Algol variables or Algol-type binaries are a class of eclipsing

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

s 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 continuous and distinctive band of stars that appears on plots of stellar color versus brightness. These color-magnitude plots are known as Hertzsprung–Russell diagrams after their co-developers, Ejnar Her ...

star well within its

Roche lobe In astronomy, the Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star. It is an approximately teardrop-shaped region bounded by a critical gravitational equipotential, wit ...

. The secondary may also be a main sequence star, referred to as a

detached binary A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

or it may 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 or reflecting light. In other words, brightness is the perception elicited by the luminance of a visual target. The perception is not linear to luminance, ...

of the binary, as viewed from

Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. While large volumes of water can be found throughout the Solar System, only Earth sustains liquid surface water. About 71% of Earth's surfa ...

, 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 Period may refer to: Common uses * Era, a length or span of time * Full stop (or period), a punctuation mark Arts, entertainment, and media * Period (music), a concept in musical composition * Periodic sentence (or rhetorical period), a concept ...

, 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 (Latin: ''revolutio'', "a turn around") is a fundamental and relatively sudden change in political power and political organization which occurs when the population revolts against the government, typically due ...

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 HW Virginis, abbreviated HW Vir, is an eclipsing binary system (of the Algol type), approximately 563 light-years away based on the parallax measured by the Gaia spacecraft, in the constellation of Virgo. The system comprises an ecl ...

); the longest is 9892 days (27 years,

Epsilon Aurigae Epsilon Aurigae (ε Aurigae, abbreviated Epsilon Aur, ε Aur) is a multiple star system in the northern constellation of Auriga, the charioteer. It is an unusual eclipsing binary system comprising an F0 supergiant (officially named Alma ...

). Over long periods of time, various effects can cause the period to vary: in some Algol binaries,

mass transfer Mass transfer is the net movement of mass from one location (usually meaning stream, phase, fraction or component) to another. Mass transfer occurs in many processes, such as absorption, evaporation, drying, precipitation, membrane filtration, ...

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 The Applegate mechanism (Applegate's mechanism or Applegate effect) explains long term orbital period variations seen in certain eclipsing binaries. As a main sequence star goes through an activity cycle, the outer layers of the star are subject t ...

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 A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

, 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 because the two component stars orbit each other, and in this orbit one component periodically passes in front of the other one, thereby blocking its light ...

s and

W Ursae Majoris variable A W Ursae Majoris variable, also known as a low mass contact binary, is a type of eclipsing binary variable star. These stars are close binaries of spectral types F, G, or K that share a common envelope of material and are thus in contact with one ...

s, where the two components are so close that

gravitation In physics, gravity () is a fundamental interaction which causes mutual attraction between all things with mass or energy. Gravity is, by far, the weakest of the four fundamental interactions, approximately 1038 times weaker than the stron ...

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 amplit ...

s of the brightness variations are of the order of one

magnitude Magnitude may refer to: Mathematics *Euclidean vector, a quantity defined by both its magnitude and its direction *Magnitude (mathematics), the relative size of an object *Norm (mathematics), a term for the size or length of a vector *Order of ...

, the largest variation known being 3.4 magnitudes (

V342 Aquilae V34, or similar, may refer to: * Brazilian corvette ''Barroso'' (V34) * Fokker V.34, a German prototype fighter aircraft of World War I * V.34, a telecommunications recommendation of the ITU-T {{Letter-NumberCombDisambig ...

). The components may have any

spectral type In astronomy, stellar classification is the classification of stars based on their stellar spectrum, spectral characteristics. Electromagnetic radiation from the star is analyzed by splitting it with a Prism (optics), prism or diffraction grati ...

, 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 with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...

Bayer designation A Bayer designation is a stellar designation in which a specific star is identified by a Greek or Latin letter followed by the genitive form of its parent constellation's Latin name. The original list of Bayer designations contained 1,564 stars. ...

Beta Persei, first had its variability recorded in 1667 by

Geminiano Montanari Geminiano Montanari (1 June 1633 – 13 October 1687) was an Italian astronomer, lens-maker, and proponent of the experimental approach to science. He was a member of various learned academies, notably the Accademia dei Gelati. Montanari's famous ...

. 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-grandfath ...

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