What is characteristic of a red giant star?

A red giant star is characterized by having expanded and cooled, which results in its luminous and red appearance. During this stage of stellar evolution, a star has exhausted the hydrogen fuel in its core and has started fusing heavier elements. This change causes the outer layers of the star to expand significantly while the core contracts. The decrease in temperature of the outer layers leads to a red hue, while the expansion contributes to its increased luminosity. This phase represents a later stage in the life of a star, with notable changes in both size and temperature compared to its earlier life as a main-sequence star.