The Red Giant Branch
The hydrogen shell source established when the core hydrogen is depleted burns its way outward, leaving
behind a growing core of helium ash that is very hot, but is initially
too cold to ignite helium. Initially the gas pressure of
the helium core supports the rest of the star, but once the helium core exceeds a certain mass it can no
longer support itself or the rest of the star against gravity and begins to contract. The gravitational
energy released in the collapse heats the interior, causing the hydrogen shell source to burn hotter. The
increased outpouring of energy goes into expanding the outer envelope, which in turn
lowers the surface
temperature. Thus, the star moves upward and to the right relative to the main sequence into the giant
region, as illustrated below.
Stars executing this rapid expansion between the points labeled
6 and 7 in the above diagram define the red giant branch (RGB) of the HR diagram.
The RGB phase is terminated by the
ignition of helium burning through the triple-alpha process
in the core. However, as we now discuss, the manner in which
the helium is ignited depends on how
massive the star is.