Stars form within the densest parts of great gatherings of gas and dust known as molecular clouds. The first stage in the process of stellar birth involves the formation of a distinct clump within such a cloud. The clump becomes detached from its surroundings and begins to contract under its own gravitational pull. At the center of the collapsing cloud, where the density is greatest, a protostellar core forms. Over a period of about a million years this core draws in more and more material from its surroundings until it achieves the status of a protostar. After a further period, the protostar evolves to become a pre-main-sequence object such as a T Tauri star.