What happends in the core that marks the birth of a star?

In the core of a molecular cloud, regions of high density collapse under their own gravity, leading to the formation of a protostar. As the material falls inward, gravitational potential energy is converted into thermal energy, raising the core's temperature. Once the core reaches sufficiently high temperatures and pressures, nuclear fusion reactions begin, primarily fusing hydrogen into helium. This marks the birth of a star, as it enters the main sequence phase of stellar evolution.