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Sheliak, Beta Lyrae

Eclipsing Binary Variable of Beta Lyrae type in Lyra

Right Ascension 18 : 50 : 04.7 (h:m)
Declination +33 : 21 : 46 (deg:m)
Distance 300 (ly)
Visual brightness 3.34 .. 4.34 (mag)
Period 12.936 (days)
Spectra type B7Ve + A8p

Sheliak (Arabic for "Harp" or - according to Robert Garfinkle's Star Hopping - "The Tortoise"), or Beta Lyrae, is a half separated (i.e. one of the stars reached its Roche volume) eclipsing binary of a cream-white color. The brightness varies from 3.4 mag to 4.3 mag every twelve days and 22 hours. As one of the two stars of this system is filling its Roche surface and ellipsoidally deformed, the lightcurve is softened in comparison, e.g., to that of Algol (Beta Persei). Beta Lyrae is the prototype of this class of eclipsing binaries, the Beta Lyrae Stars or EB variables. For Sheliak, the larger star totally eclipses its smaller companion in its main minimum, while after half the period, the smaller star occults parts of the larger one to generate a secondary minimum of about mag 3.8. Otto Struve interpreted slight changes of its lightcurve as indication for a disk of dark matter surrounding the secondary star - very probably forming streams of hot gas ejected by the gravitationally interacting stars.

Sheliak was found to be a radio star by radio astronomers Wade and Hjellming in 1971 - another indication of the matter transfer between the two stars. Astrophysicists think that matter is is flowing from the larger to the smaller star at 300 km/s.

Larger telescopes resolve Beta Lyra as an attractive double star with a blue companion of 8th mag.

Sheliak has been recognised as a variable in 1784 by the English amateur astronomer John Goodricke.

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Hartmut Frommert
Christine Kronberg