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The evolution of low-mass stars into red giants is still poorly understood.During this evolution the core of the star contracts and, simultaneously, theenvelope expands -- a process known as the `mirror'. Additionally, there is ashort phase where the trend for increasing luminosity is reversed. This isknown as the red-giant-branch bump. We explore the underlying physical reasonsfor these two phenomena by considering the specific entropy distribution in thestar and its temporal changes. We find that between the luminosity maximum andluminosity minimum of the bump there is no mirror present and the star is fullycontracting. The contraction is halted and the star regains its mirror when thehydrogen-burning shell reaches the mean molecular weight discontinuity. Thismarks the luminosity minimum of the bump.

Mirror principle and the red-giant bump: the battle of entropy in low-mass stars