How do stars start fusing?

Stars begin fusing when they reach a critical temperature and pressure in their cores, typically around 10 million degrees Celsius. This condition arises from gravitational collapse, which increases the core's density and temperature. Once these conditions are met, hydrogen nuclei (protons) can overcome their repulsion and collide, leading to nuclear fusion, where they combine to form helium and release vast amounts of energy. This process marks the transition from a protostar to a main-sequence star, enabling it to shine for millions to billions of years.