A polar jet is a phenomenon often seen in astronomy, where streams of matter are emitted along the axis of rotation of a compact object. It is usually caused by the dynamic interactions within an accretion disc. When matter is emitted at speeds approaching the speed of light, these jets are called relativistic jets. The largest polar jets are those seen in active galaxies such as quasars. Other systems which often contain polar jets include cataclysmic variable stars, X-ray binaries and T Tauri stars. Herbig-Haro objects are caused by the interaction of polar jets with the interstellar medium. Bipolar outflows or jets may also be associated with protostars (young, forming stars), or with evolved post-AGB stars (often in the form of Bipolar nebulae)