Accretion disks have been observed, or theorized to exist, in association with protostars, T Tauri stars, and other very young stars, Vega-type stars, and many interacting binary systems, including U Geminorum stars, novae, symbiotic stars, and W Serpentis stars, in which the secondary component is losing matter to the primary. They are also believed to occur commonly around black holes, including stellar black holes in close binary systems and supermassive black holes at the centers of some galaxies. In the latter case, accretion disks are thought to play a crucial role in the active galactic nucleus phenomenon.