Monday, November 9, 2015

Lyra - FL Lyrae, an Algol variable

Copyright Mike Guidry / Univ. of Tennessee

FL Lyrae
Further north from R Lyrae is FL Lyrae. It is a faint 9th-magnitude Algol variable that drops by half a magnitude every 2.18 days during the primary eclipse.

Both components are main-sequence stars, the primary being late F-type and the secondary late G-type.

The system was one of the first main-sequence eclipsing binaries containing G-type star to have its properties known as well as the better-studied early-type eclipsing binaries.

Algol variable
Algol variables or Algol-type binaries are a class of eclipsing binary stars that are related to the prototype member of this class, β Persei (Beta Persei, Algol) from an evolutionary point of view.

An Algol binary is a semidetached binary system where the primary component is an early type, main sequence star that does not fill its Roche lobe, while the cooler, fainter, larger, less massive secondary component lies above the main sequence in a Hertzsprung–Russell diagram and fills the Roche lobe. (The Roche lobe is the region around a star in a binary system within which orbiting material is gravitationally bound to that star.)
Early in its history, the secondary star would have been more massive, evolving first to overfill its Roche lobe. After rapid mass exchange, the lobe-filling star became less massive than its companion.

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 over moderate periods of time (months to years), being determined by the revolution period of the binary, the time it takes for the two components to once orbit around each other.

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.

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).

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