Thanks to James B. Kaler. These contents are the property of the author and are reproduced from original without the author's express consent because of fair use and valid educational purposes.
So many star names -- Vega, Deneb, Rigel -- ring familiarly to the ear. Sheliak, at the dim end of third magnitude, is not among them. Only because the little star is an integral part of the exquisite constellation Lyra, the Lyre or Harp, of which great Vega is king, does it even have a proper name. The southwestern-most star of the little parallelogram that makes the body of the Harp, and usually referred to as Lyra's Beta star, the name Sheliak derives from an Arabic word that refers to the whole constellation, to the celestial harp itself, which in Greek mythology commemorates the harp of Orpheus. Sheliak has an importance all out of proportion to its apparent dimness. Located nearly 900 light years away, it actually radiates the visible light of 2000 Suns. However, it is not one star, but two, a bright bluish hotter one with a temperature of some 13,000 degrees Kelvin orbiting a dimmer white cooler one (though one still much brighter than the Sun) with a temperature closer to 8000 Kelvin. Doubles of course abound in the sky. Our Sheliak, however, eclipses! The plane of the orbit is pitched so that during an orbital period of 12.9 days each star gets in the way of the other, the combined light of the system at minimum alternating between 30% and half of normal every 6.5 days. Sheliak's variations, easily visible to the naked eye by comparing the star to others in the constellation, were discovered in 1784. Such eclipsing doubles (the most famous Algol in Perseus) tell astronomers a great deal about stars, helping to determine their masses and diameters. Eclipsers are actually quite common, but again Sheliak is different. The two stars, both quite massive, are very close together. Tidal forces both distort the stars and cause streams of matter to flow from one star to the other and apparently in a disk around the fainter of two, making the pair both quite difficult and not very well understood. Such "mass transfer" is profoundly important in the lives of double stars and produces some of the more bizarre of celestial phenomena (including Sheliak!). In extreme cases, one star can actually orbit inside the extended envelope of an expanding, dying giant star, gradually bringing the two closer together and setting the stage for later stellar explosions. Others are so close they actually touch at their surfaces.