Algol variables or Algol-type binaries are a class of eclipsing binary stars where the orbital plane of the stars are coincident with the line of sight from Earth. When the cooler component passes in front of the hotter one, part of the latter's light is blocked, and the total brightness of the binary, as viewed from Earth, 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, being determined by the revolution 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).
Component stars of Algol binary systems have a spherical, or slightly ellipsoidal shape. This distinguishes them from the so-called beta Lyrae variables and W Ursae Majoris variables, where the two components are so close that gravitational effects lead to serious deformations of both stars.
Generally the amplitudes 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 spectrum, though in most cases the brighter component is found to have B, A, F, or G spectra.
Algol itself, the prototype of this type of variable star, Bayer designation 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 in 1782.
Many thousands of Algol binaries are now known: the latest edition of the General Catalogue of Variable Stars (2003) lists 3,554 of them (9% of all variable stars). Data for some interesting Algol variables are given in the list of known variable stars.
- "A consideration of close binary systems in relation to light variation.". Report of the South African Association for the Advancement of Science 1: 110–111. 1903.
- Eclipsing Binary Stars, D. Bruton (Stephen F. Austin State University)