Stellar system formation. Illustration of stages in the formation of a star and a planetary system. A nebula (cloud) of gas and dust forms (top) from the universe's primordial hydrogen and helium, and other elements from previous generations of stars. The nebula will have an overall rotational motion (angular momentum, blue arrow). A trigger, such as a nearby supernova, will start the gravitational contraction of the nebula. The angular momentum is conserved, and so the nebula flattens and forms a disc (centre). At the centre of the disc, the star ignites under its own weight. The planets (bottom) form in the disc by accretion, a slow gathering together of material by gravitational attraction.