We begin this volume on viewing double and multiple stars by defining just what they are: two or more suns placed in close proximity to each other in the sky as seen with the unaided eye, binoculars or telescopes. With the exception of stars that just happen to lie along the same line of sight but are actually far apart in space, these objects are physically (gravitationally) bound together as a system. In some cases, they are separated enough that they are simply drifting through space together, while in others they are actually orbiting around the common center of gravity of the system.
The various types of double stars are discussed in depth in the next chapter. But here an important point needs to be made. Contrary to common belief, double stars are deep-sky objects! Anything beyond the confines of the Sun's family is by definition in deep space. This includes single, variable and double stars, in addition to the more traditional star clusters, nebulae and galaxies. Also, throughout this book we will use the term "double star" to mean both double and multiple star systems.
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