lobe will fall into the other star.. 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.)