 Solution

An isochrone is the locus of stars of a common age and composition over a range of initial masses. A real cluster diagram approximates it, with scatter from binaries, measurement errors, composition spread and differential reddening. Region A is the hydrogen-core-burning main sequence. Region B contains evolved subgiant/red-giant stars, typically with hydrogen-shell burning around an increasingly helium-rich core. Region C is the horizontal branch, occupied by core-helium-burning stars; RR Lyrae lie in its instability strip.

Point D is the main-sequence turnoff: core hydrogen exhaustion drives structural readjustment toward shell burning and expansion. Point E is the tip of the red-giant branch, where a low-mass star ignites helium in its degenerate core, the helium flash, and subsequently moves toward horizontal-branch conditions. The two labeled events are different evolutionary transitions, not merely changes in apparent color from dust.