ABSTRACT
Double stars are of two types; optical pairs and binaries. The first double star to be discovered telescopically was Mizar itself, which is made up of two rather unequal components separated by 14''.5. The discovery was made by Riccioli in 1651. The first southern double star to be discovered was a Crucis, by Father Guy Tachard, in 1685. The old theory–that a binary was formed as a result of the fission of a single star–has been abandoned, and neither is it likely that binaries are due to the mutual capture of the components. If the two components of a binary system are close together, the evolution of one component may profoundly affect the other. In such a system there is an hourglass-shaped region bounded by the points where the two stars will equally affect a small particle; each of the two segments of the ‘hourglass’ encloses a region termed a Roche lobe.
