T Tauri stars are young low-mass stars (typically younger than 5 million of years), which are surrounded by a protoplanetary disk, e.g. a disk of gas and dust orbiting around the star. The disk material does not reach the central star: the dust component sublimates in the inner disk, where the temperature exceeds 1500 degrees, while the gas disk is truncated
Pre-Main Sequence stars are young (few million years) stars whose nuclei are not powered yet by the thermonuclear reactions, and that may still accrete gas from a surrounding disk (called protoplanetary or accretion disks). Even if the accretion disks are typically extended more than 100 Astronomical Units (AU, the mean distance between Earth and the Sun, equal to 150 million
In the Milky Way one of the main modes of star formation is in stellar clusters, which remain associated with their parental cloud for about 5-10 million years. This makes the star forming regions complex targets, characterised by rich stellar populations packed into small regions, sometime even with massive stars, and clouds whose morphology and properties are affected by the
