Not all double stars are binary. As we know, some can be an optical double stars, and then there are others that are close to each other and share very similar motions. These are known as Common Proper Motion stars. Many binary stars are CPM stars. The commonality of motion in these stars are thought to be a by-product of their stellar birthplace energy displayed as they travel through space together.
This lesson will assume a familiarity with proper motions.