Algol variables or Algol-type binaries are a class of eclipsing binary stars that are similar to the prototype member of this class, β Persei (Beta Persei,

Algol ALGOL (; short for "Algorithmic Language") is a family of imperative computer programming languages originally developed in 1958. ALGOL heavily influenced many other languages and was the standard method for algorithm description used by the ...

). An Algol binary is a system where both stars are near-spherical such that the timing of the start and end of the eclipses is well-defined. The primary is generally a

main sequence In astronomy, the main sequence is a classification of stars which appear on plots of stellar color index, color versus absolute magnitude, brightness as a continuous and distinctive band. Stars on this band are known as main-sequence stars or d ...

star well within its Roche lobe. The secondary may also be a main sequence star, referred to as a detached binary or it may be an evolved star filling its Roche lobe, referred to as a semidetached binary. When the cooler component passes in front of the hotter one, part of the latter's light is blocked, and the total

brightness Brightness is an attribute of visual perception in which a source appears to be radiating/reflecting light. In other words, brightness is the perception dictated by the luminance of a visual target. The perception is not linear to luminance, and ...

of the binary, as viewed from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

, temporarily decreases. This is the primary minimum of the binary. Total brightness may also decrease, but less so, when the hotter component passes in front of the cooler one; this is the secondary minimum. The period, or time span between two primary minima, is very regular over moderate periods of time (months to years), being determined by the

revolution In political science, a revolution (, 'a turn around') is a rapid, fundamental transformation of a society's class, state, ethnic or religious structures. According to sociologist Jack Goldstone, all revolutions contain "a common set of elements ...

period of the binary, the time it takes for the two components to once orbit around each other. Most Algol variables are quite close binaries, and therefore their periods are short, typically a few days. The shortest known period is 0.1167 days (~2:48 hours, HW Virginis); the longest is 9892 days (27 years, Epsilon Aurigae). Over long periods of time, various effects can cause the period to vary: in some Algol binaries, mass transfer between the closely spaced components of the variable may cause monotonic increases in period; if one component of the pair is magnetically active, the Applegate mechanism may cause recurrent changes in period on the order of ∆P/P ≈ 10⁻⁵; magnetic braking or the effects of a third component star in a highly eccentric orbit can cause larger changes in period. Component stars of Algol binary systems have a spherical, or slightly ellipsoidal shape. This distinguishes them from the so-called

beta Lyrae variable Beta Lyrae variables are a class of close binary stars. Their total brightness is variable star, variable because the two component stars orbit each other, and in this orbit one component periodically passes in front of the other one, thereby bloc ...

s and W Ursae Majoris variables, where the two components are so close that

gravitation In physics, gravity (), also known as gravitation or a gravitational interaction, is a fundamental interaction, a mutual attraction between all massive particles. On Earth, gravity takes a slightly different meaning: the observed force b ...

al effects lead to serious deformations of both stars. Generally the

amplitude The amplitude of a periodic variable is a measure of its change in a single period (such as time or spatial period). The amplitude of a non-periodic signal is its magnitude compared with a reference value. There are various definitions of am ...

s of the brightness variations are of the order of one magnitude, the largest variation known being 3.4 magnitudes ( V342 Aquilae). The components may have any spectral type, though in most cases the brighter component is found to have a B, A, F, or G class.

itself, the prototype of this type of

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes systematically with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are ...

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek alphabet, Greek or Latin letter followed by the genitive case, genitive form of its parent constellation's Latin name. The original list of Bayer design ...

Beta Persei, first had its variability recorded in 1667 by Geminiano Montanari. The mechanism for its being variable was first correctly explained by

John Goodricke John Goodricke FRS (17 September 1764 – 20 April 1786) was an English amateur astronomer. He is best known for his observations of the variable star Algol (Beta Persei) in 1782. Life and work John Goodricke, named after his great-grandfa ...

in 1782. Many thousands of Algol binaries are now known: the latest edition of th
General Catalogue of Variable Stars
(2003) lists 3,554 of them (9% of all variable stars). *DM = A detached main-sequence system. Both components are main-sequence stars and neither fills their inner Roche lobe *DS = A detached system with a subgiant. The subgiant does not fill its inner critical surface *GS = A system with one or both giant and supergiant components; one of the components may be a main sequence star *KE = A contact system of early (O-A) spectral type, both components being close in size to their inner critical surfaces. *SD = A semidetached system. One star fills its Roche lobe.

References

Further reading