A red giant has an extremely dense core surrounded by a large, diffuse envelope. The structure of a red giant is fundamentally different from that of a main sequence star, where the density and temperature rise smoothly from the surface to the center. The helium core of a red giant contracts slowly, and it grows in mass as the hydrogen-fusing shell drops new helium onto it. Eventually, the core becomes hot and dense enough to ignite helium fusion. Helium fuses into carbon by the triple-alpha process .
Some carbon fuses with helium to make oxygen.
Helium fusion raises the temperature of the core, increasing its pressure, so it expands, and cools. The luminosity of the hydrogen-fusing shell drops because of the lower temperature. More massive stars much more luminous, so they have shorter lifetimes even though they start with larger fuel supplies. The stars in a star cluster are born at about the same time, so they are all about the same age. Older clusters have redder main sequence turnoffs.