The Young Stars

The entity that emerges from a core is a young star that only vaguely resembles the final version. It is spinning, spotted and oversized. The core and envelope have been shed but a massive disk now dominates the environment. In this chapter, we describe and explore the reasons for the violent behaviour and how it is eventually harnessed. The vast majority of young stars are T Tauri stars. This is simply because they spend most time in this slowly evolving state. It can be argued that the destiny of T Tauri stars has been pre-determined in the previous collapse and main accretion stages. Therefore, this growing phase represents nothing more than a necessary stage to endure in the progress towards adulthood. However, they prove to be of immense observational significance. As objects revealed in the optical for the first time, they have provided most of our knowledge of Young Stellar Objects (YSOs). Due to their large numbers, however, they also provide us with statistical samples and spatial distributions. Finally, we find that both accretion and outflow have not been entirely eradicated but are still ongoing in a moderate form. This activity thus presents itself in manifestations explorable with our optical telescopes. As planet-forming environments, the classical T Tauri stars have now taken on extra significance. We can explore how a spinning disk might evolve out of an envelope into a stellar system in which a star is surrounded by planets. The disk geometry and the balance of forces permit a more quantitative approach here than for any other stage of star formation. Our concepts concerning the nature and evolution of proto-planetary disks can now be tested. This has led us to re-think our ideas because observations have revealed unexpected disk properties.