Algol variable
Encyclopedia
Algol variables or Algol-type binaries are a class of eclipsing
binary star
s 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, 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 variable
s and W Ursae Majoris variable
s, where the two components are so close that gravitation
al effects lead to serious deformations of both stars.
Generally the amplitude
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 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.
Eclipse
An eclipse is an astronomical event that occurs when an astronomical object is temporarily obscured, either by passing into the shadow of another body or by having another body pass between it and the viewer...
binary star
Binary star
A binary star is a star system consisting of two stars orbiting around their common center of mass. The brighter star is called the primary and the other is its companion star, comes, or secondary...
s where the orbital plane
Orbital plane (astronomy)
All of the planets, comets, and asteroids in the solar system are in orbit around the Sun. All of those orbits line up with each other making a semi-flat disk called the orbital plane. The orbital plane of an object orbiting another is the geometrical plane in which the orbit is embedded...
of the stars are coincident
Coincident
Coincident is a geometric term that pertains to the relationship between two vectors. Vectors consist of a "magnitude" and a "direction". Vectors can be said to be coincident when their direction is the same though the magnitude may be different; that is to say, they lie one on top of the other...
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
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...
of the binary, as viewed from Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
, temporarily decreases. This is the primary minimum of the binary. Total brightness may also decrease, but less, when the hotter component passes in front of the cooler one; this is the secondary minimum.
The period
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
, 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
HW Virginis
HW Virginis, abbreviated HW Vir, is an eclipsing binary system approximately 590 light-years away in the constellation of Virgo. The system comprises an eclipsing B-type subdwarf star and red dwarf star...
); the longest is 9892 days (27 years, epsilon Aurigae
Epsilon Aurigae
Epsilon Aurigae is a star in the constellation Auriga. It is traditionally known as Almaaz, Haldus, or Al Anz. Epsilon Aurigae is an unusual eclipsing binary system comprising an F0 supergiant and a companion which is generally accepted to be a huge dark disk orbiting an unknown object, possibly a...
).
Component stars of Algol binary systems have a spherical
Sphere
A sphere is a perfectly round geometrical object in three-dimensional space, such as the shape of a round ball. Like a circle in two dimensions, a perfect sphere is completely symmetrical around its center, with all points on the surface lying the same distance r from the center point...
, or slightly ellipsoidal shape. This distinguishes them from the so-called beta Lyrae variable
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. The two component stars of Beta Lyrae systems are...
s and W Ursae Majoris variable
W Ursae Majoris variable
A W Ursae Majoris variable is a type of eclipsing binary variable star. These stars are close binaries, whose surfaces are in contact with one another. They are termed contact binaries because the two stars touch and they essentially share material in their outer layers. Through the neck between...
s, where the two components are so close that gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
al effects lead to serious deformations of both stars.
Generally the amplitude
Amplitude
Amplitude is the magnitude of change in the oscillating variable with each oscillation within an oscillating system. For example, sound waves in air are oscillations in atmospheric pressure and their amplitudes are proportional to the change in pressure during one oscillation...
s of the brightness variations are of the order of one magnitude
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
, the largest variation known being 3.4 magnitudes (V342 Aquilae). The components may have any spectrum
Stellar classification
In astronomy, stellar classification is a classification of stars based on their spectral characteristics. The spectral class of a star is a designated class of a star describing the ionization of its chromosphere, what atomic excitations are most prominent in the light, giving an objective measure...
, 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
Variable star
A star is classified as variable if its apparent magnitude as seen from Earth changes over time, whether the changes are due to variations in the star's actual luminosity, or to variations in the amount of the star's light that is blocked from reaching Earth...
, Bayer designation
Bayer designation
A Bayer designation is a stellar designation in which a specific star is identified by a Greek letter, followed by the genitive form of its parent constellation's Latin name...
Beta Persei
Perseus
Perseus ,Perseos and Perseas are not used in English. the legendary founder of Mycenae and of the Perseid dynasty of Danaans there, was the first of the mythic heroes of Greek mythology whose exploits in defeating various archaic monsters provided the founding myths of the Twelve Olympians...
, first had its variability recorded in 1667 by Geminiano Montanari
Geminiano Montanari
thumb|150px|Geminiano Montanari.Geminiano Montanari was an Italian astronomer, lens-maker, and proponent of the experimental approach to science....
. The mechanism for its being variable was first correctly explained by John Goodricke
John Goodricke
John Goodricke FRS was an eminent and profoundly deaf amateur astronomer. He is best known for his observations of the variable star Algol in 1782.- Life and work :...
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.