[Equi-potential surfaces in the co-rotation frame of a binary system. Shown here is a cross-cut view in the orbital plane. There are 5 Lagragian points. The region surrounded by the Roche surface is called the Roche lobe.]
[Panel (a) is a semi-detached binary, panel (b) is also a semi-detached binary but with Roch-lobe overflow resulting in an accretion disk around the other star, and panel (c) is an over-contact binary with a common envelope enclosing both stars. ]
[The history of a system with a less massive but more evolved star]
[Nova Cygni 1975 is an example of novae, which are an explosive phenomenon of white dwarf surface thermal nuclear fusion after accumulating enough mass by accretion from its companion. Novae are a recurrent phenomenon. In the left panel the star is about magnitude 2 and about magnitude 11 in the right.]
[Novae are similar to X-ray bursters but with explosive hydrogen burning on the surface of a white dwarf, instead of a neutron star. The explosion is much less powerful than SN Ia (105L⊙ vs 109L⊙), in which explosion occurs within the whole white dwarf and blows it apart.]