What is a red giant star?
A red giant is a luminous giant star of low or intermediate mass (roughly 0.5–10 solar masses) that is in a late phase of stellar evolution. The outer atmosphere is inflated and tenuous, making the radius immense and the surface temperature low, somewhere from 5,000 K and lower. The appearance of the red giant is from yellow orange to red, including the spectral types K and M, but also class S stars and most carbon stars. Red giants are stars with radii tens to hundreds of times larger than that of the Sun which have exhausted the supply of hydrogen in their cores and switched to fusing hydrogen in a shell outside the core.
A red giant star is a star with a mass like our Sun that is in the last phase of its life. Hydrogen fusion reactions have become less efficient in the core region, and with gravitational collapse of the core, the fusion reactions now occur in a shell surrounding the core .This increases the luminosity of the star enormously ( up to 1000 times the Sun) and it expands. The outer layers then cool to only 3000 K or so and you get a red star, but its size is now equal to the orbit of Mercury or Venus…or even the Earth! After a few more millions of years, the star evolves into a white dwarf-planetary nebula system and then its all over for the star. Even larger ‘supergiants’ can outshine the sun by over a million times. These are very massive stars seen just before their supernova stags. Betelgeuse, Antares and Arcturus are such stars. The above two images show a normal red supergiant star with most of its energy coming from a shell of hydrogen fusion activity just outside the dense core.
