Dynamo theory
Dynamo theory proposes a mechanism by which a celestial body such as the earth generates a magnetic field. In the case of the earth the magnetic field is believed to be caused by the convection of molten iron and nickel, within the planetary core, along with the overall planetary rotation. This results in the creation of electric currents as well as a magnetic field. Similar magnetic fields are present in many celestial bodies including most stars such as the sun and active galactic nuclei.The membrane paradigm is a way of looking at black holes that allows for the material near their surfaces to be expressed in the language of dynamo theory.