Since Fanaroff & Riley (1974) reported the morphological and brightness dichotomy of radiogalaxies, and it became clear that the symmetric emission from jets and counter-jets in the centre-brightened, less powerful, FRI sources could be caused by jet deceleration, many works have addressed different mechanisms that could cause this difference. Recent observational results seem to indicate that the deceleration must be caused by the development of small-scale instabilities that force mixing at the jet boundary. According to these results, the mixing layer expands and propagates down to the jet axis along several kiloparsecs, until it covers the whole jet cross-section. Several candidate mechanisms have been proposed as the initial trigger for the generation of such mixing layer. However, the instabilities proposed so far do not fully manage to explain the observations of FRI jets and/or require a triggering mechanism. Therefore, there is not still a satisfactory explanation for the original cause of jet deceleration. In this letter, I show that the penetration (and exit) of stars from jets could give the adequate explanation by means of creating a jet-interstellar medium mixing layer that expands across the jet.
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