A bipolar outflow represents two continuous flows of gas from the poles of a star. Bipolar outflows may be associated with protostars (young, forming stars), or with evolved post-AGB stars (often in the form of Bipolar nebulae). In the case of a young star, the bipolar outflow is driven by a dense, collimated jet. The jet is narrower than the outflow and very difficult to observe directly. However, supersonic shock fronts along the jet heat the gas in and around the jet to thousands of degrees. These pockets of hot gas radiate at infrared wavelengths and thus can be detected with telescopes like the United Kingdom Infrared Telescope (UKIRT). They often appear as discrete knots or arcs along the beam of the jet. They are usually called molecular bow shocks, since the knots are usually curved like the bow wave at the front of a ship. Typically, molecular bow shocks are observed in ro-vibrational emission from hot molecular hydrogen. These objects are known as Molecular Hydrogen emission-line Objects, or MHOs.

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