Final answer:
Very luminous red giants can last as giants for only a few million years before transitioning to the next stage of their stellar evolution. Stars like Betelgeuse demonstrate this, as they are relatively young in comparison to our Sun but are already nearing the end of their giant phase. After exhausting their helium fuel, these stars may become blue supergiants or explode as supernovae.
Step-by-step explanation:
The time a red giant can exist in its expanded state can vary significantly based on several factors, including its initial mass. However, very luminous red giants have a relatively short lifespan in this phase. For instance, stars like Betelgeuse are only a few million years old, much younger than our Sun which is 4.5 billion years old, but are already approaching the end of their life cycle.
Luminous red giants can swell up to enormous sizes, sometimes with diameters stretching beyond the orbit of Mars if placed at the center of our solar system. These large sizes and high luminosities are temporary stages; Betelgeuse will soon exhaust its helium fuel after about 1 million years of helium fusion, leading to further contraction and heating before ultimately exploding as a supernova.
Therefore, very luminous red giants might last as giants for only a few million years before transitioning to the next stage of their stellar evolution, which could involve becoming a blue supergiant or exploding as a supernova.