As the clusters continue to grow larger and larger, they begin to break off into fragments forming PROTOSTARS. Think of protostars as embryonic stars, not fully developed but growing steadily, trying to balance the inward pull of gravity with the outward pressure of heat. When a protostar achieves this balance of gravity and heat, the FUSION of hydrogen atoms can occur. A temperature of about 107 K is needed to trigger this balance. At this point, two hydrogen atoms can combine to form something new—helium. With hydrogen successfully fusing into helium the star has achieved HYDROSTATIC EQUILIBRIUM. It has reached what astronomers call the MAIN SEQUENCE. A star is born.