Young stellar objects of both high and low luminosity emit energetic jets or winds of material that are often highly collimated and often bipolar. Near the stars, turbulent swept-up gas is observed in the emission of interstellar molecules such as carbon monoxide, and small, bright regions of water maser emission and the nebulous bright patches known as Herbig-Haro objects appear to be participating in the outflows. There are striking changes in chemical abundances associated with the attendant shocks. Probably every star goes through this phase, which may mark the end of its period of accretion.
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