Horizontal Branch - Evolution

Evolution

In main sequence stars with masses up to 2.3 times the mass of the Sun, the thermonuclear fusion of hydrogen (bearing the name of p-p chain) at the core will steadily build up a concentration of helium at a rate primarily determined by the mass of the star. In due course, the helium-enriched core becomes unable to sustain nuclear fusion of hydrogen and the fusion process migrates outward to a shell. The core becomes a region of degenerate matter that does not contribute to the generation of energy. It continues to grow and increase in temperature as the hydrogen fusion along the shell contributes more helium.

If the star has more than 0.26 solar masses, the core eventually reaches the temperature necessary for the fusion of helium into carbon through the triple-alpha process. The initiation of helium fusion begins across the core region, which will cause an immediate temperature rise and a rapid increase in the rate of fusion. Within a few seconds the core becomes non-degenerate and quickly expands, producing an event called helium flash. The output of this event is absorbed by the layers of plasma above, so the effects are not seen from the exterior of the star. The star now changes to a new equilibrium state, and its evolutionary path switches from the red giant branch (RGB) onto the horizontal branch of the Hertzsprung–Russell diagram. This term means that the luminosity of the star will stay relatively stable while the effective temperature increases, effectively migrating horizontally across the H-R diagram.