Protostars are formed through a gravitational collapse. When a region inside a cloud core becomes dense enough, it will collapse under its own gravity. This region has to have a mass greater than or equal to 0.08M☉ in order to become a protostar. Density increases and matter accumulates fastest at its center. The collapse causes the cloud core to heat up, eventually forming a core at the center, known as a protostar.
Once a protostar's core becomes hot and dense enough for hydrogen fusion to begin, it reaches ZAMS, which is the start of its main sequence life. Here is when the star first starts burning hydrogen in its core through fusion reactions and joins the main sequence on the HR diagram.
When a star is stably fusing hydrogen into helium in its core it is called a main sequence star. The difference between a main sequence star and a 0-age main sequence is that ZAMS is the very beginning of a star’s life on the main sequence, while the main sequence includes the entire period during which a star performs hydrogen fusion. When a star’s fusion creates an inward pull of gravity balanced by the outward pressure from heat (energy), the star has reached a thermo-gravitational equilibrium.
Stars that have a mass greater than or equal to the sun’s grow to become red giants. After a star runs out of hydrogen fuel for nuclear fusion to happen and leaves its state of thermo-gravitational equilibrium, it becomes a red giant, and has started the process of dying. These stars become red because the energy at their surfaces have decreased, causing their surfaces to cool which changes their color. Red giants have begun thermonuclear fusion of hydrogen in a shell surrounding the star’s core, which is when large amounts of energy are released when light atomic nuclei combine at very high termoeratures and pressures to form heavier nuclei. What happens after these stars reach the red giant stage depends on their mass.