What is the best prediction for what will happen when all of the hydrogen in the Sun has undergone fusion?

Once the core hydrogen is exhausted in 5.4 billion years, the Sun will expand into a red giant phase and Helium nuclei will split to form hydrogen, and the Sun will no longer release energy. The helium present in sun will get converted to carbon and o2 and the glowing sun will get converted to a red giant which have a life period of 120 million years of active and finally it will become a dwarf planet