We will now consider stars with masses less than about twice the mass of the Sun. As the central core contracts, the hydrogen surrounding it heats up and creates a shell which makes most stars increase in luminosity. The stars also become enormous.

The expansion causes the temperature at the surface to decrease and the star leaves the main sequence. It becomes a red giant, which can be found in the high-luminosity and low-temperature portion of the H-R diagram. In a main sequence star, if the core of the star becomes hotter for any reason, the core can expand slightly and the temperature will drop.

However, in a degenerate core, raising the temperature does not increase the pressure, but will speed nuclear reactions which raise the temperature even further. After a while all helium in the central region is used up and the core no longer produces energy.

Now we have a core of carbon and oxygen surrounded by a shell where helium is still burning and the star moves back to the red giant domain. Stars with lower mass can't compress the carbon-oxygen core to initiate another stage of nuclear burning and instead they leave behind a White Dwarf.

Since no star completes it's evolution to a red giant quickly enough for us to observe the changes, this description of stellar evolution is based entirely on calculations using models. Fortunately, we can test the calculations of stellar models by observing a cluster of stars that all formed at the same time but have different masses. One advantage of using clusters instead of individual stars is that they are at the same distance so that their luminosities can be directly compared.