What is stage 4 of star formation?

Stage 4 of star formation is characterized by the formation of a protostar, where the collapsing gas and dust from a molecular cloud begin to accumulate in a dense core. As the core contracts, it heats up, leading to the onset of nuclear fusion in the core when temperatures rise sufficiently. This stage marks the transition from a protostar to a main-sequence star, with the surrounding material forming a protoplanetary disk that may lead to planet formation. Eventually, the protostar stabilizes, and the star begins its life on the main sequence.