SHELIAK (Beta Lyrae). So many star names -- Vega, Deneb, Rigel -- ring familiarly to the ear. Sheliak, at the dim end of third magnitude (3.45), is not among them. Only because the little star is an integral part of the exquisite constellation of 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 more commonly referred to by its Greek letter name, Beta Lyrae, "Sheliak" derives from a Greek then Arabic word that refers to the whole constellation, to the celestial harp itself, which in Greek mythology commemorates the lyre of Orpheus. Sheliak is one of the most confusing, heavily studied, and important stars of the nighttime sky. Located 962 light years away, give or take 48 (second Hipparcos reduction), it radiates the visible light of 4600 Suns (assuming 0.4 magnitudes of dimming by interstellar dust, which is probably high), and when the whole spectrum is included (see below), vastly more. Sheliak, however, it is not one star, but two, a bright bluish class B (B7) bright giant with a temperature of 14,500 Kelvin coupled with a much hotter, even bluer, B0.5 dwarf at 30,400 Kelvin. There is nothing rare about binary stars. Sheliak's stars, however, mutually eclipse each other. The plane of the orbit is pitched so that during an orbital period of 12.9 days each star (including a disk around the hotter one; see below) gets in the way of the other, the combined light of the system at minimum alternating between 50 and 75 percent of normal every 6.5 days. Sheliak's variations, easily visible to the naked eye by comparing the star to others in the constellation (specifically to Gamma Lyrae just to the east of it), were discovered in 1784.

light curve The light curve of Beta Lyrae, magnitude plotted against time, shows continuous variation over its 12.9-day period as a result of tidally distorted stars and flowing gas. The deeper primary eclipse takes place when a hot B0.5 dwarf hidden by a thick surrounding disk gets in front of a lesser but still very luminous B7 giant. The system has challenged astronomers for decades. From The Hundred Greatest Stars, J. B. Kaler, NY, Copernicus Books, 2002, as adapted from Burnham's Celestial Handbook, R. Burnham Jr., New York, Dover.

Such eclipsing doubles (the most famous being Algol in Perseus) tell a great deal about stars, helping to determine their various parameters. Theoretical modeling of the visual light curve, the graph of stellar brightness vs. time, shows that once its ultraviolet light is taken into account, the B7 giant radiates with the luminosity of 8300 times that of the Sun, the B0.5 dwarf shining at 27,700 Suns, some three times more. The radii figure in at 14.4 and 6.0 times solar, and the masses at 2.25 and 13.3 Suns. The two stars are close together, about a quarter of an Astronomical Unit apart. Tidal forces severely distort the lesser B7 giant, forcing it to fill its "Roche Lobe," a teardrop-shaped "surface" along which the gravities of the stars (including forces from their whirling motion) balance each other. As a result, as the giant evolves it sends mass through the point of the teardrop at a rate of about a hundred thousandth of a solar mass per year (a billion times the flow rate of the solar wind) toward the more massive dwarf, which then gains weight at the expense of the other. The flowing matter, however, does not flow directly from the "loser" onto the "gainer," but first enters into a thick disk from which it later falls. The disk is so opaque that the hotter, more massive gainer is mostly hidden. The activity is so energetic that the opposing jets squirt perpendicularly outward from the disk. Between the distortion of the orbiting, eclipsing, stars, and the flowing mass, the brightness of the star changes continuously, whether in eclipse or not.

The greater the mass of a star, the shorter its lifetime, that is, the faster it turns from a hydrogen-fusing dwarf into a giant. Yet here it is the lesser star that has seemingly evolved first. It USED to be the more massive, but has now lost much of itself to the B0.5 gainer, which is literally consuming its companion. Such mass transfer is profoundly important in the lives of close 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. At the limit, the companion just literally disappears, not even a ghost of itself left. Even if the star itself were ordinary, it would be significant as an excellent guide to the famed Ring Nebula in Lyra, Messier 57, which lies almost exactly between it and Gamma Lyrae (Sulafat). (Data and analysis taken largely from articles by J. R. Lomax et al., Astrophysical Journal for May 1, 2012, and by R. E. Mennickent and G. Djurasevic, Monthly Notices of the RAS for April 19, 2013. (8/14/98; revised 11/07/13).

Written by Jim Kaler 8/14/98; revised 11/15/13. Return to STARS.