An empire is a political unit made up of several territories, military outposts, and peoples, "usually created by conquest, and divided between a dominant center and subordinate peripheries". The center of the empire exercises political control over the peripheries. Within an empire, different populations have different sets of rights and are governed differently. Narrowly defined, an empire is a sovereign state whose head of state is an emperor or empress; but not all states with aggregate territory under the rule of supreme authorities are called empires or are ruled by an emperor; nor have all self-described empires been accepted as such by contemporaries and historians.