The red giant star is the third stage of the life cycle of a star
The red giant star is born when the fuel of a main sequence star runs out and then the star blows up into a massive red giant star destroying everything in its path for example when the sun runs out of fuel it will become a red giant star destroying everything up until Jupiter |
Here is a diagram of the sun in the red giant stage compared to what the sun is like now in its main sequence stage.
The main sequence stars that do not turn into red giants that have low masses are still fully function able and have the convection current with molten material flowing up when it gets hot and then flowing back down once it has cooled off also they still continue to defuse hydrogen into helium for many more years after approximately 3 million years once they run out of things to keep burning they start to cool down and then they become a cool helium white dwarf.
And then we have the main sequence stars with high masses that do not turn into red giants they end up developing into supergiants and follow an unsteady track back and forth across the HR diagram and they usually end their life as a type 2 supernova. |