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A main sequence star is fusing hydrogen into helium in its core, releasing energy that causes the star to shine. This is the longest stage in a star's life.

Helium burning is the stage in a star's life when the core contracts and temperatures rise high enough for helium to fuse into carbon and oxygen.

A white dwarf is the hot, dense, carbon-oxygen core left behind after a low or medium mass star has expelled its outer layers. No fusion occurs in a white dwarf.

A red giant is a luminous giant star of low or intermediate mass (roughly 0.3 to 8 solar masses) that is in a late phase of stellar evolution. The outer envelope is expanding and cooling which results in its red color.

A protostar is a very young star that is still gathering mass from its parent molecular cloud. The protostar phase is the earliest one in the life of a star.

A black dwarf is a theoretical stellar remnant, specifically a white dwarf that has cooled sufficiently that it no longer emits significant heat or light.