Artist drawing of a Herbig-Haro object.  Protostar in the center of an accretion disk. Bipolar jets being shot out along the rotation pole of the star and accretion disk.  Eventually the two bipolar jets, notice the bullets in the jet, sweep up enough stuff, interstellar material, and balloon on up.  The bullet firing of shocked gas along the poles is thought to be a magnetic plasma MHD (Magneto-Hydro-Dynamic) effect.