A large and very bright star that has undergone the gravitational collapse of its central part and the expansion of its outer parts.
Sun type stars become red giants when the hydrogen in their centre is exhausted and the combustion of the hydrogen in the outermost parts intensifies.
The nuclear fusion reactions then start to consume helium, and the nucleosynthesis of the heavier elements (e.g. carbon, oxygen) can then begin.
As the star evolves it loses a lot of weight to the interstellar medium. After a period of great variability and activity, the stars eventually move on to become white dwarves.