The Tip of the Red Giant Branch
The Tip of the Red Giant Branch (TRGB) is observed in the empirical Hertzsprung-Russell (H-R), or color-magnitude, diagram (CMD), which encodes information about the luminosities (magnitudes) and temperatures (colors) of astronomical objects.
A low-mass star that is fusing hydrogen in a thin shell surrounding its inert Helium core will increase in luminosity and ascend the H-R diagram along the red giant branch. After the RGB star's once-inert core reaches temperatures to fuse He, a seconds-long thermonuclear runaway event is ignited. This Helium Flash injects sufficient energy to lift the core degeneracy and power a rapid evolution off of the RGB. The RGB star then migrates toward evolutionary stages that are bluer and up to two orders of magnitude fainter, i.e., the Horizontal Branch or the Red Clump (RC).
The rapid trajectory in luminosity-temperature (or magnitude-color) space leaves behind a sharp discontinuity in the observed counts of stars at the ``Tip'' of the RGB. And due to the identical levels of core degeneracy reached by stars below masses $M \sim 1.8 M_{\odot}$ at the time of the Helium Flash, their luminosities at the TRGB are almost perfectly invariant, particularly when observed through the Johnson-Cousins $I$-band ($\lambda_{eff} \sim 800$~nm).
