IMAGES of young stars 1 and supernova remnants 2 often reveal small, high-density knots of material which are interpreted as 'bullets' ejected by the source and propagating at supersonic speeds into the surrounding interstellar medium. But it is unclear how these bullets could be created and accelerated without disrupting their structure. An alternative interpretation of these features is that they condense in situ in high-velocity stellar winds as a result of hydrodynamical instabilities—such mechanisms have been proposed to explain the condensations seen in supernova ejecta 3 , and may also operate in planetary nebulae. Here we show that similar processes can also form bullets in the poorly collimated winds from young stars. We therefore suggest that bullets associated with outflow sources should not, in general, be ascribed to explosive events at the source; rather, they form as a direct result of the interaction between the outflowing gas and the surrounding medium.
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