Algol

Algol (Beta Persei, β Persei, Beta Per, or β Per) is a triple star system including an eclipsing spectroscopic binary, and whose stars show characteristics that do not fit within normal main-sequence evolution. It shows a very distinct variation and was one of the first stars recorded to be a variable star other than novae: it regularly dims full magnitude for ten hours, the dimming recurring with a period of just under three days. Its unusual characteristic is that the less massive star of the pair is distinctly larger, a subgiant: generally, binary stars are coeval, formed at the same time, in which case the less massive star will reach the subgiant and giant stages later than its binary companion. This unusual characteristic has been termed the Algol paradox. The favored explanation is that mass transfer has occurred between the two stars in their past. Characteristics of β Per Aa1:

• Perseus - constellation.
• 03h08m10.13245s - right ascension.
• +40deg57m20.3280 - declination.
• +2.12 - apparent magnitude (V).
• 94 ly - distance.
• B8V - spectral type.
• 3.17 - mass(solar).
• 182 - luminosity(solar).
• 2.73 - radius(solar).
• -0.05 - B-V color index.

(The third star is in a larger orbit and does not contribute to the unusual characteristics.)

The terms Algol variable (or just Algol) are used for a variable star type and a binary star type, for which Algol is the prototype. A star is more likely to be termed an Algol if such mass transfer is evident. Stars close enough to transfer mass are often in contact or are elongated through gravity (tidal force), something that light curves reveal. Algol shows just a small amount of such elongation, and the term Algol is sometimes reserved for eclipsing binary stars showing little or no elongation.