Introduction

Whilst the luminous jets of radio galaxies and quasars are the most powerful examples of collimated outflow in the cosmos, there are many examples of jets and outflows to be found much closer to home, within our own Galaxy. These span a great range of luminosities and collimation factors, from the optically visible jets and “lobes” associated with low-mass young stellar objects, which are morphologically very similar to the classical radio galaxies, to the poorly collimated and much less clearly denned “jets” associated with the Galactic Centre and with various supernova remnants. Galactic jet sources also include the singular object SS 433, which is known to be emitting a two-sided jet at a quarter of the speed of light. This jet is known to be associated with a binary star system, and there is some evidence that other mass-transfer binaries may also have jets.

In many cases the jet material itself is insufficiently excited to dissociate it completely, giving us a variety of spectral lines at optical, infrared and radio frequencies with which to probe the underlying kinematics, while the mere fact that these objects are close gives us greatly enhanced linear resolution. If there is a lesson to be learnt from the wide variety of systems which exhibit collimated mass-loss it is that jets are very easily formed once symmetry is broken through rotation.