Collapse from giant molecular cloud to protostar
Stars are born in giant molecular clouds, cold regions where hydrogen molecules, dust, and gas are pulled together by gravity. These clouds are typically 50 light-years across and so cold (10 to 20 Kelvin) that molecules dominate over atomic hydrogen. A disturbance (perhaps a nearby supernova shock wave) can trigger collapse in a small clump of the cloud. As the clump collapses, its density and temperature rise. Conservation of angular momentum causes the collapsing material to spin, forming a rotating disk around a central protostar. This protostellar stage lasts roughly 100,000 years, during which the central object is still too cool to fuse hydrogen and is surrounded by a thick disk of infalling material. The protostar grows hotter and denser as gravitational potential energy converts to heat.
The T Tauri phase and approach to maturity
As the protostar continues to contract and heat, it enters the T Tauri stage, characterized by intense stellar winds and violent outflows. The protostar is now hot enough to ionize surrounding gas but still below the threshold for core hydrogen fusion. This phase lasts roughly 1 to 10 million years and is marked by high variability and energetic jets. Young T Tauri stars still accumulate material from their disk, building up mass and angular momentum. Eventually, the core temperature and pressure climb high enough for hydrogen fusion to ignite. The young star then becomes a main-sequence star and enters a long, stable phase lasting billions of years. Once fusion begins, the outward pressure of radiation balances gravity, halting the collapse and stabilizing the star.