Stars begin their lives on the main sequence of the H-R diagram, where they fuse hydrogen in their cores. As they exhaust their hydrogen fuel, they evolve off the main sequence and become red giants. During this phase, they fuse helium in their cores and dredge up material from their interiors, which results in their increased luminosity and a shift to the right on the H-R diagram.
After the red giant phase, stars with masses less than about 8 solar masses evolve to the horizontal branch. On the horizontal branch, they burn helium in their cores and hydrogen in a shell surrounding the core. This results in a stable burning phase where the stars maintain a constant luminosity and temperature, hence their location on the horizontal branch of the H-R diagram.
Eventually, stars on the horizontal branch exhaust their helium fuel and either evolve to the asymptotic giant branch (AGB) or become white dwarfs, depending on their mass. Stars on the AGB go through additional stages of nuclear burning, including the fusion of carbon and oxygen. White dwarfs, on the other hand, are the final fate of stars that have exhausted their nuclear fuel and collapsed under their own gravity.
