M87 jet. Image: Hubble Space Telescope.
A jet is a bright, highly directional beam of matter and radiation associated with certain types of stars and active galaxies. Jets are observed mainly at radio wavelengths but sometimes at optical and other wavelengths; in many instances, their existence is presumed, even if they can't be seen directly, because of the existence of giant lobes of material on either side of the central object. They typically come in pairs, with each jet aiming in the opposite direction to the other, and commonly occur where accretion disks are present. On a galactic scale, jets are found emerging from many radio galaxies and quasars. Stellar jets are seen associated particularly with T Tauri stars and FU Orionis stars.