Schematic figure of a collapsing nebula

A schematic diagram of the collapse of a molecular cloud core to form the solar nebula. (a) The arrows suggest the incident angles between the solar nebula and the material that accreted to it. (b) Once the material was in the solar nebula, most of it was transported towards the protosun (long horizontal arrows). There was a critical radius in the outer portion of the nebula beyond which a small amount of material was transported outward (short horizontal arrows). (c) After the accretion of interstellar material had decreased, dust in the nebula settled to the midplane. This created a zone with a high density of dust near the midplane. A series of aggregational processes swept up this dust to form planetesimals and eventually planets.