Final answer:
Stars begin as protostars in a cold molecular cloud, evolving and accreting mass until they enter the T-Tauri phase, observed through their stellar winds and jets. This process progresses until nuclear fusion initiates and the star joins the main sequence.
Step-by-step explanation:
The life cycle of a star begins in a molecular cloud, a cold and dense region where particles and gas clump together due to gravity. Within these clumps, protostars form as the material continuously collapses and begins to heat up. However, these young stars are not yet hot enough to initiate nuclear fusion, the process that defines a true star. During this time, protostars are obscured from our view in visible light due to the dusty envelope that surrounds them but can be observed in the infrared spectrum because infrared radiation can penetrate the dust.
As the protostar evolves, it enters the T-Tauri phase, named after the prototype of its class found in the Taurus constellation. This phase is characterized by the nearing of the protostar to its final mass, after accreting almost all available material. T-Tauri stars, typically less massive than or similar to our Sun, exhibit stellar winds and jets, indicating dynamic interaction with their surroundings. This activity can help clear surrounding material and signal the transition to the next stage of star development where the star begins fusing hydrogen and joins the main sequence.
The process of star formation, from a molecular cloud to a T-Tauri star and beyond, is a complex and beautiful demonstration of the physics governing our universe. It showcases the genesis of celestial bodies from the simplest atoms to complex systems capable of supporting life as we know it.