for a proto star to be star it needs to gain enough mass in order to have enough pressure to undergo nuclear fusion.
mass is a great factor in the stability of a star. if this state is achieved this proto star became stable young stars.
A protostar is quite like a star. The main difference is that its core is not hot enough to fuse the nucleus of atoms. The protostar becomes a star when its core exceeds 10 million degrees Kelvin.
Many protostars never succeed in becoming a true star as they are not large enough. For a protostar to transform properly it must be at least 0.08 solar masses a solar mass being our sun. If they are above 200 solar masses their gravity is so strong that they literally blow themselves apart.