Stars that do not become red giants

Very low mass stars are thought to be fully convective[10] and thus may not accumulate an inert core of helium, and thus may exhaust all of their fuel without ever becoming red giants.[11] Such stars are commonly referred to as red dwarfs. The predicted lifespan of these stars is much larger than the current age of the universe, and hence there are no actual observations of these stars aging.

Very high mass stars instead develop to supergiant stars that wander back and forth horizontally over the HR diagram, at the right end constituting red supergiants. These usually end their life as type II supernovae.