Orbiting bodies revolve around their common center of mass (green). If the center of mass is inside one of the bodies, it will seem to wobble. This wobble can sometimes be detected for distant stars, which indicates the presence of an orbiting body, such as a planet that cannot be observed directly.
Snapshot 1: In some systems, the center of mass, which is also known as the barycenter, is found well inside the larger body. This occurs, for instance, if the larger body has a considerably larger mass than the smaller one. As a result, the larger body wobbles slightly as the two bodies revolve around the barycenter.
Snapshot 2: If the two bodies are closer in mass, with the barycenter still inside the larger body, the wobble will be more pronounced.
Snapshot 3: If the two bodies have roughly the same mass, the bodies rotate around the barycenter without appearing to wobble.