 Once formed , the stars within the nebula emit a stream of charged particles known as a stellar wind . Massive stars and young stars have much stronger stellar winds than the Sun . The wind forms shock waves or hydrodynamical instabilities when it encounters the gas in the nebula , which then shapes the gas clouds . The shock waves from stellar wind also play a large part in stellar formation by compacting the gas clouds , creating density inhomogeneities that lead to gravitational collapse of the cloud . 
