W Serpentis star
W Serpentis star(ser-pen -tiss) (Beta Lyrae star) A close binary star system where matter is being transferred very rapidly from one star to the other. This occurs when the more massive member of a close binary evolves to become a red giant (the previous stage may be an RS Canum Venaticorum star); the mass transfer can be so efficient that 85% of the red giant's mass is transferred to the other star. The system ends up as an Algol variable.
Observationally, W Ser stars are spectroscopic binaries characterized by emission lines from an extended gas envelope that is hotter than either of the stars in the system; the emission is thousands of times brighter than the lines from a star's chromosphere. This gas represents part of the giant star's mass that is lost to the system during the rapid transfer. The bulk of the gas forms an accretion disk around the giant's companion, and as this gas spirals inward the inner regions can heat up to 100 000 K and supply the radiation that ionizes the extensive tenuous gas producing the emission lines.
The outer cooler regions of the accretion disk can camouflage the accreting star and make it look larger and cooler than it actually is. This star will thus often have the appearance of a giant rather than that of the underlying main-sequence star. When mass transfer is most rapid, as in Beta Lyrae, the disk conceals this component entirely. Thus in Beta Lyrae we detect only the expanding giant star, a supergiant of spectral type B8.5 that is elongated towards its companion because it fills its Roche lobe (see equipotential surfaces). The companion is hidden by a disk about twice as wide but only half as thick as the supergiant's diameter. The system is an eclipsing binary, with disk and star alternately eclipsing one another; the ellipsoidal shape of the supergiant causes the light curve to peak between eclipses, when the maximum extent of the star is seen. The B8.5 star is losing mass at a rate of about 10–5 solar masses per year, and this causes an increase in the orbital period (13 days) at a rate of 19 seconds per year.