When the temperature of a protostar rises higher than 10,000,000 K, it becomes a true star. At this temperature, hydrogen fusion begins to happen. Fusion heats the star, contracts the core and causes the star to emit light. A protostar must have 0.08 solar masses to become a star.
A protostar starts as a cloud of gas. As its high gravity causes the cloud to collapse, it starts to spin and creates a magnetic field. At this point, the protostar is usually hidden by gas clouds. As the magnetic field creates stellar wind, it blows the gas and particles away, revealing the forming star.