Figure 11-31Three Close Binaries Sketches of light curves for three eclipsing binaries are shown. The phase denotes the fraction of the orbital period from one primary minimum to the next. (a) Algol, also known as β Persei, is a semidetached binary. The deep eclipse occurs when the giant star (right) blocks the light from the smaller but more luminous main-sequence star. (b) β Lyrae is a semidetached binary in which mass transfer has produced an accretion disk that surrounds the detached star. This disk is so thick and opaque that it renders the secondary star almost invisible. (c) W Ursae Majoris is an overcontact binary. Both stars therefore share their outer atmospheres. The short, 8-hour period of this binary indicates that the stars are very close to each other.