A
satellite galaxy is a galaxy that
orbits a larger galaxy due to
gravitational attraction. Although a
galaxy is made of a large number of objects (such as
stars,
planets, and
nebulae) that are not connected to each other, it has a
center of mass, which represents a weighted average (by mass) of the positions of each component object. This is similar to how an everyday object has a center of mass which is the weighted average of the positions of all its component
atoms.