Final answer:
A protostar evolves into a main-sequence star if it has enough mass to ignite hydrogen fusion. Without sufficient mass, it becomes a brown dwarf. Main-sequence stars go through life stages that eventually turn them into red giants.
Step-by-step explanation:
The two outcomes for a protostar depend on its mass. If a protostar has sufficient mass, typically more than about 1/12 the mass of the sun, it will become a main-sequence star. As a main-sequence star, it spends most of its life in a stable state, fusing hydrogen into helium in its core. This is the stage where a star like our Sun is currently in. After exhausting its hydrogen fuel, a main-sequence star with a mass similar to the Sun will expand and cool to become a red giant.
Alternatively, if the protostar does not have enough mass to ignite sustained nuclear fusion (less than about 1/12 the mass of the sun), it becomes a brown dwarf, an object intermediate in size between a planet and a star. Brown dwarfs do not sustain significant fusion reactions and thus remain relatively dim and difficult to detect.