Mass loss is the outflow of particles and gas from a star, occurring at varying rates and by a variety of processes, throughout its lifetime. Bipolar flows are often associated with protostars, while powerful stellar winds blow from T Tauri stars, giants, and supergiants. Ejected material builds up as a disk around a Be star or as a shell around a Gamma Cassiopeiae star; in late evolutionary star it may give rise to a planetary nebula. More violent ejection accompanies a nova or supernova explosion. Mass loss may also occur through the outer Lagrangian point in a contact binary or an overcontact binary (see close binary).