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.
Image: Hubble Space Telescope