Pulsation harmonics reveal the origin of the century-old Blazhko effect

The Blazhko effect – a quasi-periodic modulation of pulsation amplitude and phase observed in all subtypes of RR Lyrae stars – has defied physical explanation for over a century. Here we show that pulsation harmonics of the Blazhko RRab star V783 Cyg, traditionally regarded as passive replicas of the fundamental mode, encode a direct observational signature of the effect’s origin. Using Kepler short-cadence photometry, we identify a subset of harmonics – here termed ``disharmonized harmonics’’ – whose amplitude variations are anti-correlated with those of lower-order harmonics and of the fundamental mode. These disharmonized harmonics map precisely onto the base of the convective envelope, where convection–pulsation interaction is strongest, and their behaviour directly traces the strength of turbulent convection. We interpret the Blazhko modulation as arising from quasi-periodic variation of turbulent convection in the stellar envelope, which regulates both the efficiency of energy transport and the thickness of the $\kappa$-excitation zone. This interpretation naturally accounts for the occurrence of the Blazhko effect across RR Lyrae subtypes and its absence when the convective envelope extends only shallowly into the He~I/H ionization zone, establishing an observationally grounded framework that links harmonic behaviour to the convective dynamics underlying this long-standing puzzle.