Tip of the red giant branch কী?

The tip of the red giant branch (TRGB) is the sharp upper limit of the luminosities of the red giants of an old stellar population, set by the ignition of helium in their cores, the helium flash; in the I band it lies at nearly the same absolute magnitude in every old population, $M_I \approx -4.05$ at a colour $V - I = 1.6$, depending only weakly on chemical composition.