Stellar system formation. Illustration of a stage in the formation of a star and a planetary system. A protostar has ignited at the centre of a nebula (cloud) of gas and dust that is contracting. This contraction has led to the formation of a disc by the conservation of the original rotational motion (angular momentum, blue circular arrow) of the nebula. Planets may eventually form in this disc. The ignition of the star has also led to two jets (blue arrows) forming perpendicular to the plane of the protoplanetary disc. This is caused by strong stellar winds blowing outwards in what is known as the T-Tauri stage of stellar birth.