An interacting binary is a binary system in which the two stars orbit around each other so closely that they strongly affect each other's structure and evolution, most notably through mass transfer to the primary from the secondary, 'donor' star. A typical interacting pair consists of a degenerate, or compact, primary – a black hole, neutron star, or, most commonly, a white dwarf – and a secondary that has either filled its Roche lobe or has a large stellar wind. Matter is pulled toward the compact object and forms an accretion disk around it. Material cannot land directly onto the compact object as it has to get rid of its angular momentum – hence, the accretion disk – but when it does, the gravitational potential energy of the material is converted to electromagnetic energy as it falls and/or undergoes explosive fusion on the surface of the compact recipient.