SHELIAK (Beta Lyrae). 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.