Stellar Processes and Evolution
Stars form out of nebulae – giant clouds of gas often found in the spiral arms of galaxies. The gas (predominantly hydrogen) can exist in a cloud for many millions of years, but if it is somehow disturbed (by the blast from a nearby supernova, or through intercloud collision for example) the cloud may collapse in on itself.
As the density of the core region increases the collapse accelerates due to the ever-increasing gravitational attraction. The central region will start to collapse faster than the outer regions and so a cloud of gas shrouds the core. If the cloud is particularly large it may produce multiple stars. This process of cloud collapse signifies the start of star formation.
Under continuing collapse, the gas in the cloud begins to warm up and gradually brightens. Eventually the core region reaches a critical temperature such that nuclear reactions can begin and the body evolves from a protostar into a true star.
The remaining gas and dust from the original interstellar cloud starts to condense and planets can form. A strong stellar wind, however, drives away some of the matter. Such stars are called T Tauri stars (from the first star of this type to be observed).